PB 197 373

DOT HS-800 378

MAXIMUM SPEED LIMITS

A Study for the Selection of Maximum

Speed Limits

INDIANA UNIVERSITYINSTITUTE FOR RESEARCH IN PUBLIC SAFETY400 EAST SEVENTH STREETBLOOMINGTON, INDIANA

Final Report

Volume I of IVOctober 1970Contract No. FH-11-7275

PREPARED FOR:

U.S. DEPARTMENT OF TRANSPORTATION

NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

WASHINGTON, D.C. 20591Reproduced by

NATIONAL TECHNICALINFORMATION SERVICE

Springfield, Va. 22151

The opinions, findings, and conclusions expressedin this publication are those of the authors andnot necessarily those of the National Highway TrafficSafety Administration.

Reproduced frombest available copy

I---·------------~i

~,---~_._-_._--_._--,------...;

14. Sponsoring Agency Code

I

FH-11-727 5

Final Report, Vol.

11. Contract or Grclnt No.

!"H-11-7 27 513. Type of Report and Period Covered

--------~

I VOLUME I LIMITS , ,6~~~f:~'~geO~go:z:t:o~ Co Ie ------,

I A Study for the Selection of Haximum Speed Ll l tsL-- ~ _~--j

17, Authoeisl Kent B. Joscelyn, Principal Investl- 8, Pe,fo,mlngOegan'zation ReooetNo '

I Ralph K. Jones qatorI Patricia A. Elstonr9 Pedoemlng Oeganozoholl Name and AodeessI Institute for Research in Public SafetyI Indiana University: 400 East Seventh Street, Room 519I Bloominqton, Indiana 47401112~Sponsorin9Agency Name and Address·- ----

, Depart:rrent of TransportationNational Highway Traffic Safety Administration

I Washington, D.C. 20591

I~,",,·m.",..,,.,.,I

16. Abstract

This study recommends a method to establish maximumspeed limits based on the 85th percentile of travel speeds.The conclusion is supported by an extensive literaturesearch and analysis of traffic flow data collected by aunique Computer-Sensor System. Such data indicate thatrisk increases with deviation from mean speed. Suchincrease is minimal until approximately the 85th percentilewhen the slope of the ~isk curve starts to rise sharply.

18. Distribu'~ion Statement

Maximum Sneed LimitsOptimum Speed Limits

1--;-;::-'7'---::::--,-;-~;-;--:-----:-,-----r-;;;;-'7'--:---::::-'-:-;--;-';-'7-:--~---;-----":;-;--7;--',-;;---,.,.-:--=-------.~

'._s_e~~i~~~'_S_if_'_(O_f_t_hi_._,_ep_O_'_D ~_~20_'_s_ec_u_"_.t_y_c_IO_S_S_if_,_IO_'_t_hi_s_P_o,g__el ~~' ,.... •• "n

Form DOT F 1700.7 (8-69)

Kent B. Joscelyn

Ralph K. Jones

Patricia A. Elston

Project Management

Principal Investigator

Project Director

Assistant Project Director

Research Staff

Quantitative Analysis

Computer data collectionand programming

Survey Analysis

Literature Search

Gunda OpferFrancis J. ConnellyWilliam J. Kennedy, Jr.Donald M. Goldenbaum

Robert RockenbaughGary Fox

Linda S. Buczek

Gary R. Cagle

ABSTRACT

This study recommends a method to establishmaximum speed limits based on the 85th percentileof travel speeds. The conclusion is supported byan extensive literature search and analysis oftraffic flow data collected by a unique Computer­Sensor System. Such data indicate that riskincreases with deviation from mean speed. Suchincrease is minimal until approximately the 85thpercentile when the slope of the risk curve startsto rise sharply.

iii

A STUDY FOR THE SELECTION OF

MAXIMUM SPEED LIMITS

Index

1.0 INTRODUCTION ... i

1.1 General Objectives

1.2 Specific Objectives ..

1.3 Background ....

1.4 Scope and Approach

2.0 SUMMARY REVIEW OF THE LITERATUF~ .

1

2

3.0 SURVEY OF SPEED LIMIT PRACTICE~:. . 29

3.1 Response Rates

3.2 Analysis

3.2.1 State Survey..

3.2.2 General State Profile.

.. . .29

30

30

31

3.2.3 Cities. 34

3.2.4 General city Profile.

3.2.5 Comparison of cities to the States inWhich They are Located.

4.0 SPEED LIMIT CONCEPTS.

35

38

45

4.1

4.2

4.3

General Concepts

Speed Limits and the Traffic Law System..

Speed Limits and Risk ....

45

46

48

4.4 Speed Limits and Driver Acceptance 51

4.5 Factors Affecting the Development of aGeneral Method . . . . . . . . . . . .

v

52

4.6 Factors Affecting Implementation of a GeneralMethod .

4.7 eri teria for a General r!icthod of Establi:3hin'JSpeed Limits . . . . .

5.0 DTSCUSSION OF SELECTED METHODS FOR ESTA13LISHIJ\jC:':;PPED LIMITS .

5.1 Screening Analysis

5.2 Analysis of Selected Methods for EstablishingSpeed Limits .

5.2.1 Taylor's Theory of Speed Di::::trib,ltiOl!Skewness. .

5.2.2 Oppenlander's Cost-Oriented Approach.

5.2.3 The 85th Percentile Method.

6.0 ANALYSIS OF IRPS DATA RELATING SPEED AND l,CelDENTS

6.1 Introduction

6.2 Approach

6.3 Results.

6.3.1 The Effect of Location.

6.3.2 Accident Frequency Distribution StudlCS

7.0 CONCLUSIONS.

8.0 THE RECOMMENDED ~~THOD .

8.1 Background ...

8.2 Sampling Method.

8.3 General Comments .

9.0 COMrvt.ENTS ON SPEED LIMITS AND SPEED CONTROL

9.1 Special Speed Limits for Trucks and Other Veh c

9.2 Minimum and Advisory Speed Limits ...

vi

9.3

9.4

9.5

10.0

Enforcement Tolerance.....

Measuring the Effectiveness of Speed Limits ..

Recommendations for Further Research .

REFERENCES .

· 103

· 106

· 109

. . 113

APPENDICES . . . .

Appendix A, Glossary of Terms ..

· 131

· 133

Appendix B, Sensor System Description . · 139

Appendix C, Speed Limit Practices Survey Questionnaires.. 169

Appendix D, Data Tables for Speed Limit Survey Responses .. 177

Appendix E, Selected Speed Distributions from theIRPS Sensor System . . . . . . . . . . . . . . . 187

vii

SUMMARY

This report documents work conducted by the Indiana

University Institute for Research in Public Safety under

sponsorship of the National Highway Safety Bureau of the

U.s. Department of Transportation to recommend a method for

establishing maximum speed limits that could be widely imple­

mented utilizing existing technology and manpower resources.

The research plan includes an extensive review of exist­

ing research literature, an evaluation of identified methods,

collection and analysis of data utilizing a Computer-Sensor

System to validate existing methods, and the development of a

programmed instruction text to implement the recommended

method.

The literature review considered more than 300 docu­

ments relating to the history of speed limits; the relation­

ship of speed and speed limits; driver speed behavior and

variables, other than speed limits which influence it; the

relationship of speed, speed limits, and accidents; and

methods for establishing speed limits. The review revealed

three major approaches to establishing speed limits, one

based on measure of prevailing vehicle speeds, another

based on characteristics of the speed distribution, and a

third based on cost.

ix

A national survE~Y of practices used by states ,,;nel r~J L <:'

to establish maximum speed limi ts was conducted to clei:crIT'i,lf'

existing methods, technical resources, and the manpower

,involved in establishing speed limits. Questionnai res \<It r,'

sent to traffic engineers of all state highway departmen~s

all cities over 100,000 in population (130), and 52 select((;

cities with populations under 100,000. The response (88 )

indicated that the following items were most frequently con-

sidered in establishing speed limits:

* 85th percentile* ball-bank indicator data* accident experience* length of zone and adjacent zone* design speed* pace* spacing of intersections and driveways* traffic volume* presence and condition of shoulders* average test run speed* presence of pedestrians* traffic signals and controls

The survey also showed the general availability of

radar, vascar, ball-bank indicator, and vehicle counters to

measure vehicle speed, traffic characteristics, and roadway

features.

The various techniques for establishing speed limits

identified as a result of the literature review and the

survey of jurisdictions were subjected to a scrcenlng ana ~

to identify those methods worthy of further consideration

x

for full-scale implementation. The analysis revealed these

such techniques:

* Taylor's theory of speed distribution skewness* Oppenlander's cost-oriented approach* The 85th percentile method

Data collected using the IRPS Computer-Sensor System

were analyzed to provide a further basis for selecting a

recommended technique from ~he three identified by the

screening analysis. The analysis clearly showed the strong

relationship between devia~ion of the speed of the accident-

involved vehicle from the mean speed of the traffic stream.

The analysis also showed that the cumulative accident

involvement rates were acceptably flat (i.e., independent

of speed) until the speed deviat~on reached a point corres-

ponding to the 85th percentile speed plus rounding and

enforcement tolerances, after which it started to rise at a

precipitous rate.

The final result of the study effort was to recommend

that maximum speed limits be established on the basis of the

85th percentile of travel speeds. Such a limit is:

1. Fundamentally fair in the context of the TrafficLaw System.

2. Related to risk of dysfunction in the SurfaceRoad Transportation System.

3. Accepted as reasonable by drivers.4. Applicable to a wide range of highways.5. Capable of being implemented \vi th existing resources.

xi

The recommendation of the 8:.th percen tile i s ~~ I:l,;:'()rt "j

by a substantial portion of the technical literature as weI]

as by the data and analyses of the present study.

The final project report is presented in four volumes.

Volume I contains the technical portion of the report;

Volume II presents an extensive review of the literature;

Volume III provides an imp leme:1tation program for the

reconunended method of establishing a speed limit; and,

Volume IV contains a condensed explanation of the recom­

mended method for the experienced traffic engineer.

xii

1.0 INTRODUCTION

This is the final report of research on establishing

speed limits conducted by the Indiana University Institute

for Research in Public Safety (IRPS) under contract FH 11-7275

with the National Highway Safety Bureau of the U.S. Depart-

ment of Transportation.

This report is presented in four volumes. Volume I

contains the technical portion of the report including a

surmnary review of relevant Ii terai:ure. Volume II presents

an extensive review of the literature concerning speed and

speed control. Volume III presents an implementation program

for the recormnended method of establishing an appropriatc:;

speed limit. Volume IV is a brief explanation of the recom-

mended method to be used by the experienced traffic engineer.

This report documents research conducted during the

period June 20, 1969 through August 31, 1970.

1.1 General Objective

The general objective of the study was to recommend a

method for establishing maximum speed limits that could be

widely implemented utilizing existing technology and man-

power resources.

1.2 Specific Objectives

The specific objectives of the study were to:

- 1 -

1. Present. a comprehensi'lE::' summary of existinq ,,";(? Ul')

ology relating to spcp-d control and the cstab i"il~

ment of speed limits.

2. Survey state and local jurisdictions to identifycurrent practices in cstablishment of speed limits.

3. Identify the availability of technical resources(at state and local levels) that could be utili:2,clin establishing speed limits.

4. Develop an operational explanation of the functionand objectives of speed limits.

5. Select the best, real-world conceptual approachfor the establishment of speed limits.

6. Conduct analytical investigations to validatesuch concepts.

7. Develop a statistically valid method of establis ll1

speed limits that can be implemented utilizing"existing technology and manpower resources.

8. Identify such research as may be necessary to fur­ther refine the "state of the art" of speed controland the establishment of speed limits.

1.3 Background

At present t.here appear to be three distinct methods

of establishing spee6 limits. They arc: arbitrary methode,

political or constituent pressure methods, and traffic

engineering methods.

Arbitrary methods are generally carried out by count

highway departments or city street commissions that arf--,

handicapped by lack of funds and are generally unable to

secure competent personnel. The result of this handicap

takes the form of speed limits which are posted bv judgm~~t

rather than traffic analysis.

- 2 -

Political pressures, such as driver complaints and

accident pUblicity, often indicate that a speed limit should

be changed. Such pressures, however, give no means for

determining the amount of the change. Political pressures,

furthermore, are highly irregular and do not necessarily

meet the needs of the traffic.

Speed limits determined by utilizing traffic engineer­

ing theory take their impetus from the 85th percentile, pace,

and average test run concepts. These methods are rather

widely used and are documented in most traffic engineering

texts. It should be noted that although these methods

are widely accepted the selection of these methods is, in

most cases, arbitrary.

Independent of the method used in determining the

limit, there are three distinct types of speed limits used

in conjunction with a basic speed rule. Every state has a

statute covering what is known as the "basic speed rule."

The substance of such laws is that: a driver shall always

operate his vehicle at a speed that is reasonable and pru­

dent under existing conditions.

The three types of speed limits that are used to sup-

plement the "basic speed law" are:: absolute limits, p~ima

facie speed limits, and advisory speed limits. Of these,

- 3 -

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Such data allowed detailed examination of existing

speed limit concepts and validation of the recommended statis­

tical method for implementation.

- 7 -

:2.0 SUMMARY REVIEW OF THE .Ll n2ERATURE

This section presents a summary review of selected

literature dealing with the subject of speed and speed con-

trol in the context of high\'v'ay safety.

In excess of three hundred publications were reviewed

during the extensive literature search conducted for the

Optimum Speed Limits study. Several facilities were used

during this search, including the National Safety Council,

Highway Research Board, and North\ves tern Universi ty Trans-

portation Center Library, in addition to the Indiana Univer-

sity Library System.

A number of aspects of speed control have been con-

sidered. Among these are: the history of speed control;

the relationship of speed and speed limits; the variables

which influence driving speeds; the relationship of speed,

speed limits, and accidents; and methods for setting speed

limits.

Deta~led results of this extensive search can be found

in Maximum Speed Limits~ VoZume II~ The DeveZopment of

Speed Limits: A Review of the Literature. h brief summari-

zation of the body of literature in this area follows.

The following chronology of highlights in the history

of speed control in America illustrates the progress that

has been made in this area.

Preceding page blank

Tho hi:; tory of speed lirni ts In the LJni tee! State'; L,::.d:.c:;

back to 1678 when speed restrictions were imposed on horses

in Newport, Rhode Island ( 4 ) European speed control J-Licl

its effect on American policy, and in 1901 tht'? firc;t ::ijJc:f"d

limit for automobiles in the U.S. was enacted in Connecti-

cut. (6)

Early speed control campaigns were colorful and dramatl

But in addition to the radical denunciations of speed and

"speed mania" came more rational argurnc'nts supportinq ',. -1 ~ r

.... 11;-_

adjustment of drivinsr speed and speed requlationr::: to cxic;t;;

condi tions and condemning reckle,os driving rat> r tl1(].n speec

per se.

1I_long v;i t.h speed restrictior: lc,q:isli:1tio,~ C'ITn techno];:

gical developments fC1r speed measureffiC>LL

a need had developed for minimum speed limits in adJition

to maximum limits.

'1 ••• By the 1930's efforts were begun to study SDC

control and take a mc)re realis t:ic aplJroach tC) it ..... 'I ( ·3~ \

The organized traffic safety movement began ii' the rni -30's.

causing both cities and states to institute rigid aCCl

reducticm campaigns, the most notablt, bO.i.119 PTc:vicl<~

Rhode L; 1 and. (63, 65) Already in the 1al=:' Itp0' ' .. '.:

states such as Colorado had speed limits as hi.

- 10 -

During World War II an effort to conserve gasoline and

rubber brought the entire nation under a 35 mph limit. (74)

And in 1948 radar became a part of the traffic control system.

(80)

Tracing the development of the present day speed limit,

it can be seen that many of the ideas which generate contro­

versy today were proposed quite early in the history of

automobile regulation.

A study conducted at the University of Illinois in

1947-1948 concluded that traffic ignored speed limits and ran

at speeds which drivers considered safe and that most posted

speed limits were ineffective because they were unreason­

able. (79)

J. Edward Johnston suggested in 1956 that the 85th

percentile was reasonable and encouraged uniform speeds. (85)

Since the 1950's several major concepts have prevailed

in the controversy over the effects of speed limits on

driving speeds. The first is that speed limits have little

or no effect -- that drivers ignore them and drive at speeds

which they consider reasonable and safe. (~,~, ~~, 104,

!05, 109) An opposing view states that speed limits do have

an effect (101, 107), however, different studies have shown

varying effects, including decreased pace (97), reduction of

- 11 -

excessively fast or slow speeds causing vehicles to t:Ci:'/ 'i

closer to the same speed (77, 114, 175), and a random

effect. (99)

"For any given road there is an opt:imum speedlimit which will have the greatest effect onspot speed. 'I'his value is usually between the80 and 90 percentile of the free-flowing speedas plotted on a cumulative-frequency curvc,:. Ii (77)

Other articles seem to concur that the 85th or SOL p

centile is a reasonable guide for setting speed limits.

99, 113)

It has been stated that absolute speed limits receive

the highest observance, while advisory limits are excecdcu

more often than either absolute or regulatory limits. (90)

Urban limits appear to be violated more often than rural

limits. (105, 109)

The study of driver speed behavior has illustrated a

number of interesting findings.

A 1954 study found that about a third of all driver

exceeded the 85th percentile of the spot speed distributic

If a set of drivers was observed as many as seven times,

more than half could be expected to exceed the 85th per r

tile speed at least once. (116)

f\ study to inves,tigate drivers I speed perccpt,LOnn"F

the abi Ii ty of eight subj ects to halve or doub lic' thc_lr

- 12 -

on conunand. In this study drivers seemed to underestimate

their speed when decelerating and overestimate it when

accelerating. (119)

A Los Angeles study to assess the effectiveness of

written warnings as compared to citations involved the stop-

ping of motorists who exceeded 40 mph in a 25 mph zone.

By following these drivers after citations or warnings were

issued, the study found that:

(1) Where a citation was issued, accompanied by noconversation between driver and officer, 32 of100 cases recorded exceeded the limit againwithin five miles.

(2) Where a safety message accompanied the citation,33 of 100 cases exceeded the posted speed withinthe next five miles.

(3) Where a written warning and safety message wereissued in place of a citation, 43 of 100 motor­ists had exceeded the limit within two miles and22 more within five miles, for a total of 65. (122)

Numerous publications discuss the factors which affect

speeds. The following factors and relationships have been

identified:

Studies of driver variables found that speed increased

with trip distance (126, 136, l:~); non-local vehicles trav­

eled faster than local vehicles (138, 139, 143, 144, 145);-- -- ----~ -- ---

male drivers drove slightly fas1:er than female drivers (2-36,

137, 143, 145); and, drivers wii:h passengers in the car

drove slightly slower than those without passengers (137,

- 13 .-

L 4 } , lJ', i:'"':" \-L -i ---.J) '"

veL!?; ,--:xp(~cted arrivdl time (1·1\;)

')i,inion of the :;pccd 1 mit (14(;) and ai~1()UI1L 0 f dr i Vl nq ! )

ilcctcd \..jj th the cJrivcr"'3 work (146).

V\c:hicle varialdcs sho\t!cd that v(:hicl(~ type (l37! "

traveling faster than oldi"r Oil0S (126, 13(,).

cla:3sificaLion or road t'/pe (137), ,:;urf'ac!:=, type and con-

di.tion (137, 138, 140, J42, 147') I numL,er of lanes and Id'lc

width 112CJ -3 7\__-=-_' .~.:...~_~_~. f140, 142),

type (h~2, lj_-O, access contLol (137, 139, 1421 ,- frequ(~ncy

f . t .0 .. ln ersect.lons (137, 139, 140, 147), shoulder -vj dth a:li:

condi tion (!.!~. L~_~ r 1:..:1.2), and de~; i qn ,-;peed (.~47)· Other

factors identified include geographic locat.ion (137), siqh~

distance (137_, 13..§.: 139, 140, 142, 143), curvature; (137,

138, 140), gradient (1:}_?), length of grade (137, 140),

lateral clearance (1~;7, 142), horizontal and vertical rC':;i~;.

tance (13~, :!:i~), marginal friction (138), vertiea 1 aod

horizontal alinement (142), traffic signals and control

devices (~Q, .~4? f~.~l) f parking (140 f 147)

of pedestrians (140, 147).

and pre80r1C','

Traffic vari abl(,: s shm'Jn to af fect~ ,,,peeds werC\fO i.~11'l> '

(129, l~O, 138, 140, 142, 147)? pCl3Sinq man(~1:\1(-:rs (137)

opposing traffic (l:}J) , and percentage of commercial vehicles

(137, 142, l11). A study a Iso indicat.ed that veh i cl efi in

each lane tended to adjust their speed to the speed of the

slower parallel lane (107).

A study of environmental factors which affected speeds

found that objects on the road shoulders had little affect

on speeds unless the lane widths were less than 20 feet (132)

Other environmental factors include weather (137, 142, 146),-_.-.~- --

time (133, 13I, 142), and roadside establishment (138, 139/

140, 142, 147).

It can be seen that a number of studies have been con-

ducted to identify the above mentioned variables and that

there is considerable agreement in this area.

Although discussions of the speed-accident relationship

date back to the early use of the automobile, serious

research in this area seems to have begun in the 1930's.

From 1930 to 1939 the speed capability of the average vehicle

sold rose from 55 mph to 84 mph. (126)

A number of the early studies in this area attributed

the occurrence of accidents to speed (252, 151/ 153) while

others did not find a relationship between high driving

speeds and accidents (126, 152). One author even went so far

- 15 -

;.vdY de~~ ign. He \vrote:

"r. t i:o CiS cos c; 8!'LL a 1 to s[)(::nd effort tOe mprovcI he speed of transpor.(tion as it L; to ~3f1end

it to reduce a:"ci(]cn!>~. 'The automobile \4a5

not rna.nuf2ct U',":'l to '3(~V'~ 1 j '.,1e:=;, or the rr);1d-',,' a y b u i 1 t t) P r:: ')en t J 1 u:r: Lc' :'0 • " ( 1 :-, 4 )

;;,tt: opinions are pole,xiz2d on the '1uestil)l1s '::Jf ~:;o"'<l

::ffc:ct.ivc mean" () dccident reductic'J.

caus,:.; at accident,s (4) and cth:':cs ";"0 indi.cat.e that aecider

increase wi th :i ncreased speed ,0; (J 5 7, 1. (,::) •

;\1. Earl Ca.rnpbcl1 prc~scnL:: .,~n L: tc°rC' ting approach to

, . 1 .speea-aCCl(lent comparIsons. 30, 1964 11(~WS rC.l.eas0

from tho California Divisi011 of Hi..qL\·FlY::; statccd that dlthou

1911, this is a fc1tdlity rate of 3,')'1(' n,"-r 10(' m~lJion

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ranC1('~~. Speerl and accidc:nt,·, cannot: b(~ rcL:lLc(ito each other simply as a tvw-dimo:::n:;ionalrelationship: the relationship is always multi­dimensional and may include one hundred otherfactors ... the t:hird diTr(~nsion of traffic den­Slty must always be included if any validrelationship is to come out of the analysi~~. II

(1:33)

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of ~)(,,'(-'.ds of alI th(~ cat-: in Clccc:a]_ trafc:;i:rc'a11', and (2) fixed d)jc~cts ane} oth,r~r rO:l 1 -; L

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t:hat the hazard _[reb: above and bclc\! t:h.-: d'/:::;r,,'!t';-; 1) l,_'~ c~ (} 0 f tra ~ .f i r: f J ()h' in Cl'(~a ~:~ c' s fa, S -t-,("~ 'r- t~'~lc 1'-1. '1 t ~--i ,'-I.

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(:I>_~ ct~::) C~:; in 1-1 a 7:~ a.rc1 () f t-:- r-l~;(111CTl C::""27' c) f i. ~'~ \IO 1 \,.7; ~,l n ',~~

i,1j_t.~1 j_rlC~l~c:c1.~~(~l irl spc·c~(J is fOU11(:j in. t:"l l~-!C, -t' ~

records of th(~ :Jing-;e-·v(~hicle acr;ident. ... 'Till:; ]',')[ accident constitl,tc;,; mon' thaD o!','-tJl'Lrc:J c'::t-hc accidents resultinq in fat(11it,ics, and it_ lS

j ncrcClE;ing at i'[ ra tee c;i,s:rni £i can -'-1'. f as tr.-.r thai;t.11 (-; <l\t~:~ l- agt: r a 1:-0 o.f in c rc~ as E.:~ () f -~ cl. t a 1 (J.{~ (::.i.. Ctt~ 1-\ L::', ,cE;pccial1y in urban areas." (l e;3)

A 1956 study stated:

II [:f) as ter dr iver,s hav(~ more ac:cid,:'l1 ts than s} C'i"'2~­

drivers, especially when judgpd hy their spep~s

in the afternoon. The inch v:id\Ell speeds of t_h:'drivers wi th accident records are sTirill tTy [ll-,;!H:r:

tha" those for the drive rs wi theiu t accLden t rc c-­Or(:l~3; \vhile il1 -ttl(~ mC)l~ning, it .is t;~c: cJl:-:~V(?rs

',vi t:hOll t accidcn t recorcl_~; whose' ,)t'-c€'cb dl-'-' s Li,<;-; t Ivhigher." (158) ",,- ",~~~,;.'

Another idea having sevcro.l propo;-',C:'ilt:S L-; tL~'t lc',--

speed drivers are more likely to he involved in accid?~c

- 17 -

than high-speed drivers. (159, ~65, !~) Chance of being

involved in an accidEmt is lowest about 65 mph, highest

for low-speed dri.vers, and increases over 65 mph. (163)

" •.• [Olnly eight percent of all fatal motor vehicleaccidents are reported to have happened at speedsabove 60 mph in 1962. On the other hand, morethan half of all fatal accidents in urban areasreportedly occurred at speeds under 30 mph, thatsame year." (.!1~)

.,. .'A very rational stand taken by several authors is that

speed is a causal factor in accidents, but it is one of a

number of causes and not necessarily the most important.

( 156)

"Speeding is outrageously overrated as a CAUSEof accidents. The fact is that it is inattentive­ness or errors in judgment or lack of driving skillwhich contributes most heavily to the causes ofaccidents. Seldom does a single deficiency causean accident." (178)

The Proceedings of NationaZ Highway Safety Bureau Priori-

ties Seminar gives the following explanation.

"Does speed cause accidents and produce casualties?Obviously, considering speed of itself, theanswer is often no. As a matter of fact thereare occasions when the capacity for speed mayeven aid in the avoidance of a crash or the miti­gation of its results (The capacity to passquickly in a suddenly developing tight situationand the minimizing of speed differentials inrear-end collisions, for example). On the otherhand speed can very often compound the task ofaccident avoidance if not precipitate a crash.Further, and without question, speed aggravatesthe consequences of the crash.

- 18 -

"In discussing speed in relation to accidents itis well to delineate the several senses in whichthe term might be used. It is useful to thinkof speed in the following contexts:

"a) very [sic] high speed -- speeds approach­ing and exceeding 100 mph. Under these conditions,the speed factor dominates as a causative agent,since few if any of the elements of the overallvehicle-driver-highway system have been designedto accomodate travel at thi~3 speed.

"b) Excessive speed for conditions -- speedsranging anywhere from zero to design speeds.This category of speeding encompasses many ofthe speeding citations issued in connection withaccidents.

"c) Differential speed or speed gradients inthe traffic stream -- in part, an overlappingset with excessive speed but: also includes in­adequate speed. Differential speed and the relatedvariable "acceleration noisE~" figure prominentlyin the safe and efficient flow of traffic. Largespeed differentials are seldom if ever cited asa contributing cause, the factor being implicitin other improper driving cat.egories such as fol­lowing too close and reckless driving.

"With these connotations of speed in mind it canbe appreciated that speed is very often not asingular or an explicit variable in the accidentequation. Thus, efforts to treat speed as anaccident cause are often reduced to treatingsymptoms arising from the synergy of speed andmany other system factors." (179)

There are differing views as to the effect of speed zon-

ing on accidents. Some studies have shown that speed limits

reduce accidents either in number or in severity.

164, 168, 170, 172, 174, 176)

(114, 155,

In 1960 John Baerwald wrote: "No evidence exists to

indicate that accidents are increased when speed zones are

- 19 -

d':'n t;; are rna teri alLy H".ducl.;,j by' e stabl ishing zones, al thow);,

'i dOvJ!1vJard trend is indicat:d." (96)

Other studi,~s hav(' sho,..ln that specc:c1 limi t practices

"Jere inc ffecti ve a t reducing accic.1en ts of any type'" (177

()nc: of the newe r theor.l eO) in the area of SPCl~ci and aCC.L-

dents "1 c.'-- ,) that accidents dre related to speed dlftercnc£s.

(189, 191) Probably Ute mo::.;t ricted study to support. this

theory was conducted by David Solomon. He concluded:

1. "The ac<:idc:1t·-involvemcnt, injuc/ .. aridproperty-O(imagc' rates werE: highc~;t. at very10\..] speedE;, lowest at about the aVl'Tdqespeed of all traEf lC, and incl'c"scc] at thevery high speeds, particularly ~t niaht.Thus, the greater the varia~jon in spec'cl ofdny 'leh] cle from the averaqc f3pe(~d of alltraffic, the greater its chance of boinginvolved in an accident."

2. "The severi i:y of accidents increased asspeed increased, especially at speeds ex­ceeding 60 miles per hour."

3. "The fat.alit:y rate was hiS]hest at_ verl hi;,:,spE~eds and low(:st at about thE? c3'Ieraqc ,';i:;c~eCi."

4. "Pairs of :passenger car drive:t":,; invc'lvedin two-car, r('ar-end colli:,i J)", were: mucnmore likelyr.o be tra'JeilJlc:j at. speed (lif-T '·:crencer; CJY::(l·i~ly ir! CXCf':':.: of thnse ()]),~(:rvc

for pair:; (.IF cars in normal traffic ... '.

~~1. "r)ri\lC~rS c)f r\(i:::;S(~nSJl~r CZ1r~~ ,hZ_l\ling l(}\~" hc;.rs(~­

[/)wer had ];JJhcr invol Vl,men t- rate::: t:::anelr i ver:, 0 I· car:, havin] hieher horsepower, ...Thi:, may )),-, related to the r,.'l.:ltjVC:~' ''/ l.'Doracceleration capability at hiq!lv/ay speedsof car~.; having 10\-,7 horsepower."

2 "- u

6. "Nearly hEtl f of a] 1 ace: i.dc'i1 t involvcrnentswere either rear-end collision or same­direction sideswipes. However, the propor­tion of these accident involvements decreasedas travel speed increased. Single vehicle,noncollision accident involvements contri­buted an increasingly greater proportionof all accident involvements as speedincreased, particularly at speeds of morethan 70 miles per hour. At speeds of 80miles per hour, non-collision acci~ents con­stituted half of all involvements. Althoughangle collisions usually were less than 15percent of the total, at speeds of less than25 miles per hour they constitutcJ more thanone-third of all accident involvements. Theproportion of head-on collisiuns or opposite­direction sideswipes incroased as speedincreased; but this typo of accident involve­ment always was less than 20 percont of thetotal regardless of spc'cd dnd (;ay ('1 nightcondi tions . " (167)

A recent study by RTI-IRPS indicated " a lU'-shaped re-

lationship between involvernen t rate and speed. dr,'V j i1 tion ...

These results confirm the hypoth(~sis that: slm<! driving as

well as fast driving increases the like1.ihood of being

involved in an accident. However, the curvature of this U-

shaped relationship is not as pronounced as that given by

Solomon ... "

The RTI-IHPS study also gave suppori to the w:;e of the

85th p\~rc()nti le cri terion ~~; ta tinq:

"'J'ho 3tal1dard dcviatic'1i. of Lh'· speed distributionis from:; to 7 mph. l\pprox.i.mately ~j~:& of thedrivers drive below tll\~ mean plus one o;tandanldeviation. The: driveL; havinq SIH-·(!r)r; bc,tweenthe mE~an and one standard d(~viation abovc~ the

) 1 .-

T"'l' -'-cqion betwe(:"~n on'? ':lnd hie standtll---,j dcvj·:lti_OD

ill )C)"''. th c mean s:Deed cnCU'T',pa~:;~:e,,; approximu tely] () pe I~cpn t a f the' driver:; ;,nd doC's not haV0 asignificantly oreater inv()lvc~pnt rat0 thCln ntmean speed. ~his reqion from the end of thefirst to the end of the ~oconrl standard dov~a­

tion is approximately t~p tolerance level allowed~)y police ,"1genoie::-:." (]()U

A 1966 articlc' sho\'7o r1 theit on the fi"c SCcC

(182)

198, 199)

i_D-LD four main catC'o::Jries in the ::1'tic1; "1\:1 lnrorma-

tional Fe'port on Snc'cd Zoninci "

spoen limits should be ha~;ed on

7his article suqqests th ",-

! l ()']

In

(C' ,-

\ .) I

-)', ',-, \) , 20G, 2 () (3 , 209)

I'-inlTu1 I-(essler qj\,(~S the folln\riinq -'1)';(.,[;,' lion fo)

tho 85th nercontile fnr ] i T,j f- C' •"L,,; ..

-- 22-

"The 85th percentile speed is based upon thetheory that the majority of motorists travelingupon a city street or highway are competentdrivers and possess the ability to determineand judge the speed at which they may operatosafely; further, that motorists are responsibleand prudent persons who do not want to becomeinvolved in an accident and desire to reachtheir destinations in the shortest possibletime. " ( 204 )

A 1969 "Resolution of the annual meeting of the American

Association of State Highway Officials" states:

"The review of existing practice revealedthat most of the member departments use, pri­marily, the 85th percentile speed. Some agen­cies use the 90th percentile speed, and ofsecondary consideration are such factors asdesign speed, geometric characteristics, acci­dent experience, test run speed, pace, trafficvolumes, development along the roadway, fre­quency of intersections, etc.

"On the basis of the foregoing review, theSubcommittee on Speed Zoning recommends to theAASHO Operating Committee on Traffic for consid­eration as an AASHO Policy on Speed Zoning that:

"The 85th percentile speed i.s to be given primaryconsideration in speed zones below 50 miles perhour, and the 90th percentile speed is to begiven primary consideration in establishingspeed zones of 50 miles per hour or above. Toachieve the optimum in safety, it is desirableto secure a speed distribution with a skewnessindex approaching unity." (207)

A California publication "Speed Zoning -- Why and How"

discusses the results that realistic speed zoning may pro-

duce.

- 23 -

"A. Reduce the speed dtffL:;rential in a tLlfficstream when there is a large variution 0,

speeds. This makes driving easier, incronscscapacity and reduces the likelihood [sic] ofaccidents by encouraging most drivers to travelat about the same speed.

D. Give enforcement officials a good guide as towhat a reasonable and prudent speed is undernormal conditions and permits concentrationof enforcement agilinst real traf fj c viola­tors.

C. Give motorists a speed limit which they canrespect and obey. \""hl:~;'i drivers respectspeed limi t:s in arca.~::; with which they arcfamiliar, they are more likely to pay atten­tion to limits in unfamiliar areas.

D. Assist traffic courts by providing a real­istic guide as to normal, reasonable andprudent: speeds.

E. Give local residents a realistic pictureof the actual speed of most traffic. Thereis no safety in blind reliance on a 5~eed

~imit inconsistent with speeds actuallytraveled by traffic.

F. [r:.sure t.hat: all speed zone:': ::3atisfy ti,.'requiremc)nts of state law." (208)

More recently developed areas of discussion concern tho

uniformity of speeds or the speed distribut.ion. (85, 214)

William Taylor [las said that assuminq that var Lation oC

speed distribution and hig11 accident rates on cortain cc--

tions of highway arc a result of drivers' i lability to prop-

erly evaluate the driving situation, it seems that the

,>peed di:; tr ibu tion would bc~ helpful in det.ermi'linq '.-Jhc l:('

speed zoning might be effective.

centile may not be lnfluenced by each driver's inahilitv ~G

:2 4 --

make a proper evaluation of conditions in the way that skew-

ness and kurtosis of the speed distribution are. (210 )

Taylor's theory states that a relationship exists

between the rate of occurrence of accidents and the distri-

bution of speeds on a section of rural highway, and that the

effectiveness of speed zoning in reducing accidents depends

on the speed distribution before and after zoning.

His study concluded:

(211 )

"1. There is a strong relationship between therate of occurence of accidents and the speeddistribution on rural state highways.

2. The best parameter to use in determiningnon-normality is the skewness of the distri­bution.

3. Changing the speed distribution from non­normal to normal results in an accidentrate reduction which is about twice thatfound under any other set of before andafter conditions.

4. warrants for speed zoning should be estab­lished which include the speed distributionas a factor.

5. The 'Before' speed distribution alone isnot adequate as a warrant for speed zoning."(177)

A Tennessee Department of Highways Study based on

Taylor's theory concluded that speed limits below 50 mph

are best represented by the 85th percentile, while limits

of 50 mph and above are best represented by the 90th per-

centile. (212 )

J. C. Oppenlander proposed a cost-based method of

establishing speed controls. His theory entails:

- 25 -

1. The se lect ion of an optimel1 speed th;] t mini·­mizes the cost of hiqhway transporLucion,taking into consideration monetary, time,safety, and comfort factors.

2. An adjusted speed is derived from the optimalspeed by subtracting the reduction in speedoccasioned by driver, vehicle, roadway,traffic, and environmental variables thatmodify vehicular speeds.

3. Statistical relationships between upper andlower speed limits and adjusted speed pro­duce t.he posted speed regulat:ion. (200)

Jack C. Marcellis attempted to apply part of Oppenlan-

derls theory, calculating the t.otal cost. of t.raffic movemen,

as the sum of operation cost, time cost, and accident co,;t.

The optimal speed for urban streets was sca.lr:~c1 accordinq

to frequency of stops. For passenger cars, optimal 1.Jrharl

speed ranged from 42 mph for 0 stops per mile to 27 mph for

16 stops per mile during the day. Niqht optimum J. S s l.-:'qh t 1 \."

lower. Commercial vehicles would move at an optimum speed

of ·37.5 mph at 0 stops per mile / and at 8 stop:') per mile

the rYJtimum would he 25 mph. (215)

~hus, it can he seen that there are three major approaci

to establishing speed limits, one based all. measures of prc-

vailing vehicle speeds, another based on characteristjcs of

the speed distribution, and a third based all. cost.

In summarizinq the body of litc~raturc conccrninc! :.:op,' d

and speed control, the reader can probably be certain

only one thing -- the controversial nature of many of the

- 26 -

Llndlngs in this area. However, in considering the most

rational and best supported approaches to various aspects

of the speed problem, the following conclusions would

seem reasonable.

1. Many of the basic premises concerning speedbehavior and its control are not new; they appearearly in the history of speed regulation and theautomobile.

2. Numerous factors relating to the driver, thevehicle, the roadway, "traffic, and the environmenthave a determining effect on driving speeds.

3. The main element in determining whether driversobserve a speed limit is their perception of thereasonableness of the limit.

4. Speed limits, taken as a whole, are beneficial,or at least appear to have no detrimental effecton accident occurrence.

5. Speed may playa large role in the severity ofaccidents, but is merely one of many factors inaccident causation.

6. At present the most widely supported criterionon which to base a speed limit is the 85th per­centile speed.

7. The theory that accidents increase as the value ofthe standard deviation increases, i.e., that speeddifferences play a causative role in accidentoccurrence, is a promising one, as evidenced by ahigh accident rate at both low and very high speedsand a lower accident rate around the average ornormal driving speed.

- 27 -

3.0 SUR\IEY OF SPEED LIMI'I' PRACTICES-_._--------

A survey of the practices used by states and cities to

establish maximum speed limits was undertaken to determine

existing methods, technical resources and the manpower in-

volved in actually establishing speed limits throughout the

nation.

Questionnaires (see Appendix C) were addressed to the

Chief Traffic Engineer of the Highway Department of all

states, of all cities over 100,000 in population (130) and

of 52 selected cities with populations under 100,000.

It must be understood that the summary that follows

represents a summary of responses and not necessarily a

summary of actual practice. In general, the 85th percentile

concept has apparent widespread acceptance and speed-

measuring and volume-counting devices are available in

nearly all jurisdictions surveyed.

3.1 Response Rates

An overall response rate of 88% was achieved. Of the

questionnaires sent to each of the 50 states, 48 were

returned as a result of either our initial request or follow-

up letter. One hundred thirteen of the cities of over

100,000 population responded to the survey. All of the

surveys from the states and the larger cities are thus

included in the quantitative response analyses.

- 29 -- Preceding page blank

Tne 45 surveys received from the cities of less than

100,000 population are incJuded in a qualitative evaluation

The responses from these cities were generally of

CllV'st:.l.onahle rel lab! Ii t~y! and they were thus excluded from

the: qucmtitative anal.ysis.

3 ') J\.na1y s.:h~

?_~. 2 ~} ..__ §. t a !:.~_~l~~Y_~Y

Information from the state survey was analyzed in ~wo

ways. Tables of response breakdowns are presented in

Appendix D.

quecotion.

Table I sho,,,,s the numlJer of re-spc~nscs to CtlC}l

For example, on question I, no one selected

response .. l," tnrec selected response ":2 T II and 4(, scI ':: :.J" 1

rcsplJnsc !f 3 . !I In cases where more than one rLsponse wns

pcr;nitted (questions I, II, III, IV, VI! VII, and VIII) ,

res[onses to each alternative were tablllated separately.

For example, if the responses to a partic~llar quc;stion

were 1, G, 18, and 20, each was viewed as a separate rcs-

ponse. The frequency wi th "'lhic11 each such comC.lnatJOn

occurred was not determined, due to the great number of

possible combinations for each question.

~;C'cond, for qucstionsl.n which multIple rC'sponsc:c ':Jeri'

allv..'::'u, t~c avc~raq(' number of respon:,c:3 r'las tabulat_cc11l1d

th~ information presented in Table II .

. 30 ..

question V, for example, this indicates that an average of

10.31 different factors out of a possible 21 are used by

states in setting or altering speed limits.

3.2.2 General State Profile

In an overwhelming majority of states, numerical speed

limits are set by state statute, with provisions for changing

or setting speed limits by other agencies or jurisdictions.

Nowhere, are numerical speed limits set by state statute

without some such provisions allowing other agencies, pri­

marily the state highway commission, county, or municipal

administrative agencies, to change or set speed limits.

Most states reported that any changing or setting of speed

limits by local agencies had to be supported by an appro­

priate engineering or traffic study.

In 10 of the 12 driving locations listed under question

VIII, the states reported that they relied heavily on

traffic surveys and engineering methods as aids to setting

speed limits. On residential streets, traffic surveys and

engineering methods were used with the same frequency as

local officials or an agency representing a local juris­

diction; in business districts local officials were most

commonly used to set speed limits, with traffic surveys a

close second choice. Though few states specified other

locations where speed limits were set, in those cases where

- 31 -

en':1ineerinq m,_,thods were u:;cd by a slim majority as aids

i.n c tdh~j :;hin9 .limits. In all types of driving locations,

citizens' petiLions weI:' rarely used <1:3 aids i;-i the estab-

ilsfllnq of spceci 1 i;Tlits .

1\11 s tat~es but one rf:portcd i:ha t L.he 85th pOTeen ti Ie

15 used in engineering or traffic studios prior to the

alteration or establishing ot a speed limit.

cons idered Wl':!re, in orde:c 0 r dee.n~asing frequency: aeci-

dent. expcric'Ilce at:. that 0:( ':'I.mllar locations, ball-bank

indicator, length of z()ne and l:ffect of dd:iaccnt zone,';,

design ';Dc~pd, pac and spacinq of intersecLi. C',S and Clri'1e-'

ways. Pl':c:;ciltage: oj commercial vehi,' :eo:~ was :::un:::-.i.dcrc'.l

least frequently.

Most state:'; cC}Jorted a eli f f oren j-J a j sp.'ed Iimi t for

trucks and cars, primarily for Lensons of safety. In state

'7here cars at1.d trucks had the'; samc:3pCE.::cl limit, facili tatL

of traffjc flow was the primary considccation. In TIC' case

did any state report that trucking or other ;ndustries

directly lnfluenced truck S;'2i::,d limi t:

l.ndi Cd. '~~or~, .

. ,-- 32

reported that vehicle coun ters were avai lable to th,~m.

Very few states listed additional available devices wlu ch

might aid them in setting speed limits. Where additL'J.dl

devices were listed, computers were most frequent, but by

a very slight margin.

Nost states did report making attempts to evaluate

the effect of new speed limits. Answers as to how this

effect was measured were quite varied, but most contained

similar elements: before and after studies were used

extensively, and were related to other factors such as aCC1-

dent experience, 85th percentile, radar checks, and speed

checks. While reasons for not measuring the effects of

new limits were not requested, one state did provide such

an explanation, namely, that the beneficial effects of

reasonable limits have been well established in numerous

studies.

In summary, responses concE!rning speed limit prac-

tices at the state level may be summarized as follows:

1. State statutes set numerical speed limits and makeprovisions for changing or setting speed limitsby other agencies or jurisdictions.

2. Authority to set or change limits is largelydelegated to two agencies or authorities: thestate highway commission and county or municipaladministrative agencies.

3. When local agencies set or change speed limits,the change must be supported by an engineeringor traffic study.

- 33 -

4. For rC'~;identi()l ~3tn'et_~;, traffic surv,-_'ys anci localofficials set speed limits; in business distrlcLSlocal officials set speed limits; at all otherlocations traffic surveys are used exclusively.

5. The ten elements most frequently considered intraffic or engineering studies of speed limits are,in order of frequency:

85th percentJle speed- accidEmt experi(~nce at that_ or ~~in'cllar

locations- ball-bank indicator data- len(Jth of zone b.nd effect of ad~iaccnt

zones- design speL,d- pace- spacing of intersections and drive-

ways- tra.ffic volume- presence and condi tion of :iC~j ; (1ers- average test run speed.

6. For reason~3 of safety i:h(~rc lS d :3EJccd limit oif£,;(-­ential for trucks and cars.

7. Four instruments; radar, vascar, hall-bank indi­cator, and computer, are generally available tomeasure vehicle speed, traffic characteristics,and roadway features.

8. Attempts to measure the effects of new or alterodspeed limits are made with t.he ~lSC oj beforc­and-after studies.

3.2.3 Cities

Information about the cities with population over

100,000 is tabulat0d in Appendix D. Dat~ for cities WIt

populations less than 100,000 were not tabulate~ due'

response size and response validity considerations. As for

t.he states, the responses were counted to obtain the n'lITJ:Jer

\4

of responses to each question. Table III shows this infor­

mation. The first two listings on this table show that

there wery 90 "1" (" yes ," in this case) and 15 "2" (" no ,"

in this case) responses for question I. Again, each number

in a combination response was counted individually.

Second, on questions where a combination response was

possible (questions II, II, IV, V, VIII, and IX), the average

number of responses was tabulated. Table IV shows that on

question II, for example, the average number of responses

was 1. 02.

Third, all cities were classified into groups, based

on the state in which they were located. A profile of

each state's cities was then developed by counting the

responses of the cities, and comparing the cities of each

state among themselves to find similarities and differences.

Fourth, the general profile of each state's cities was

then compared to that state to find similarities or dis­

similarities in the answers.

3.2.4 General City Profile

A great majority of the cities reported that states

delegate authority to them for establishing or setting

speed limits in their jurisdictions. This authority, granted

- 35 -

\\7il.::h state highway conunissions, county or muniC!_pal traCf"

engineers, and other agencies, such as the city council.

Though each of these three agencies can set speed limits.

most limits are actually established by county or municipo_

engineers.

Ilinimum speed limi ts an:; SE.' t. U1 most, ~"1 t c:ertcLi._ rlly

not all cities. ~heir use is largely confined to express

ways and urban expressways.

All but three ci ties report 1lsing enC.fLi:.":r J."CT s tv' i(

to determi no speed I_imi ts, and the 8 r,th p(~r

important consldcrat:ion in nearly ever,)' (';St'.

tors, including accident experience at that or ~imilar

locations, length of zone and effect of adJacent zones,

presence of pedestrians, and traffic signals ~n

traffic controls, are considered much less eCten trla:l thE'

85th percentile.

That traffi c surveys and engineering stuc1iC'~: an

wide 11' used again reveals itself in the ci tics I repor-t:; tl, \ '­

at (~v(:r:l location these methods are most eften uSC'cJ to :-,ct

or chdnge a speed limit.

RalJar was ~lvailable to every Cl~)! but one, ond t- ct:

city listed it as being helpful for u ,_ure 'N'ork. 'IF:hiclc

counters and ball-bank indicators were also avallabl~ to

- 36 -

many cities. While there was no extensive listing of desired

but unavailable instruments helpful in dealing with speed

limits, computers and road sensors were most requested.

Most cities did report attempting to learn the effects

of new speed limits, relying heavily on before-and-after

studies for work in this area.

In general, a typical city had the following character-

istics:

1. Authority to set or change speed limits, as providedto the city by the state legislature throughstatute.

2. County or municipal traffic engineers having thisauthority and actually setting most of the speedlimits within the jurisdiction.

3. Use of minimum speed limits, primarily on express­ways.

4. Use of engineering studies in setting or changingspeed limits.

5. Eight factors commonly used in such engineeringstudies are, in order of frequency:

- 85th percentile- accident experience at that or similar

locations- length of zone and effect of adjacent

zones- presence of pedestrians- traffic signals or other traffic controls- traffic volume- design speed- spacing of intersections and driveways

6. At every location, traffic surveys and engineeringmethods are most commonly used to set speed limits.

- 37 -

7. Radar, vehicle counters, and Lcd L-[) lnkidj,:a;are available for use in measurin~1 vf:hl eTc p''''(traffic characteristics, and roadway features.

8. Effects of new speed limi ts are studic'c' bv !-<",and-after tests and surveys.

Although spe:cific questions on thE~ state and ci ::::tn'

vel's vary sligh tly, the qusstionnai l:"t::o rc:que~~L ver-y ,,1 E' L·.

kinds of information.

For purposes of this comparison r r'spcl',;"'''' L' t::

lowing questions are discussed:

II

IIIIIIII

IVV

VIIIIX

IIII'

VIVI IIVIII

VIIIX

For convenience in the remainder of this di,,;cus~~i::;;

stat.e questions are preceded by "s," and cj t'.' qUC"'tiUL

a "c."

vvhen the ci t.ics of a s t~ate were ;;o·lipa.;:'\?d teo ,1,

dgreement was found in the following cas,,,s:

- 38 -,

c i

sI .... cI

sI .... cII

sII .... cIII

sI II .... cVI

sIll .... cVIll

There was agreement when the statereplied "1" (no delegation ofauthority to set speed limits), forall such cit1es replied "2" (no dele­gation of authority to set speedlimits). There was also agreementwhen the state replied "2" or "3"(delegation of authority to setspeed limits), for all such citiesreplied "1" (delegation of authorityto set speed limits).

There was agreement when the statereplied "2" or "3" (delegation ofauthority to set speed limits), forall cities replied "I," "2," or "3"(specification of the agency whichgrants authority to set speed limits).

There was a'Jreement when the state andall cities specified the same agenciesto whom authority to set speed limitswas delegab~d. (Since in sII, "I" 1Sno delegation of authority, stateresponse "2"' matches city response"I," state response "3" matches cityresponse "2," etc.).

There was aqreement when the stateresponded "5" (cities must use engi­neering methods to set speed limit),for all cities responded "1" (engi­neering studies are used). If thestate responded "2," "3," or "4"(specification of other constraintson the cities) in addition to "5" andall cities responded "1," there wasstill agreen~nt, for the other stateconstraints do not conflict with therequirement of an engineering study.

There was agreement when the stateresponded "5" (cities must use engi­neering methods to set speed limits),for the city responded "2" (use ofengineering methods to set speed

- 39 -

sIV .... cVIII

sV .... cVIII

sVIII .... cIX

sIX .... cX

limits) in every case a-m. If thestate responded "2," "3," or "4"(specification of other constraintson the city) in addition to "5," andall ci ties responded "1," "3," "4,"or "5" (other factors considered insetting speed limits) in addition to"2," there was still agreement, forthe other constraints or methodsdo not conflict with the state require­ment or the city use of engineeringmethods in setting speed limits.

There was agreement insofar as thestate and all cities considered thesame factors or used the same methodsin setting speed limits (parts a-mwere considered separately: stater(~sponse "1" matches ci ty response"1," state response "2" matches cityresponse "2," etc.).

There was agreement insofar as thestate and all cities considered thesame factors in setting speed limits.(State response "1" matches cityresponse "1," state response "2"matches city response "2," etc.).

There was agreement insofar as thestate and all cities used the sameinstruments to measure vehicle speeds,roadway characteristics, etc. (stateresponse "1" matches ci ty response "1 f"

state response "2" matches city res­ponse "2," etc.).

There was agreement when the stateresponded "1" (s tl.ldy of the ne\v speerllimi ts is made), for all c1 ties re~;pon(>

ed "1" (study of new speed limits i~;

made). There was also agreement whenthe state responded "2" (no study ofnew limits), for all cities responded"2" (no study of new limi ts) .

- 40 -

A comparison of sI (state question I) to cI (city ques­

tion I) and cII shows general agreement between states and

cities. A great majority of states report that authority to

set or alter speed limits has been given to the cities by

statute. The cities corroborate this information, for they

report in appropriate cases that the states have, in fact,

delegated this authority to them. Further, cities most

commonly report that this authority has been statutorily

granted.

In sII the states report that the authority for setting

or changing speed limits is most often deleg~ted to the state

highway commission, and then to the county or municipal

administrative agencies. Cities are in slight disagreement on

this point, however, for they indicate in cIII that first,

county or municipal traffic engineers, and second, the state

highway commission, most often have this responsibility. Only

one state selected county or municipal traffic engineers as

having this authority; the cities indicated the authority of

county or municipal administrative agencies with about medium

frequency.

In sIll the states reported that the cities must support

speed limit changes or alterations by engineering or traffic

studies. Cities do, correspondingly, follow this constraint.

In cVI all responding cities reported using engineering

.- 41 -

studies ln the determination of speed limits.

cVIII, cities reported that at every location traffic and

engineering methods were used more commonly than other

methods of setting or changing speed limlts. In addition, in

eX most cities indicated that the effects of new limits were

stucied, usually by engineering or traffic study methods.

Answers from the states and cities on questions sIV and

cVI II are virtually identical. As mentioned, ci t.ies prefer

to do traffic surveys and engineering methods at every loca­

tion. States preferred these methods in every case but two;

of these two, engineering methods were used either as often

as local officials and agencies or were a close second choice.

Citizens' petitions were rarely utilized by states or cities.

with the exception of engineering methods and citizens'

pet.ition, then, all other choices were selected with similar

degrees of frequency.

With regard to factors included in engineering or

traffic studies, states included an average of 10 factors and

cities an average of 8. Using the information from sV and

eVIl, the fact.ors in this question can be ranked. The factcr

selected the most frequently is assigned rank number 1, the

factor selected next most frequently is rank number 2, etc.

wi th a few exceptions, the rankings are not simi lar. S tate~)

-- 42 -

and cities ranked 85th percentile and accident experience at

the location, first and second, respectively. In both cases,

fac":ors least often considered were ranked identically. For

example, slipperiness of pavement, roughness of pavement,

percentage of commercial vehicles, and other factors were

ranked 18, 19, 20, and 21, respectively. With these excep­

tions, the rankings vary widely, as Table V illustrates.

Responses to sVIII and cIX were similar in two ways.

First, both states and cities indicated that an average of

three of the instruments cited were available to them.

Second, both selected the same kinds of instruments as being

most commonly available. Radar was most frequently available

to both states and cities. In order of availability, the

states then selected ball-bank indicators and vehicle

counters; for the cities the availability of these two was

reversed. States and cities did not report that the remaining

instruments were equally available, however.

In response to sIX and cX, most cities and states indi­

cated that the effects of new speed limits were studied.

Similar methods, such as before-and-after studies of accidents,

85th percentile, etc. I were utilized in both cases.

In general, states and cities agreed in their responses

to most of the survey questions.

- 43 -

4.0 SPEED LIMIT CONCEPTS

The following discussion of tht2 objectives and func-

tions of speed limits and the establishment of speed limits

is intended to provide the reader with conceptual framework

necessary for critically examining the literature and under-

standing the arialytical objectives of this study.

4.1 General Concepts

Certain generalizations can be made that highlight

the interplay of factors that constrain the development of

a methodology for the establishment of speed limits:

1. Speed limits are established by the operation ofthe legal system. Such establishment must beconsistent with the objectives of the TrafficLaw System.

2. Speed limits are intended to reduce risk withinthe Surface Road Transportation System. Thus,the speed limit must be related to hazard.

3. Drivers tend to ignore limits that are perceivedas unreasonable. Thus, an effE~ctive limit mustbe perceived by the majority of drivers asreasonable.

4. There are an almost unlimited range of variablesarising from the man-machine-highway mix thatimpact on the determination of a reasonable speedlimit.

5. Few jurisdictions have manpower resources ortechnology to permit implementation of a methodfor establishing speed limits requiring sophis­ticated data collection and analysis.

- 45 - Preceding page blank

The above-m(~ntioned factors are di scusscd in more

detai.l in the following sections. It is hoped that the

reader will realize the complexity of issues presented,

and '.vi 11 accept the inherent di fficul ty accompanying a

short dissertation on a complex subject.

_,±-~~pced Limi ts an(~__-t:l1e~£~f~i.~ Law ~y~stem

The establishment of speed limits may be regarded as

an operational act of the Traffic Law System (TLS) consis­

tent with the basic objective of the TLS -- risk management

of the Sur face Road Transportation Sys tem (SRT).

The TLS is the basic social control system applied to

manage risks within the SRT system. The TLS operates in

four basic functional components; Rule Making, Enforcement,

Adjudication, and Sanctioning.

Thus, when the operation of a motor vehicle at a speed

inappropriate for existing conditions is identified as a

risk to the basic operation of the SRT System, the TLS 15

called upon to operab2 in risk management mode. In general,

this is done by the establishment of a speed limit, the

enforcement of such a rule with appropriate adjudication

and sanctioning of offenders. Theoretically, sanctions

act to correct the offender and serve as a deterrent to

others similarly inclined.

- 46 -

If the TLS is to effectively function as a risk manage­

ment system, the rule making component must precisely and

correctly identify risk. If this is not done, the remain­

der of the system inefficiently allocates resources in

dealing with individuals who technically violated a rule

but in fact did not engage in hazardous activity.

An examination of enforcement activity indicates that

the bulk of traffic citations are given for speeding offenses.

Accordingly, the bulk of court activity is taken up with

speeding offenses and the bulk of sanctions are imposed for

speeding.

If it were clearly established that all speed zones

were precisely established to define risk, the above-mentioned

allocation of resources could be defended as appropriate for

a risk management system. Regrettably, the opposite seems

to be the case in many instances.

Speed limits which are improperly posted, particularly

those which are set artificially 101117, tend to be ignored

by the majority of drivers and thus have little effect on

SRT risk. At the same time the limit makes technical

violators of a high percentage of drivers. Frequently, the

high number of violators draws enforcement presence and

concurrent citations.

- 47 -

Such action violates the basic concepts of risk manage­

ment inasmuch as resources are being diverted to deal with

low risk behavior when they should be focused on high risk

behavior as a priority. Not only are enforcement resources

diverted but the resources of the courts and administrative

agencies are also clogged.

Perhaps more important than the damage to the TLS

system in a sheer cost sense is the damage to the TLS as a

control mechanism. The TLS, as part of the Criminal Justice

System of our society, is dependent upon public support and

must maintain a position of fundamental fairness to operate

e ffecti ve ly. Inappropriate rule making which creates

fundamentally unfair enforcement, adjudication and sanction­

ing constitutes a detriment to society.

The impact of such improper setting of speed limits

can be evaluated only when one recognizes that more citizens

have contact with the Criminal Justice System and its con­

stituent agencies through traffic violations than through

any other single cause.

4.3 Speed Limits and Risk

The basic objective of the Surface Road Transportation

:':;i'stem is to facili..tate the flow of goods and people from

.- 48 --

point to point as safely as is possible. It might be more

appropriate to think of the basic objective as having two

components~

1. Maximize Flow

2. Maximize Safety

It should be obvious that some conflict exists in the

concept of the two components. The flow rate would theoret­

ically increase as speed increased. Similarly, it could be

hypothesized that safety would decrease as speed increased.

Examination of the real world reflects that a trade­

off has occurred as drivers have made heuristic judgments

in arriving at the speed they travel. Thus, it appears

that a discussion of the risk of flow disruption or the

risk of an accident or other potential safety threat must

consider such real-world activity. One could picture the

concept of risk as a curve with a minimum point as shown in

Figure 4-1. Available data indicates that the slope of the

curve is quite flat near the minimum point indicating that

there lS a range of speeds with nearly the same risk value.

It would seem desirable to encourage drivers to operate

their vehicles at speeds within the speed band with minimum

risk.

Theoretically, maximum speed limits could be set at

the upper end of the speed band, minimum speed limits at

- 49 -

FIGURE 4-1

/!

_. 50 -

Risk

Band

Minimal

Speed

t:' I I~I4-~ I' i Io ! I

i~ I I-~ I~j I :

I I\r---Hr-----f+-I-----!i

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>-.(f)

the lower end of the speed band, and advisory speed limits

at the middle of the speed band.

Establishment of enforceable maximum limits lower than

the upper end of the minimum risk band would not act to

reduce risk within the SRT system and would be inconsistent

with the objectives of the TLS system.

Thus, it appears that a method for establishing a

maximum speed limit should result in the selection of a

value for the speed limit that would fall at or near the

upper end of the minimum risk band.

4.4 Speed Limits and Driver Acceptance

There has been a tendency on the part of many indi­

viduals associated with rule-making and highway safety to

assume that the correct met.hod of dealing wi th a particular

problem is to enact a law making the undesirable behavior

illegal and it would cease. The illusory nature of this

concept is perhaps nowhere better illustrated than in the

case of speed limits.

While it is quite possible to compel response to

unreasonably low speed limits by the presence of over­

whelming enforcement resources, such a level of resources

simply does not exist in the Uni ted States. 1'he chances of

a violator being detected and apprehended are so low that

- 51 -

travel speeds are selected by most drivers quite independent

of considerations of the illegality of exceeding a speed

limi t. Studies have reflected that unreasonably low or

high speed limits are ignored.

Unfortunately, all drivers do not ignore unreasonable

speed limits. The net result of improperly selected speed

limits appears to be a widely dispersed mix of speeds.

Some drivers ignore the limits others obey them. The

resulting speed distributions are often characterized by

wide differences in travel speeds.

This result is inconsistent with the studies that

indicate risk increases with deviations from a mean speed.

It is also inconsistent with those studies which indicate

that a normal distribution with a small standard deviation

is desirable.

Thus, it appears that for a speed limit to minimize

risk effectively, the speed limit must be accepted by the

majority of drivers as reasonable and must be voluntarily

obeyed.

4.5 Factors Affectina the Development of a General Method______ _ :...2__________________________ . ,_. _

The search for a general method to establish speed

limits has occupied the attention of traffic specialists

- 52 -

since the wheel was invented. The literature is replete

with discussions and methods.

Most discussions recognize the almost infinite set of

variables arising from the man-machine-highway mix that

interact to influence the choice of an appropriate speed

limit for a particular highway. The type of drivers, the

mix of vehicles, the geometry of the particular highway are

just a few of the factors that could be considered.

While a method for establishing speed limits for a

particular class of highways might be based on a complex

evaluation of such variables such as number of intersections

per road mile or nature of roadside development or any

other number of physical characteristics, such an approach

would be incapable of being widely generalized.

Researchers in the field of speed control have generally

recognized the futility of attempting to base a general

method on characteristics of a particular highway environ­

ment. Instead, they have recognized that the general behavior

of the driver mix serves as an indicator of the influences

of highway environment. Each driver considers, consciously

or unconsciously, a range of factors and selects an appro­

priate travel speed for a particular highway. An examination

of the distribution of travel speeds of a sample of safe

- 53 -

drivers appears to serve as the best indicator of an appro­

priate speed limit. The impact of the innumerable other

variables are reflected in distribution of vehicle speeds.

'['lie weight of expertise supports the concept of such

a general approach and accepts that a value selected by such

a method represents a best estimate. Such an estimate would

then be judgmentally evaluated in light of local conditions

and minor corrections made if warranted by factual data.

A general method has definite value for the overall high­

way safety field. It promotes consistency in a wide range

of jurisdictions which In turn could be expected to produce

better driver response or compliance.

While such an approach is accepted by the majority of

researchers, the literature still contains references to

"optimal methods" which are conceptually inconsistent with

the approach of a general method. If one accepts the

common scientific interpretation of the term "optimal" as

the single best value, one must also conclude that it canno·t

be appropriately applied to a general method designed to

select a value at or near the best value. Thus, this report

focuses on a general method for the establishment of a

"reasonable" or "appropriate" speed limit as opposed to an

"optimal" speed limit.

- 54 -

4.6 Factors Affecting Implementation of a General Method

If the hypothesis that wide implementation of a general

method of establishing speed limits is desirable to promote

commonality and consistency within the SRT system is accepted,

one must be concerned with the development of an implemen­

tation scheme that can be widely used.

Two basic considerations underlie implementation of any

technique in the highway safety field. One, can it be

implemented by existing manpower resources? Two, can it be

implemented utilizing existing technology?

In general, the manpower available for the establish­

ment of speed limits possesses limited scientific education

and training. Major urban areas and state highway depart­

ments are usually staffed by compe'tent traffic engineers.

Smaller urban areas often assign traffic engineering duties

to individuals without formal education or training.

The actual mechanical establishment of speed limits,

even in the major jurisdictions, is often assigned to non­

engineering personnel. The range of judgment exercised by

such individuals is evident in the mix of speed zones on

our highways. It is not uncommon to discover the function

of speed zone establishment assigned to law enforcement

officials. While there are notewor-thy exceptions, in

- 55 -

general, such individuals are not familiar with traffic

engineering practices and tend to rely on intuitive judg-

ment. Such judgment is often conditioned by years of con-

cern with speed as a primary perc~ived accident factor.

Frequently, the result is a speed limit lower than appro-

priate for existing conditions.

The implementation of a general concept will be

dependent upon its presentation in terms that can be easily

understood by an individual without engineering or scien-

tific training.

Speed measuring devices and traffic counters are

almost universally available in most jurisdictions. The

cost of such devices is relatively low so that acquisition

or expansion of existing equipment could be accomplished

with minimal difficulty. More sophisticated equipment is

not generally available nor is the manpower available that

could use such equipment.

The method developed for implementation must rely on

data that can be coll,ected by existing manpower utilizing

presently available or easily acquired instrumentation.

4.7 Criteria for a General Method of Establishing SpeedLimits

The discussions in the prior sections have indicated

certain factors and constraints which structure the

- 56 -

development of a general method of establishing maximum

speed limits.

A general method should meet the following criteria:

1. Be fundamentally fair in the context of theTraffic Law System.

2. Be related to risk of dysfunction in the SurfaceRoad Transportation System.

3. Be accepted as reasonable by drivers.

4. Be capable of general use on a range of highways.

5. Be capable of being implemented by existingresources.

Such criteria have been considered in the analyses that

led to the development of the recommended general method.

- 57 -

'i.o DISCUSSION OF METHODS FOR ESTABLISHING SPEED LiJ'll.., ..>

In previous sections of this report numerous techni,:!uc.

for establishing maximum speed limits have been described.

The present section is concerned with:

1. Screening these techniques to eliminate thosewhich clearly fail to meet the five criterialisted in section 4.7.

2. Further analysis of the techniques surviving thepreliminary screening process.

5.1 Screening Analysis

A total of thirteen techniques for establishing maximULi

speed limits were identified in the literature review and

the survey of jurisdictions. The screening analysis matched

each of these techniques against the five criteria listed

in section 4.7. Those techniques which clearly failed to

meet anyone of the five criteria were eliminated from further

analysis. The results of the screening analysis are

summarized in Table 5-1. Note that numbers 6, 7, 8, 9, II,

12, and 13 in the matrix are only elements that may affect

driving speeds, and thereby fail to meet all the necessary

criteria.

In considering this presentation, the reader should

bear in mind that the criterion "Be related to risk

of dysfunction in the SRT system" means that a causal rela-

tionship between dysfunction and speed must be shown to

- 59 - Preceding page blank

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criterion "Be accepted as reasonable by drivers" is intencicu

to imply acceptance by a larg~ ~aj~)ri~ of drivers.

The table shows that, based on elementary considera-

tions, only three of the techniques: can be considered to be

generally in consonance with all five criteria:

* Taylor's theory of speed distribution skewness

* Opperlander's cost-oriented approach

* The 85th percentile method

It is noted that criteria 2 and 3 are the ones most

conunonly not met by the various techniques.

The pace speed failed the screening test by the

narrowest margin. It was rejected because, by definition,

it is accepted as reasonable only by m~!e drivers than

any other speed increment. Thus, the drivers traveling

within the pace speed do not necessarily comprise a !?rge

majority of the drivers, and criterion 3 is not met.

5.2 Analysis of Selected Methods for Establishing SpeedLimits

The three approaches for establishing speeds limits

which survived the initial screening process are subjected

to further analysis in this section.

- 61 -

Taylor's theory (177) states that accldent frequency

increases with increasing deviation of the speed distri-

bution from normality, where the deviation is increased by

the non-symmetry of the speed distribution. Thus, a speed

limit may be said to be "effect1ve' 1£ the skewness of the

resulting speed distribution is small.

The Tennessee Department of Highways has applied this

theory in a study which undertook to define a method for

determining speed limits and to presc~nt proof that th.c

recommended speed leads to a speed limit which scrvcs its

stated purpose, namely t.he reduction of accident rates. To

do this, speed data were collected at many speed zones.

Those zones with a small value of skewness (i.e., non-

symmetry) were selected as having effective speed limits.

Next, the Tennessee study attempted to determine which

characteristics of the speed distribution (i.e., 85th per-

centile, 90th percentile, mean speed, or median speed) wus

most clearly represented by the posted speed limit. The

result of this investigation, based on 384 locations, 189 of

which showed normally distributed speeds, was that speed

limits below 50 mph are best represented by the 85th per-

centile, and those above 50 mph are best represented by the

62 -

90th percentile. From this result the conclusion was drawn

that the 85th or 90th percentile should be reconunended where

the appropriate conditions prevail.

Several questions arise regarding Taylor's theory and

its application by the Tennessee group. The first is a

basic one regarding the nature of speed distributions.

Taylor states that:

In a situation where all drivers are able todetermine and evaluate the conditi.ons that existat that time and at that location, the resultingspeed distribution is normal with no skewnessand no kurtosis. (177)

unfortunately, no data supporting this statement can be

found nor has there been any analysis of how the probability

distribution is influenced by speed, traffic obstacles,

roadway obstacles, and other factors. For example, there

is evidence that speed distributions may be bimodal in the

vicinity of intersection. This effect is shown most vividly

by the IRPS data collected in Monroe County, Indiana.

Figure 5-1 represents data collected on SR 37 over a two-

hour period. The sensor site was located within 50 feet of

an intersection. A distinct bimodality is noted with peaks

occurring at 24 and 40 mph. Figure !;-2 indicates bimodality

may occur also at locations where there are other types of

traffic flow impediments. The sensor site was located near

a bridge under repair where traffic vIas being s lowed by a

flagman.

- 63 -

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It i~ apparent, then, that before Taylor's assumptlo~~

regarding normality, skewness, and kurtosis can be accepted,

we will need more data descrlbing the probability distri-

bution.

jnc appllcatlon of the theory by the Tennessee group

als n raises a question. The Tennessee study used as the

mea:)ure of rear-mali ty the skewness index S. I., where S. I.

is deflned as

_ 2 (P 9 3 - P 5 0) andS . r. - --p 9 3--------pr

P7 = 7th percentile speedP50 = 50th percentile speedP93 93rd percentile speed

Till S lndex provide s a mr=" as ure of symme try, not nor:nali ty .

For example, applying the skewness index to a normal dlstri-

bution will result ln a skewness index of 1.00, but the

same result will also be Obtained for any other symmetric

probability distribution.

It is also important to note that Taylor's theory

does not provide a measure for distinguishing the effects

of left skewness from the eftects of rlght skewness. A

curve skewed to tne right (i.e., with most of the probabilit

distribution to the right of the peak of the distribution)

will have more cars going faster than will one skewed to

the left, and it would seem that the severity, if not the

- 66 -

frequency, of accidents would be greater for a right skew

than for a left skew. It is believed that this factor

should be considered in any theory of speed distribution.

Thus, considerable work needs to be done before Taylor's

thf:ory can be accepted as a basis for a methodology for

determining speed limits. One believes that considerable

insight could be gained by analysis of those 195 Tennessee

locations which showed non-normal distributions. The Ten­

nessee study does support the concept that for symmetric

speed distributions, speed limits below 50 mph are best

represented by the 85th percentile, those of 50 mph and

higher, by the 90th percentile.

5.2.2 Oppenlander's Cost-oriented Approach

A major criterion of a speed limit is the degree to

which it results in a reduction in accident rate. Oppen­

lander's approach takes this factor into account, along

with several others, all of which are reduced to a common

denominator, cost. He seeks the spE~ed limit which mini­

mizes total cost, where the cost components are the opera­

tion cost, the time cost, the accident cost, and the cost

of comfort and convenience. Operation cost is assumed to

be a decreasing function of speed; accident cost is assumed

to be very small relative to the other two and is assumed

- 67 -

,lear]',' const.ant; and LlI" Cr)st of comfort and convc;nicnc('

J:; d,::~umcd Lo lH; non--quanLlli,1h]p. vJi Ii, these' ass umpt Lons ,

i)pl'enl<lnc1cr creat.os a total cost function and finds its mini-

mum at some "optimum speed." He then selects upper and

lo\vcr speed limits "in such Cl. manner that the average speed

()f travel on the roadway section being speed-zoned coincides

with the adjusted (for roadway environment conditions)

speed."

The advantage to Oppenlander's approach is that it makes

the problem of determining optimum speed limits a proble;n

of public choice, with the cost function beinq the cost to

the: pub1i c as a whole. Tn such a CO;1t~(;xt it is pc:~sibJ('

t.u analyze such questions as the track·-of-f ,:; b:~tW,;C:'l costs

of increased la"v enforcement and benefits of increased traffic

flow, if increased traffic flow is shown to be the result of

more stringent law enforcement. It is also possible to

analyze the costs and benefits of conducting speed surveys

in order to determine the speed limit which most facilitates

traffic flow, if such a speed limit does indeed exist.

Tllus, Oppenlander's approach is regarded as having

considerable interest conceptually and one deserving of

fclture study. Such fu·ture efforts must place a heavy EJ,>

phasis on the determination of the cost components. The cost

of the political and emotional impact of high accident rates,

- 68 -

as well as vJays of assessillg f,lOr(' accu.cat.ely the relative!}'

concrete cost components (e.g., vehicle, loss ot earni~gs,

insurance) must be treated.

IN If,USt, nevertheless, concJude that Oppenlander's

approach is not sufficlently developed to serve as a baSlS

for present lmplementation. Future research may change thlS

.:::onclusion.

5.2.3 The 85th Percentlle Method

The 85th percentile speed is mentioned in most of the

Ilterature dealing with the establishment of speed limits.

It is a speed which uses the speed data usually collected

by traffic specialists; it is a speed easily calculated

once data has been obtained; and it is a number which

reflects the judgment shown by most drivers in their reac-

tions to the environmental conditions of the roadway.

Further, a substantial body of data has been collected

which indicates that the method may be consistent with the

oDjective of low accident involvement rate (number of

accidents per 100 million vehicle-miles).

A study by Solomon (167) shows that 85th percentile

speed is in the speed range where the accident involvement

rate (number of accidents per 100 million vehicle-miles) is

lowest. This conclusion has been given added weigHt by d

- 69 -

:::e.1" YeCt.l~t. study pc,rformcc1 1)\· U

Lute ~nd supported by IRPS. 'l'hi <; studv c"'.;nclude~;:

Tn"x' \·;a::.' not :~uf f i..cicnt delta ava.llable toallow a full analysis but it appears that thisstud~ reinforces the setting of speeds [8ic]liwits at the i:35th pcrccnLilC' sp(-;ed. The stan­dard dc'vi,J.tion of L-:.he speed di:3tribution is from:) t.o -;' mph. l\.pprox imate 1y 8 S% of the' driversdrive helow the mean plus one standard deviation.Ttl" '[1: ;vr:r:.--, havinq speeds b(·t\,v~cn thc' mean anden ~.~ ~:; -:._ ;111 (~(.'~ .r(] (lev i cl t: i Cj 11 a l.JC)\f(~ t 11 c~ rn(~ a a r c_ elf:: f 1

nit.cly III d lcn,y-involvcmcnt group. 'l'hc r<)giol,,~tv,7(',:n ')11(' and t.wo:::tan<1dn.: dc'vLatio0s clbovctl',; m"cJJl speed (,ncompasc:-;cs ajJprox i..mc:tTe ly 10 :'cy­"cnL r)f the driver::; and does not have: d ~;ignif.i..­

cantly greater involvement rate thai') at r;lc~an

speed. This region from the end of th~ :irstto the end of the second standard deviat~on lS

approxin'latcly the tolerance level a.llow('c] Lvpolice agencies.

It minimum speed limits are set a simllarargumc" I. h'8uld lead to the conj ecturc' that trwliilJi t c:hould be placed at about the' 15th per­centile speed with enforcement at ahout the5th percentile" (191)

)lew:<-, C'.lrn'nt Ii tera::'ure indicates that the 8Sth perccnti.i..c

c;pE,,'ci limi t i~j one fallinq- in the speed in terval wi thin

\iuch few accid(;nL3 occur.

The 8Sth percentile, by definition, also reflects the

J,-.lCJi'lent of lTlljSt dri\lers. This fact is hrought out in

"Spc~ed Zoning -- ~-vhy and How":

This [the 85th percentil~ speed] is the speedat or below which 85 percent of the traffic

is moving. Experience has S;lO\07n that this isthe one characteristic of traffic speeds whichmost nearly conforms to a reasonable limit.Speed limits set higher than the critical speedwill make very few additional drivers "legal"for each five mile per hour increment of speedincreased. Speed limits set lower than thecritical speed will make a large number ofrec-sonable drivers "illegal" for each five mileper hour increment by which the speed isreduced. (208)

It seems remarkable that the speed range which 85%

of the drivers do not exceed is also the safest. Appar-

ently the public's perception of hazard is a valid one.

The arguments, then, for the use of the 85th percentile

speed as a speed limit are that, first, the 85th percentile

speed is a speed below which the probability of occurrence

of an accident is low; second, the 85th percentile reflects

the safe speed for given environmental and traffic con-

ditions as reflected by the judgment of most drivers. The

85th percentile has the additional advantage that it is

easily obtained with present equipment and with a minimum

of computation.

- 71 -

6.0 ANALYSIS OF IRPS DATA RELA'l'ING SPEED AND ACCIDENTS

6.1 Introduction

This section of the report introduces data collected

by the ~RPS Computer-Sensor System. Analyses of the data

are presented in order to provide a further basis for

choosing a method for establishing maximum speed limits.

The three methods discussed in the previous section will

then be re-examined in light of these analyses, and a

recommended method will be presented.

Since May of 1969, IRPS has been collecting extensive

speed and accident data in the vicinity of the Indiana

university in Monroe County, Indiana. The speed data were

collected auton:atically by sensor loops lnstalled at

seven locations on Highway SR 37, near Bloomington, Indiana,

and at four locations within Bloomington. The sensor

loops are connected with an IBM 1800 computer. T11e arrival

time and speed of each car which crosses a sensor loop are

stored on magnetic tapes. These data are availaole begin­

ning in May, 1969, up to the presen1: time. A detailed

description of the sensor loop system is given in Appen­

dix B.

In addition to these speed data, there are accident

files for SR 37 as well as for the areas surrounding two of

- 73 - Preceding page blank

the four locatlons wi t~hi n Bloomingi:oYl.

Ii!' C' n ;-;: cpt sinc e th e beq inn i n Ci 0 t 19 6 8 .

These files n~ve

Among other infor-

;-n3 Lion, the exact posi t.ions of the accidents are lis ted.

6.2 roacn

The analytical investigations were conducted in two

Fart.s. F1.rst, the interrelationshLp bebveen the paramet(?r"

vihich define speed distribution and accident frequency we,re

studied from the viewpoirlt of sensor location. For each

of the seven sensor Sltes on SR 37, these parameters were

computed, and statistical comparisons were made. Thu5,

for each pair of parameters studied (e.g., mean speed and

accident frequency), seven data points were available.

The other approach taken was to lump the data from

all sensor sites and examine the parameters derived from

the aggregate. In thlS approach, it was possible to study

the accident rate distribution as a function of speed.

6.3 Results

6.3.1 The Effect of Locatlon

For each of the seven sensor sites on SR 37, SlX

characteristics describlng vehicle speed distribution were

computed using standard statistical techniques.

parameters are defined as follows:

v Average speed

0 V Standard deviation of speed

- 74 -

These

v = Coefficient of variation

V85 = 85th percentile speed

V85- l5 = Difference between 85th percentile and15th percentile speeds

Sk = Pearsonian coefficient of skewness

The results of these calculations, plus the number of acci-

dents occurring within 0.4 miles of each sensor site are

summarized in Table 6-1. Statistical analyses of these

data led to the following conclusions:

- 75 -

TABLE 6-1

SPEED DISrrRIBUTION CHARAC'fERIS'.rICS AT DIFFERENT LOCATIONS(MEAN VALUES OF LOOPS OPPOSITE TO EACH OTHER)

LoopNLlJ.'11ber v

v _8~ V NA

-~_.~ -- +--'-,'--'---" ------.--_.__._---- ._0___'_"---"- _____.'_._.. _ .•.. _..._______ -----,-'--- _. __._--_..._.__._--_ .._...._._-

10/14 58.3 8.3 16.0 66.4 0.14 -0.12 13 9.80

9/13 55.4 9.1 17.4 64.2 0.16 -0.13 21 17.09

8/12 53.9 7.6 13 .1 60.8 0.14 --0.25 12 9.02

0/4 40.7 8.7 17.8 4ti.7 0.18 -0.50 16 10. ,- ...,::ll

1/5 46.0 6.9 13.4 52.9 0.15 -0.02 9 10.97

2/6 50.9 7.9 15.7 58.7 0.17 0.05 4 4.19

3/7 54.9 7.6 14.3 62.2 0.17 -0.07 6 7.13

Notes: 1. = Total number of accidenLs within 0.4 miles ofa sensor site durlng 1968 and 1969.

where: = 'I'otal number of vehicles pasSlngsensor site durlng 1968 and 1969.

- 76 -

1. Parameters which measure speed spread (e.g., a)

tend to increase with increasing number of acci­

dents.

2. There is no apparent relationship between traffic

mean speed and number of accidents.

3. The speed distributions at all of the sensor

sites are nearly normal.

4. A large increase was noted in numbers of accidents

with increasing absolute value of speed distribu­

tion skewness. This increase was observed even

though the maximum skewness that occurred was only

0.5. These findings are consistent wlth Taylor's

theory. (177)

Thus, it seems clear that speed distributions containing

a large spread of speeds may be associated with high acci­

dent rates. An analysis of the fine structure of accident­

speed distributions was conducted next to determine the

amounts of spread that may be tolera"t:ed as relatively" safe,"

and that which the traffic law system should seek to eliminate

by the implementation of suitable speed limlts.

6.3.2 Accident Frequency Distribution Studies

This analysis considered how accident involvement rate

varies with the deviation of the speed ot the accident-

- 77 -

involved vehicle (AIV) from the mean speed of the traffic

stream. The ratlonale followed was that in order to achieve

a high level of trafflc flow, the highest possible speed

limit would be sought consistent with acceptably low acci­

dent rates. Thus, the speed limit would not necessarily

h~ve to occur at the speed which resulted in absolute mini­

mum accident involvement but could be higher, if the accident

involvement did not increase more than a few percent. Speed

limits occurring in regions of rapidly increasing accident

invol'Tement w'ere not permitted.

The basic data used were those collected by IRPS in

Monroe County, Indiana, and reduced and presented by RTI in

reference 191. Since we were seeking a maximum speed liml t

rather than a minimum limit, a cumulative distribution was

used which showed accident involvement rates at all AIV speed

deviations less than a given value.

The data are presented in Figure 6-1. Curves are shown

for SR 37; SR's 45, 46, 48 (total); and all SR's in Monroe

County. Turning vehicles nave been excluded to make the data

applicable to our purpose. Note that regardless of which

roads are being considered, the slopes of the curves are

nearly constant and :ceasonably low for AIV speed deviations

of less than 10 mph from the mean speed of the traffic stream,

A precipitous rise is noted in accident-involvement rate in

- 78 -

EFFECT OF AIV SPEED DEVIATIONON ACCIDENT INVOLVE~£NT HATE

FOR VARIOUS STATE ROUTES INMONROE COUNTY I INDIANA

FIGURE 6-1

(non-turning vehicles)

/6

+

'--_--0----0 --

o I I_ '-2"":"""0-----_-1=-O:---------lo---------J1

LO----------l

20

II 1+

+ _---+-----t--rlO~-~-_-_---------_-6-_-6---[16

90th Percentile /5085th Percentile7 0

o SP 45 I 46 I 48

t76SR 3740

+ All SRs in Monroe Co. I I

2

30

Deviation of AIV froM ~1p an Sp'?ed of Traffic Stre <'lIn < )\1 P. H .

- 79 -

the speed deviation region 10 .::. \lVAIV < 20 such that, for

example, vehicles traveling at speeds up to 20 mph gr."eater

than the mean have an accident-involvement rate nearly

twice that of vehicles traveling at speeds up to 10 mph

greater than the mean!

It is therefore clearly important that maximum speed

limits be set to reduce the likelihood of travel at speeds

much greater than 10 mph from the mean speed of the traffic

stream.

Consideration of a mei:hod to implement this concept

must take into account bolO real world practices. First,

speed limits are almost universally established in five­

mile increments. Thus, the impact of rounding values is a

consideration.

Second, enforcement practices allow a tolerance, that

is a range above the posted limit before enforcement action

is taken. This is justified partially on the basis of

possible measurement error either by the officer or the

driver. The fact that actual travel speeds vary so that a

well-intentioned driver may slightly exceed an absolute

limit is also considered.

Thus, any value selected as a function of speed dis­

tributions is llkely to be altered in implementation. ~vhile

the impact of a lower implemented value may not have great

- 80 -

risk significance due to the relatively flat slope of the

risk curve, the opposite is true of a higher value.

The 85th percentile speed (85, 77, 98, 22., 113, 140,

203, 204, 205, 206, 208, 209) appears ideally suited for

such a speed limit. First of all, it. is a speed which, by

definition, 85% of all drivers do not exceed, so that

enforcement action need be targeted at a maximum of 15% of

the drivers. If, however, enforcement is set at the 85th

percentile plus 5 mph (i. e., mean spE~ed plus 12 mph), only

about 4% of the drivers will be enforcement targets. Further,

the resultant accident-involvement rate will still be

acceptably close to the region of lo~rest accident involve­

ment.

The case for a 90th percentile speed limit is not as

persuasive. It is, to begin with, uncomfortably close to

the high accident involvement region, which is also a region

of great sensitivity to small changes in speed deviation.

Additionally, even a small enforcement margin places one too

high up on the accident involvement curve.

Some researchers have stated that a 90th percentile

limit may be more attractive in situations where the mean

speed of the traffic stream is greatE!r than or equal to

50 mph (207). At first glance, the Honroe County data seem

- 81 -

to confirm this (figure 6-2). However, if percentage in­

crease in accident-involvement rate is th~:> cri terion, then

the 90th percentile limi t: fares no better for mean speeds

over 50 mph than it does for those less than 50 mph. Con­

sider; for example, t.he percentage increase in accident-

nvolvement rates obt.ained from increasing the enforcement

speed from 85th percentile plus 5 mph to 90th percentile

plus 5 mph. For mean speeds of less than 50 mph, there is

an 11% increase in accident-involvement rate, while at

mean speeds of greater than 50 mph, a practically identical

10% increase is noted. Thus, the 90th percentile limit

would appear to be preferable for speeds over 50 mph only

if absolute accident-involvement rate is the criterion and

accident-involvement rate, rather than accident-involvement

disutility, is to be minimized.

Actually, in any real world application, the effect of

setting a speed limit at the 90th percentile rather than the

85th percentile would be difficult to detect. In the first

place, IRPS data indicate that the speeds associated with

these points will not in all likelihood differ by more than

2 to 3 mph (see appendix E). Secondly, the practice of

rounding speed limits upward to the nearest 5 mph provides

a band of tolerance which, if the limit were set at the 85th

percentile, would include the 90th percentile .

.- 82 -

EFFECT OF AIV SPEED DEVIATIONON ACCIDENT INVOLVEMENT RATE

FOR TWO RANGES OF TRAFFICMEAN SPEED

(non-turninq vehicles)

FIGURE 6-2

O~ ---'L..- -..J --J, -..,.I

-20 -10 0 10 20

/o

90th Percentile

85th

o u <SO m.p.h.

~ u > 50 m.p.h.

~.------6----~ I

5

20

15Q)+Jttl~

+Js::Q)e~M III0 Q)

> Ms:: ~H

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s:: MCD u'0 0.-10.-1 ~U

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> 0.-10.-1 r-i+J r-ittl 0.-1M X

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U IJ.l

Deviation of AIV from Mean Speed of Traffic Stream, M.P.H.

- 83 -

..

7.0 CONCIJUSIONS

The concept of establishing a maximum speed limit on

the basis of the 85th percentile of travel speeds on a

highway is recommended. Such a maximum speed limit meets

the risk management objectives of a speed law and can be widely

implemented utilizing existing manpower resources and tech-

nology.

Such a limit is:

1. Fundamentally fair in the context ofthe Traffic Law System.

2. Related to risk of dysfunction inthe Surface Road Transportation System.

3. Accepted as reasonable by drivers.

4. Applicable to a wide range of highways.

5. Capable of implementation with existingresources.

The recommendation of the 85t:h percentile speed limit

concept is supported by a substantial portion of the technical

literature as well as the data and analyses of the present

study.

A method for implementation of the 85th percentile

concept is explained in the next chapter and presented in

detail in Volume III of this report.

- 85 - Preceding page blank

8.0 THE RECOMMENDED METHOD

This section discusses in detail the recorr~ended imple­

mentation method for setting a speed limit and the rationale

behind various aspects of the method.

8.1 Background

In order that the findings of this study might be trans­

lated into a usable form, it was decided that an implemen­

tation manual would be structured to apply to any level of

traffic personnel involved in the setting of speed limits,

but primarily it would be directed toward those people without

a college engineering background who are responsible for

setting speed limits in smal.ler communities and jurisdictions.

As was stated in section 3.0, the survey results indicated

that often an enforcement or public works official is res­

ponsible for determining speed limits in such jurisdictions.

Since the findings of this study indicate that at

present the best speed limit is a "reasonable" one, this

manual is an implementation of the 85th percentile method.

According to the conclusions of this research, the 85th per­

centile criterion produces an appropriate speed limit value

in terms of the objectives of a speed limit defined in

section 4.0. Also the literature and the survey indicate

- 87 - Preceding page blank

that the 85th percentile criterion is not only reasonable

but is also familiar and acceptable to numerous people in

the traffic field. Among those who responded to the question­

naire survey, awareness of the 85th percentile criterion

was nearly universal.

Because the 85th percentile is familiar to many traffic

specialists, it is undoubtedly put to use in various ways

throughout the country. The purpose of this manual, then,

is twofold. First, it is to teach traffic specialists how

to correctly derive a speed limit from the 85th percentile

criterion using statistlcally valid procedures. Secondly, It

is to promote uniformity throughout the country in the

setting of speed limits.

Before discussing t.he implementation method, itself,

it is important to understand how the method will be presented

to the individual traffic specialists. Since one of the

primary objectives of the implementation package is to promote

uniformity throughout the country, it is obvious that the

implementation method must be distributed to a widely dis­

persed audience. It is also desirable that the presentation

be relatively simple and understandable, as well as low in

cost. Because of these three qualities, ease of distri­

bution, ease of understanding, and low cost, a self-instructic),;

-- 88 -

educational method -- programed learning -- was chosen. It

was decided that the educational tool would be most effec­

tive if presented in two parts. Thus, the manual consists

of first, a programed educational tE~xt which teaches the

traffic specialist the 85th percentile method, and second,

a field workguide for him to use as a checklist while

actually setting a speed limit. This package has the addi­

tional advantages that no special instructors are needed and

that the educational program can be worked at the convenience

of the individual traffic specialist. The programed imple­

mentation text can be found in Volume III of the report.

In addition, a brief presentation of the recommended

method can be found in Volume IV of the report. This manual

was designed for the experienced traffic engineer, and

contains a short discussion of the recommended implementation

of the 85th percentile method, plus the field workguide

that is presented in Volume III.

- 89 -

8.2 Sampling Method

The recommended method of setting a speed limit is

based on a statistical procedure known as systematic samplinc:.

The choice of a sample da.ta method is based on a real-world

constraint of utilizing existing instrumentation to collect

speed data. Devices available to local jurisdictions are

normally radar, VAS CAR , or switch-actuated timing devices.

The use of these instruments makes it literally impossible

to measure the speed of every vehicle under many traffic con-­

ditions.

The systematic sampling technique best meets our requl-cc'

ments for a sampling method for determining the 85th per­

centile speed, namely, that it be feasible, accurate, easy

to use, and easy to understand. The data for use in systematic

sampling is a speed sample the traffic specialist can collect

in a reasonable time interval using readily available instru­

ments. The accuracy of systematic sampling in yielding an

85th percentile speed which closely approximates the 85th

percentile speed of the population is shown elsewhere in this

report. Systematic sampling is easy to use -- only simple

hand calculations are required. Finally, the method of

systematic sampling is easily understood -- it is, if any

method can make that claim, the natural ·.Nay to take a sample.

- 90 -

To take a systematic sample from a list it is necessary

to determine the number of units to be skipped. This number

is usually determined by the desired size of the resulting

sample. For example, if a sample of size ten is desired from

a list of one hundred speeds, a logical skip size would be

ten, i.e., after the first speed in the list is chosen,

every tenth speed thereafter is also chosen, the sample of

speeds being made up of all such selected speeds. Such a

sample will have ten speeds, as desired. The position in the

list of the first speed chosen can be determined arbitrarily

or with the use of a random number table, so long as that

position is in the first ten at the beginning of the list.

In this example, the first speed in the sample might be the

seventh speed in the list. Then the other speeds chosen

would be those occupying positions 17, 27, 37,. , 97 in

the list. Choosing the speeds in this way gives a "syste­

matic" sample.

A gOOd discussion of systematic sampling is given in

Hansen, Hurwitz, and Madow, Sample Survey Methods and Theory,

(John Wiley & Sons, Inc.: New York, 1953) Vol. I, pp. 503­

505. There it is pointed out that systematic sampling is a

simple and proven technique when periodicities do not exist

in the data or when no multiple of the interval between

samples is a multiple of the period of the data. In the data

- 91 -

collected in this study, no problems arose with SUGh pcrj0~'

ities.

To use systematic sampling i.s determi.nj.ng the 85th

percentile, it is first necessary to determine when the sarnpl(

speeds should be taken. It is recorrunended that this deci.~lon

be based on a plot of traffic volume versus time, a plot of

data easily obtained through the use of a traffic counter.

With this plot, the traffic specialist can determine whlc

time intervals correspond to the traffic volumes of inter~s~.

Such a plot is given in Figure 8-1. The traffic speci3l~

must use his own judgment to classify the traffic flow i:-ILC:

high, average, and low 'Jolume.

From each of the time intervals corresponding to the

desired volume of traffic, the traffic specialist mca~ures

speeds in the hour falling in the middle of the period or,

if the period is less than an hour long, for the entire

period. Data from each time interval with the same volume

are kept separately. The traffic specialist counts the

number of speeds measured during each intervaJ. He the

divides the number of speeds of each intervaJ either by IG(

or ny the smallest ;HImber of speeds in uny interval if

number is less than 100, und rounds the result of nis

division down to the next intE::>ger.

- 92 -

'l'his i"tpQer ;.5 thc· " kl

5432

FIGURE 8-1

Midnight - 6 a.m.

Low or Light Volume

11 12 1noon

7 8 9 10Time of Day

- 93 -

6543

2 p.m. - 3 p.m.(Sample 2 p.m. - 3 p.m.)

6 a.m. - 1 p.m.(Sample 9 a.m. - 10 a.m.)

1 p.m. - 2 p.m.(Sample 1 p.m. - 2 p.m.)

3 p.m. - 4:30 p.m.(Sample 3:15 p.m. - 4:15 p.m.)

Average Volume

90

VOLUME PLOT LOOP 26

:::~j High or Heavy VolumeI

220 .

50

70

60

150

200

210

30

40

Ul(lJ

..-1o

•.-1

.c& 1204-lol-i(lJ

§ 100z

31212" referred to above. \\lJth this skip sizi?, the' traffic

specialist takes a systematic sample from U1C speeds he has

collected and calculates the 85th percentile speed. Figure

3-2 and Figure 8-3 give tabulations of both sample and popu-

lation speeds and the resulting 85th percentile speeds.

Figure 8-2 is for the average volume part of Figure 8-1;

Figure 8-3 is for the high volume part of Figure 8-1.

In Figures 8-2 and 8-3 the difference between the 85th

percentile speed when calculated using systemJtic sdmplinq

and when calculated using the entire speed population (as

gathered with the corr~uter sensor system

dix B) differ by only one mile per hour.

..., • 1 ., •

QC;SC.Cl[)(:::O In

The Institute has applied the technique iust describeD

to data from loops in traffic zones with speed limits of 20,

40, and 55 mph, for high and avernge traffic volumes. In

In every c~se, the 85th percentile speed ci\lculated fr~nl

--:ystema-:,ic -aTnpli;~'c:; differed uy no more tr-:·l: -'t.'r

high and medium volumes the difference '1J(l:~ nc'I< mol"" L.llc:-1

one mile per hour.

In summary, the 85th percentile speeC:Ls a good lnCi-

~a~or of an appropriate speed limit, and the method of

systematic samplinG 2? presented above has been shown to

- 94 -

Total Total~;~ed:,> 6am-lpm 2pm-3pm~G- SP(~cds _~ ,1 n:.=-_!-O i1n..1-? p m-]1~:.:5...:-

1-39 26 6 32 ] -1 f) 1 -) 4}

40-54 330 ')9 421 40- 54 32 29 6555 63 15 499 S5 9 7 81

7\ 56 76 17 592 56 1 8 90V 57 72 13 677 57 5 4 99E 58 73 22 772 58 4 7 110E 59 99 24 895 S9 5 9 124A 60 102 10 1007 60 4 6 134G 6J 58 6 1071 61 3 1 138E 62 70 15 1156 62 3 4 145

63 50 12 1218 63 2 5 152 FIGURE 8 --

V 64 69 9 1296 64 6 2 1600 0 43 14 1353

~5 4 169

L 66 32 5 1390 3 3 175U 67 29 2 1421 2 1 178M 68 31 7 1459 68 4 3 185E 69 25 6 1490 69 2 0 187

70-79 76 12 1578 70-79 8 4 19980-89 8 0 1586 80-89 1 0 200') () + 1 0 1587 90+ 0 0 200

x .85 x .85

f 1349 170

fpopulation sample

~ 3pm- Total ~3:15pm- Total

Speeds Ipm-2pm 4~30pm c. f. Speeds Ipm-2pm 4: 15pm c. f.

1-39 10 16 26 1-39 3 7 1040- 54 83 142 251 40-54 26 29 6555 14 42 307 55 5 8 7856 19 42 368 56 7 4 89

II 57 18 47 433 57 7 8 104I 513 16 34 483 58 7 1 112G 59 23 42 548 59 8 7 127II 60 11 38 597 GO 6 4 137

61 17 25 639 61 6 8 151V 62 17 31 687

~5 6 162 FIGURE 8 -'. ..)

0 @ 9 20 716 3 5 170L 11 21 748 2 2 174U 9 19 776 65 2 5 181M 66 4 15 795 66 1 2 184E 67 5 8 808 67 1 0 185

68 7 3 818 68 3 1 18969 8 8 834 69 3 1 19370-79 15 19 868 70-79 5 2 20080-89 1 0 869 80-89 0 0 20090+ 0 0 869 90+ 0 0 200

x .85 x .85739 170

- 95 -

a s tatis tically sound and accurate method for c]etcLc, j '1 t, ';

the 85th percentile speed.

8.3 General Comments

Several addi tional points included in the recom.rnendc(,

method must be discussed.

No specific equipment is recommended for measuring

speeds, however, it is critical that the measurements be

taken in a manner that is not obvious to the drivers on the

roadway. It should be noted that the survey results of this

study indicated that nearly all jurisdictions have access

to radar units. It was also shown that most jurisdictions

had traffic counters, and, of course, if in any case such

counters are not available, a manual vehicle count is

possible.

It is recommended that in most cases speeds be measurec.

during clear, dry weather and average volume condition8.

Generally the traffic specialist will want to set a spr20

limit to deal with the driving situation most conducive to

the faster speeds that could be expected on the road. Most

drivers will automatically be slowed down by poor weather

conditions and by increasing traffic volume. As volume

inc"::·'.~ases on a road, the driver tends to flow with the

traffic stream rather than obey the speed limit. It is, of

- 96 -

course, stated in the educational manual that predominant

weather conditions and specific problem volumes may be used

when appropriate.

After the 85th percentile speed has been computed, the

speed limit is determined by rounding to the next higher

S-mph. The purpose of this is to include as many vehicle

speeds as possible under the speed limit consistent with

the discussion in section 6.0 of risk.

In its application the method put forth in the imple­

mentation package is not to be viewed as rigid. Much of

the substance of the programed unit consists of common

sense and good judgment. There are no hard, fast rules for

such activities as selecting a representative measurement

site; however, the purpose of the text is to alert the

traffic specialist to the factors which should be taken into

consideration. Thus, the entire method should be employed

with careful judgment and good sense. The speed limit

resulting from the application of this method should be

subject to evaluation in terms of the traffic situation with

which the limit was to deal. If, for example, the new limit

creates an entirely new traffic speed pattern or increases

the accident picture, additional studies must be conducted

and a more appropriate speed limit reached. It must be

- 97 -

clearly understood that traffic behavior is dynamic, not

static, and that a new speed limit may change traffic

patterns enough to require additional changes in the limi

In any case, the speed limit must meet the need created

by the individual traffic situation.

- 98 -

9.0 COMMENTS ON SPEED LIMITS AND SPEED CON'l'ROL

Prior sections of this report have presented recommenda­

tions and conclusions that the authors believe are supported

by data, the analyses, and the literature. In this chapter

certain concepts and recommendations are discussed which the

authors deem appropriate for consideration but are not supported

by sufficient emperical data to warrant advocacy as proven.

In essence such commentary represents the authors' insight

into the problems of speed limits and speed control.

9.1 Special Speed Limits for Trucks and other Vehicles

Particular interest was expressed by the Sponsor in the

rationale for separate maximum speed limits for trucks and

other vehicles.

The survey conducted of various jurisdictions and reported

in section 3.0 contained questions focused on this issue. In

general jurisdictions with the same maximum limit for all

vehicles responded that this was done to avoid impeding the

flow of traffic. Those jurisdictions with lower maximum

speed limits for trucks, buses etc., responded that this was

done in the interests of safety. No jurisdiction gave any

indication that any empirical data were available to support

any position.

Conceptually, an argument can be made against a separate

speed limit on the grounds that it would produce a separate

- 99 -

s~eed distribution which mlght resLlt in the low end of the

distribution heavily represented in the high risk area of the

overall traffic distribution. In the same sense it miqht be

said that the lower limit would produce platooning on two

lane highways which would impede flow and generate passing

activity which in turn might increase the potential for acci­

dents.

An argument can be made for a lower limit if it can be

established that trucks and other like vehicles can not travel

safely at or near normal maximum limits. This is in effect

stating that the risk curve for trucks rises more sharply for

speeds above a mean speed than does the curve for cars and like

vehicles.

The data collected in this study do not provide an answer

to these issues. Examination of truck speeds at those points

where free flow was possible does not indicate a significant

difference between truck speeds and that of other vehicles.

Lower speeds are noted on grades. It should be noted

that speed limits would have little impact on such behavior as

it stems from vehicle limitations not driver choice.

One is led to suspect that concern over maximum limits for trL

buses, etc., may be a misallocation of "worry". Our data, and oV,c

studies (159, 165, ;1-90) indicate the slow moving vehicle to

be of high risk. It would appear more satisfactory to con~;jr3er

- 100 -

the problem of the slow moving truck. Pragmatically road

geometry may provide the basic speed control mechanisms. Thus,

some response is required to identify the slow moving truck

on grades. New York and some eastern states have required

vehicles traveling on limited access roads at speeds lower

than 25 mph below the speed limit to flash both rear blinkers.

It would seem this simple practice could be widely implemented

with probable risk reduction.

At the same time it would appear relatively simple to

gather basic data on speed behavior of trucks on highways

(1) with the same maximum limit (2) with different maximum

limits for trucks and cars. The distribution of speeds of

trucks and all traffic should be compared for bot.h sets of

highways.

If it were apparent that the result of a lower maximum

limit were to cause a significant number of trucks to travel

at a lower speed so that the overall distribution of traffic

was skewed or the standard deviation significantly larger,

one would conclude that the lower limit was inappropriate.

It is perhaps more probable that the distribution of

truck speeds would be more narrow and fall within the minimum

risk range of general traffic. It is also possible that one

would find that the lower maximum limit did not fall at the

actual 85th percentile of truck speeds.

- 101 -

In absence of sound emperical data we would neither

support nor deny the reasonability of lower limits for truck~

and buses but would require those seeking to impose a lower

limit to preseIlt clear evidence that risk management is serve

by such action. Imposition of the limit without such persuasl~

evidence can only be regarded as an inappropriate act that wjl

cause misallocation of Traffic law system resources.

9.2 Minimum and Advisory Speed Limits

The establishment of minimum and advisory speed limits

was not included within the scope of the study. fIowever, it

was impossible not to review the literature on such limits or

to consider them in the context of the theoretical hierarchy

of speed control. Adequate data collection elements were not

included within the research design to allow the formulation

of supportable emperical conclusions concerning such limits.

Certain insights are suggested by both the literature

and theoretical considerations. First, if one accepts the

concept that it is desirable to have as many vehicles as posslr

travel at or near the speed which is associated with the

minimum risk, it would appear logical to set advisory speed

limits at that value. The best evidence available suggests

that the minimum risk value is the mean speed of traffic.

Accordingly, it would seem that advisory speed limits should

be established at the mean speed of traffic.

- 102 -

Secondly, the litc:rature and the HTI-IRPS study clearly

indicate the danger created by slow moving vehicles. In fact

there is some evidence to indicate such vehicles constitute

as much or more risk than do those traveling faster than the

maxim~~ speed limit. Examination of the risk curves indicate

that the risk starts to rise at about one standard deviation

below the mean speed and rises sharply at two standard devia­

tions below mean speed. It would seem logical to place

minimum speed limits at the value associated with two standard

deviations below the mean speed.

9.3 Enforcement Tolerance

The practice of allowing some range of speeds above the

posted speed limit before enforcement action is initiated

is well known in law enforcement circles and equally well

perceived by most drivers.

Unfortunately, the precise nature of the reasoning which

governs the formulation of police on tolerances is not well

established or widely known. Attempts to encourage law en­

forcement policy makers to write or speak on the subject have

not met with much success. The precise tolerance allowed in

a particular jurisdiction is a function of many factors not

the least being the mood of the particular enforcement

officer observing the violation.

- 103 -

It appears quit\~ clear that-the naSJ,C concept :lnij"Ct

the decision not to make an arrest in every possible instan

when a speed limit is exceeded is sound. The problem lies",-~:

deve.Loping a general rule that can be consistently and widej

applied.

If once accepts the fact that enforcement ro Durces arr

quite scarce and should be selectively assigned against

highest risk some of the data takes on particular significano

Examination of the risk curves indicates that while the risk

starts to increase at approximately t.he 85th percentile (wh

is approximately one standard deviation from the mean speed I

increases even more sharply at blO standard deviations from i:h

mean speed. Thus, drivers traveling at a speed 'two standard

deviations from the mean speed are a clear risk and should b

subject to enforcement response. In the cases examined the

standard deviation had a value of about 7 mph. Thus, a typical

distribution would have a mean speed of about 58 mph, a speec

limi t at the 85th percentile of 65 mph, and the second stand:n

deviation would fall at about 72. The risk curves would

suggest 72 mph as a resonable point to ini tiate enforcern(~nt

action.

What is of perhaps more interest is the fact that the us

of the two standard deviation concept gives a rule that can

be applied as traffic flow behavior on a particular roadway

- 104 -

changes. Traffic at 8 am or ~ pm will have a different mean

speed than will traffic at 3 am. Examination of mean speeds

might result in the decision to enforce at or close to the

posted limit during times when the mean speed dropped and to

allow a much wider tolerance during low volume hours when

mean speeds are higher.

The data are not sufficient to allow a precise conclusion

that the tolerance should be established at the value associa­

ted with two standard deviations from mean speed. What should

be understood is that for a particular highway the risk does

vary as a function of mean speed. A single tolerance does

not satisfy the concept of risk management and selective

assignment of scarce law enforcement resources.

- 105 -

9. 4 Meas ur ing the~_ff~~t:i:.2i:.,=-nc:.::o:~__~~ f __§..2..~t:~~_~imi ts

A great deal has been written and said about the impact

of speed limits but almost no empirical data exists that can

be used to quantitatively establish the precise effect of a

speed limit. In the absence of such knowledge it becomes

equally impossible to state quantitatively a method to measure

the effectiveness of a speed limit.

If one thinks of the objective of a speea limit 28 reau-

lating traffic in the risk management sense to achieve the

objectives of the traffic law system and the SRT system, tKC-

measures of effectiveness are suggested. One, has the flow

rate increased? Two, has the accident rate increased'?

The measurement of flow if thought of in terms of through­

put may be evaluated in terms of increase in average or mean

speed. Readings of speed distributions before and after alter-

ation or establishment of the speed limit may be expected to

give some insight.

much more complex.

However, the issue of accident reduction lS

Accidents are infrequent events no matter

how numerous they may seem. Further, the factors involved in

accident causation are numerous and many variables are at play

that cannot be influenced by a speed limit. The accident experl-

ence might actually rise following a change to precisely the

best speed limit due to the action of intervening varlable~:':.

Thus, dependency upon accidents for an evaluation of the

effectiveness of a speed limit on a particular highway is at

- 106 -

best a lengthy process of uncertain reliability.

Obviously, some interinl measure must be identified. The

work of Taylor and the Tennessee Department of Highways is

most promising in this regard. Consideration of the shape of

the resultant distribution and accompanying parameters holds

perhaps the best clue for a measure of effectiveness.

Conceptually, it would be ideal if all vehicles traveled

at or near the same speed. This would minimize the potential

for conflict of traffic traveling in the same direction. This

concept is limited in the real world by differences in the

capabilities of vehicles and drivers as well as the speed dif­

ferentials that are created by entering and leaving the highway.

For a given highway there exists some set of vehicles at

any instant of time that must travel at a speed lower than the

mean speed of traffic simply because of vehicle condition or

driver destination. Thus every distribution of speeds will have

a definite set of vehicles to the left of the mean. The extent

of the distribution to the right of the mean is more difficult

to identify. The decision to travel faster than the average

speed is a function of individual driver judgment and includes a

mix of risk perception, time pressure/ individual driving confi­

dence, vehicle confidence, familiarity with highway, etc. Such

decision-making process may be more susceptible to influence by

speed limits but the extent, mode or manner is not now clear.

It would seem generally desirable to reduce the nw~er of

- 107 -

drivers traveling at ~;oceds eli. f"fcr(~nt than the mean spfc;ccL.

Thus, a speed limit which produced a distribution that was

more nearly normal with a smaller standard deviation would

seem morc desireable or more effective. Unfortunately,

data are not available to clearly support this position no

matter how logical we may perceive it.

We believe tha.t considerable effort should be made by

the leaders in the field of Traffic Safety to encourage the

implementation of the 85th percentile concept. Such imple­

mentation should be accompanied by post-implementation evalu­

ation to determine the change in the parameters of the speed

distributions. Such changes should be correlated against

throughput and accident experience over a lengthy period of

time with every effort made to identify the impact of inter­

vening variables. Accident experience may be regarded as a

poor indicator for a single highway, but should be useful

for evaluation over a broad highway set over an extended period

of time. Local jurisdictions should be required to collect

post-implementation data at periodic intervals in a common

manner to allow cross-correlative evaluation.

- 108 -

9.5 Recommendations for further Research:

It is a characteristic of every research report that it

concludes with a recommendation for further research which is

needed to solve the question the project was originally funded

to answer. It would indeed be difficult to fly in the face of

tradition and hope to maintain any status as a researcher.

However, we are constrained to question the wisdom or

feasibility of conducting relatively small scale research

projects on the subject of speed limits or speed control. The

research to date has pointed quite conclusively to a method that

can be broadly implemented and may be expected to have desirable

results. It seems pointless to engage in further research

unless it can clearly advance the state of the art.

The "state of the Art" stops at the point one asks the

question "what is the effect of a speed limit". Such a question

can not be answered by a small scale research project. One must

either alter the speed limits on a fantastically large set of

highways while measuring the effects or settle for a smaller

set of highways and engage in continuous variation of the speed

limit. The first prospect staggers the imagination when one

considers the real world problems inherent in implementation.

The second prospect which would involve instrumenting a

set of representative highways and measuring the response to

variable speed limits is not an impossibility but the cost

- 109 -

f3ctors associated

certainly be wi thin tJle milliorl dc)11ar :r'ange.

One would question the value of such an expenditure

Vlirl-JiCl"l at best could, })(? (~xpec:t:.(~d t() adcl anly sliglltl~l greater

precision to the 85th percentile method. If the cost could

be justified it would likely to be terms of increased flow

and in such case would be relevant to only a minimal set of

the highway system, primarily urban expressways or feeder

highways where maximizing volume is a critical issue.

The issue can probably be better met in t'C':.:rms of the

need for a general information system for the nations highwayc.

The use and value of process control computer systems has been

adequately demonstrated in the private sector: Initial efforts

in the utilization of computer systems for signalization have bec~

successful but have represented relatively trivial demands on

the inherent capability of computer technology.

It would be far more logical to think in terms of develo;:

ment of a comprehensive computer based information system for

a highway set. Such a system would collect basic data, evalua.

it, provide information to highway users, and those respons i i·

for highway safety, traffic control etc. Such a system waul

logically include internal measures of effectiveness that we

evaluate driver respcmse to a range of stimuli merely one of

which would be speed limits.

- 110 -

The cost of such a system would not greatly exceed that

required for evaluation of a single variable such as speed

limits and holds considerable prom.ise for overall traffic

safety management.

It is also believed that cost-oriented methodologies

offer promise as a longer range approach to determining opti­

mal speed limits. Properly directed, this techniques could

place a large complement of highly developed and widely applied

analytical tools in the hands of highway analysts, planners,

and operational personnel. These tools could be of great

value not only to the establishmen"t of speed limits, but could

also be applied to a host of other highway problems. Perhaps

even more importantly, the approach could provide a means for

determining SRT priorities and of allocating resources among

them. Certainly, such a model would be most useful in struc­

turing and designing a highway information system such as that

described above. A considerable research effort will be

required, however, to develop the cost-oriented approach to the

point of practical utility.

- III -

1. Scott-Montague, ,John, "Automobile Legislation: ACr i ticism and Review," North American Revie'N, Vol.179, No. 573, pp. 168-177, August 1904.

2. "Regalation of Motor Cars," Quarterly Review, Vol. 205,No. 409, pp. 511-530, October 1906.

3. "Photographic Methods of Determininq Speed of Automobiles,"Scientific American, Vol. B3, p. 262, October 27, 1900.

4. Ladd, Walter D., Organizing for Traffic SafetyCommunity, Charles C. Thomas, Publisher:pp. 10, 63, 132-133, 1959.

in YourSpringfield,

5. "The Electric Car," Atlantic Monthly, Vol. 89, No. 536,p. 802, June 1902.

6. Labatut, Jean and Wheaton J. Lane, Highways in Our NationalLife: A Symposium, Princeton University Press:Princeton, p. 95, 1950.

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90. Webs ter, Lee A., and vvayne 'I. Gruen f [\ Summary 0 f thi..c,

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98. City of Nashville Traffic Commission, "Effectiveness ofSpeed Limit Signs," Nashville, Tennessee, 1956.

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106. 11tJews B.ricfs, 'I !=~~_r,:lf'~~i.,~~ r\~

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_____ . .• __..• .."'-.. ",. .. J_.

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109. Oga~"va, T., B.S ..Fj.s}-lcr, ci11'--J ~i.C:,~ ()ppe~':l{_lrl(j(~r, ~?r.-~:~~~

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110. Bezkorovainy, Georgy, Effects of Adviaory Speed ~imits ~t

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111. Bezkorovainy, Georgy and Chih-Cheng K~, The InfJuence ofHorizontal Cur've Advisory Speed Lim-its em Spot-Speeds r

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112. Fisher, Ewen Stuart, An Appraisal of a Theory of VehiculaSpeed Regulation~, Ve11icular Sp'eed -i:Zeq~~C;t-i.on -ResearchProject, Dept. CLvil EngineerinQ, University of IllincJanuary 1963.

113. Roberts, Robert E., "'::'he Influence ofUrban Speed Distribution Parameters, !I~: :1.'),,'

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114. Road Research Laboratory,Majesty's Stationery1963.

115. 'rho. Eno Foundation for Ui "ra'[ic ");;'1-']

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39-40, 1948.

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116. Lefeve, B. A. / "Speed Babi ts Observed 011 a r~ural High'J'lay, "Highway Research Board, Proceedings Thirty-thirdAnnual Meeting, t'Jal_ -ionai Eesearch Council Publ ication324, Vol. 33, pp. 409-428, 1954.

117. Michaels, R.M. and David Solomon, "Effect of Speed ChangeInformation on Spacing Between Vehicles," HighwayResearch Board, Bulletin 330: Driver Characteristics,National Research Council Publioation 1010, pp. 26-39,1962.

118. Ritchie, MaJcolm L., William K. McCoy, and William L. WeIde,"A Study of the Relation between For"lard Velocl ty andLateral Acceleration in Curves Durlng Normal Driving,"Human Factors, Vol. 10/ No. 3/ pp. 255-258/ 1968.

119. Denton, G.G., "A Subjective Scale of Spe\~d when Driving aMotor Vehicle," Ergonomics, Vol. 9/ >Jo. 3/ pp. 203­210, 1966.

120. Gutshall, Robert W., Charles Harper, and Donald Burke,"An Exploratory Study of the Interrelations amongDriving Ability, Driving Exposure, and SocioeconomicStatus of Low, Average, and High Intelligence Males,"Exceptional Children, Vol. 46, No.1, pp. 43-47,September 1968.

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125. Missouri Traffic Survey of 1934/ pp. 105-118, 1934.

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128. Taragin, A. / "Driver Performance on Horizontal Curves,"Highway Research Board, Proceedings Thirty-thirdAnnual ~1eeting, National Research Council Publication324, Vol. 33, pp. 446-466, 1954.

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130. I'orbes, T.VJ., "Speed" Headv!dy, and Volume Fe1atlonship,~

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132. Taraqin, A., "Driver 8;cc'hav.Jcr as Affe:ctc'(::l O!.)ject~; enIf i g Il't.,v a l' S h. () tIl. (} e r S f II Il.t \1:;~1\-v (J. ')' }~e ~-; c~ Z:~l I~ C:'1 j3c, eJ. r ,j

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n \lC;:blic~!133. Heyer, Robert L., ";Icuc]ySpeeds OD a Rural HIp. 345, July 1951

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Oppe n 1 a nde r, ,J. C., 1~L.L.__!-i ''Z2;:~) ate l~!:!:,'i1X:~;.~__._~) :.Li cu l.e'.!:-.Speeds, Vehicular Speed ~egulation Research Project,Dept. Civil Engineering, University of II i~ois,

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139. Wortman, Robert H.. , !i_ Mul!..l vari_~~·te L~~~lJ...)~~~~ .._..Speeds on FOlJr_._~::.~~ne Fural i[lgl2.Yv,,~_.. \7

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140. Wilson, James E., "Speed Zon:inCj cn California Highways,"California Division of Highways, originally preparedJanuary 26, 1956, revised Allgust 20, 1964.

141. Onsi, Mohamed, Evaluation of Statistical Methods forDetermining Significant Variables in Rural Vehicular~pot-Speed Prediction Equations, Vehicular SpeedRegultion Research Project, Dept. Civil Engineering,University of Illinois, October 1965.

142. Bunte, William F., Methods for Evaluating Highway Featureswhich Influence Vehicular Speeds, Vehicular SpeedRegUlation Research Project, Dept. Civil Engineering,Univers i ty of Illinois, Sept.ember 1959.

143. Horkay, A.T., Effect of Horizontal Resistance on VehicularSpeeds, Vehicular Speed Reg~lation Research ~roject,

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144. Oppenlander, J.C., Influence of Out-af-State PassengerCars on Spot-Speed Characteristics, Vehicular SpeedRegUlation Research Project, Dept. Civil Engineering,University of Illinois, May 1962.

145. Ku, Chih-Cheng, Driver Characteristics Correlated withSpeeds Observed on Horizontal Curves of Two·-LaneRural Highways, Vehicular Speed Regulation ResearchProject, Dept. Civil Engineering, University ofIllinois, May 1965.

146. Webster, Lee A., Driver Opinion and CharacteristicsRelated to Measured Rural Spot Speeds, VehicularSpeed Regulation Research Project, Dept. CivilEngineering, University of Illinois, July 1965.

147. Fields, Marvin, "Speed RegUlation," Traffic EngineeringHandbook, Third Edition, Institute of TrafficEngineers: Washington, D.C., pp. 530-549, 1965.

148. "Stop Sign Speed Control," Traffic Engineering, Vol. 34,No. 12, p. 41, September 1964.

149. Coburn, T.M., "Trends in Speeds on British Main Roads,"Reprinted from the Proceedings of the Third Confer­ence of the Australian Road Research Board, Vol. 3,Part 1, pp. 593-603, 1966.

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ot Technology:: 3els JnKl., :::>C1 en t If lC l~e sc arche s No.22 1967.

nS~;.1/~t?d as a CauE;e ():[ ITraffi,:: l:.c(=:Ldc:~l·tE; J It rrl~lC' l\Irl(;T--_._._.._ _..-._---_ =.Vol. 50, p. 73, February 1933.

2" A Survc;"}t of Tra:~:fic C'(J t.iCJ113 .tT: thE' C~l,,·t ·-)f :=;~J113~~· 'v,l

Checks of Obedience t~ ~ra~fic 1,2JWS s~)(:)nsDrod

City of .Dal1E~\s P()l.icE~ DCI='c;J:.'"tTIlent, ;-}939J" r-)~ 90.

153.

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i_,<) i;"_~ T?:-ll)1l~.:)1-'j.j_llg CC).

Richmond, I)-:-':riYr~- 19 J C'7--

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.l....J .~ C-4.l_ \.J.! ..L. ...../.)_ ,J..l, "j ... (V ..1 .1 ....... .LJ..,. \ __._,. __ 1....L ..

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Brittenharn, 'r.G. I D .. l\'l. Glartcy-, L.tn.ll L'~~-l ..of In\Tcstig-atir,g di ~L'rat

Research Board Bullet

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r¥l\:~ -1-

162. "Improper Speed a Big Fact.or in 'Traffic Accidents," TrafficSafety, Vol. 52, No.4, p. 36, April 1958.

163. Prisk, C.W., "The Speed Factor in Highway Accidents," TrafficEngineering, Vol. 29, No. 11, pp. 16-17, 25, August1959.

164. Minnesota Department of Highways, "A Comparison of RuralSecondary Road Accidents Before and After theInstallation of Lower Speed Limit Signs and AdvisoryCurve Speed Signs," January 9, 1961.

165. Moore, John 0., "A Study of Speed in Injury-ProducingAccidents: A Preliminary Report," Traffic Safety,Vol. 54, No.6, pp. 16-21, June 1959.

166. Case, Harry W., and Roger G. Stewart, "Inventory SpeedResponses and Prior Traffic Records as Predictors ofSubsequent Traffic Records," Highway.Research BoardBulletin 240: Highway Accident Studies, National .Research Council Publication 726, pp. ~3-56, 1960.

167. Solomon, David, "Accidents on Main Rural Highways Relatedto Speed, Driver, and Vehicle," Bureau of PublicRoads, July 1964.

168. Davies, Ernest, Roads and Their Traffic, Blackie and SonLimited: Glasgow and Philosophical Library, Inc.:New York, pp. 107, 118-121, 269-270, 1960.

169. Swedish Council on Road Safety Research, "Report on theResults of Temporary Speed Limit on Road Accidents 1nSweden," Stockholm, no date.

170. Road Safety Committee, Temporary Speed Limits in RoadTraffic During 1961, Stockholm, 1961.

171. Harper, B.S.C., "An Examination of the Effect of Raisingthe Speed Limit in Built-Up Areas in Victoria,"Proceedings of the Third Conference of the AustralianRoad Research Board, 1966, p. 647, 1966.

172. H~kkinen, Sauli, and Urpo Lepp~nen, Speed Limit Trial inHighway Traffic in October-November 1966, TALJA, TheCentral Organisation for Traffic Safety in Finland:Helsinki, 1968.

173. Wadsworth, J., "The Effect of Conventionally EnforcedMaximum Speed Limits on Motor Vehicle Accidents,"National Research Council of Canada, Motor VehicleAccident Study Group: Ottawa, Technical Note No.9,November 1966.

- 125 -

174.

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Prisk, Charles W., "Accident Rates on NotoDvays cspecial1.~'

1-.] i th reference to: (]) Speed Di [ferences (b) ;1inin:uidnd Maximum Speed Limits. Genp~:al Report," Interna c;Eoad Safety and Traif 1 c r<,eview, '/01. XV, 'Jo. -2, --p:::J ~.­24- 33, Spr ing 19(') 7 .---~------_._--

Automot i V(? Safety Foundati en, 'fraf f ic Contro:md j~uc',d\~.~z.

Elements: The r Relationshio to Hiqhwav Safety,. --:=-_.,.--;::;-;:----"'J.~.-----.----~---., _=_

Washington, D.-i:=:--, pp. 67-70, 1963.

T a y lor, 1,h 11 i am C., _'"I:',l,,1:..=C'_.-:L,=c:..'.f E.,-:e=-,','::;:__-,c_. _ ,(,)__ f, ~;_.,..±:,-;-,_:_.. .. _._,,-~_,_.,_:..-= -c:~__._ .

Operations,- CIIno Depacmentof Public Eoad,;, FebrJc~ry q(,

"Speed-Related ACCIdent Countermeasurc's ano '':'1:.1: Pr:i:'­Proceedings of National rIiqhway :~afety 130r,CluPriorities SeTT~rDi~r!, Vol. ~~t=ecl::-rlcK'smTl:'i-f\~Jrcr:r' ciJuly 18-30, 1969.

180. Willett, T.C., Criminal on the Hoad, Tavistock Put)]jcationo;'London, pp. 20-21, 19~4.

181.

182.

Baker, J. Stannard, "Research and AccIdents," ':;'1'3 f fie Di:;jeand Eeview, Vol. 12, No. 12" p. 12, DecemFer~-r96-~--'-----

"80 m. p. h. Speed Sharply Increaces Acciden t [(isk \vi th LJ, t_ t l

Saving in 'I'ime," Traffic Digest and,_~~:,T:rJ.:~~Y'~' Vol. 11!No.6, p. 16, June 1963.

183. Campbell, M. Earl, "Highway TrafficPossible?" Traffic Quarterlv.3 39 - 3 4 6 r J u 1 Y-I96~5-:--'-------_-d'·

Safety _.. ,; It\Tcll to 19 I\i(~ ~ 3 f

184. Goen, Richard L., "Drastic j\leasun~~;, for ;;:':::ducjnej 'l'raffJcCasual ties," Stanford Eesearch lnsti t:u tc: l\lenl() lacCalifornia, p. 20, June 1960.

Li \]03185. Goen, Richard L., "'1'h,-:' Tradeoff BetweenTinlL," [)(:.tper p~t'f;:::,entc(:i a.t. the 29thof the Operations Research Soci~ty

18, 1966.c)f -", -~, -

-c..:..\(".-'

rl.'VI!19

t_l

186. Poulsen, Roy G.Safety, Vol.

- 126 -

187. National Highway Safety Board Staff, Maximum Safe Speedfor Motor Vehicle~, January 31, 1969.

188. Mitchell, H. Richard, "Freeway Accidents and Slow-MovingVehicles," Traffic Engineering, Vol. 37, No.2, pp.22-25, November 1966.

189. Cirillo, J.A., "Interstate System Accident Research StudyII, Interim Report II," Public Roads, Vol. 35, No.3,pp. 71-76, August 1968.

190. "Traffic Accident Facts, 1969 Edition," Traffic Safety,Vol. 69, No.8, pp. 11-14, August 1969.

191. Research Triangle Institute, ?~~d and Accidents, Vol. II,Final Report, "Summary and Conclusions," May 1970.

192. Reeder, Earl J., "How Slow is 'Slow'?" The American City,Vol. 46, pp. 111-112, May 1932.

193. Carter, F.M., "Speed Zoning," Proceedings of the FirstCalifornia Institute on Street and Highway Problems,Berkeley, California, pp. 151-156, January 31 toFebruary 2, 1949.

194. Smith, Wilbur, and Associates, Report on Traffic, Transit,Parking, Metropolitan Winnipeg: New Haven, Connecticut,pp. 71-73, 1957.

195. Baerwald, John E., "Theory of Speed Zones in DevelopedAreas," Traffic Engineering" Vol. 28, No.3, pp. 20­23, 39, December 1957.

196. Ohio Department of Highways, Let1:ers and forms on speedzoning and speed control, 1957, 1960, and 1963.

197. Anonymous, "An Informational Report on Speed Zoning,"Traffic Engineering, Vol. 31, No. 10, pp. 39-44,July 1961.

198. Donigan, Robert L. and Edward C. Fisher, "Know the Law:Speed Zoning," Traffic Digest and Review, Vol. 14,No.9, pp. 19-24, September 1966.

199. Rowan, Neilon J., and Charles J. Keese, "A Study of FactorsInfluencing Traffic Speeds," Highway Research BoardBulletin 341: Accident Analysis and Speed Character­istics, National Research Council Publication 1023,pp. 30-76, 1962.

- 127 -

200.

201.

Oppcnlander, J.e.,Vehicula:r' SpcCivil Engin2cri~g, University

Baerwald, John g., "300 -HorsepO'.ver Roads -- Hors(:BUC~lr,.J'::: Spe"e<'1' "0>,,"" " 'T'raf'fic E'ng;TjO""Y.'l'J1Q \Tell'.• ~I_ _ .L.JC< \ ::, , ~_r, •• ~ ' '_':":.::':.'=._:... _=_' .20-22, June 1964.

a.nd34, [-JIJ ~

202. Kloeker, D.L., "Speed R.ezc~;inq,"

Road School in 1968.

203.

204.

205.

J3randt, Warrerl 11. f II Sl~eed I,irni t._E~~ ._- 1'nc()r.yr anci .:Jre-lct,ic(_:, f?

FBI Law Enforcement aulletin, Vol. 3S, No. na. 2-62 2 - 2 5, i\Ia r c:~--P3 6r;:-------.-----

Kessler, Warren L., The Effect of Speed Zone Modlficati TIS

Occas ioned by i~E(; Ilf·Criofs---~s?e-(2·CCJ~23,.:)·,,·,T{:~t-;T;;1-L3:r-S---

Regulation Res~~arch Project~Dept~:---CivilEngineer in9,University of Illinois, February 27, 1959.

Deen, 1"1.B., "Effect.iveness of S[)eed IJ.in1it ~~,i(Jrl ,II ].lJ:affi~_

Engineerin~, Vol. 29, No.7, pp. 22-24, 62, April 19SQ.

206. Tennessee Department of Highways, Traffic EngineeringDivis ion, 1', Summary of Practi ces in Es t~ab] i sll i ng SpeedZone~, Na s tlV i lJ_ e J~ Jun E~' 19 6 ff:"-------------·- --------------

207. Sub-Committee on SpE~ed Zoning, Flesolut~ion of t.ne AnnualMeeting of the }I,.merican Association of Stah' Highwayuificials, 1969.

208. "Speed Zoning, Why anel How -- A Guide to Es tab 1 i shingRealistic Speed Limits," Automobile Club of SouthernCalifornia and California State Automobile Association.1965.

209. Johnston, J. Ed'j·.rard, "Speed ControJ and ReguLatiun,"I.T.E. Proceedings, Institute of Traffic Engineers,pp. 141-148-;-1965.

210. Taylor, William, If}', New Concept in Speed Zon.i.nq f"

Engineering, Vol. 34, No. 12, pp. 38-39,1964. .

Tr at fie:C?!TIoe.r

211. 'Taylor, William C., "Speed Zoning;Traffic Engineering, Vol. 35,50, ,January .19(5 .~-'

A Theory and its p~

No.4, pp. 17-19, ~18-

212. 'l'ennessee Departmen t of HiglPt!ays, Traf fi.c F'n] .111'." 1.inG

Divisi()rl, I:~"\laltlatic)n elf th(~ 2fft~ L \"' r"' :::;s c,f c.~:ci()u:·;

Spot Sl)ced--Pa-r- ::3.rnE~~ ters -~Used in .f~s t ,:J~~j~~~-_i). n;~J S;;JC~~~-:__0­-Zones-~ashv·:'-,·>:;-- M",·· 1 qKQ__> _' l\1 '--... J...1- t:,- f' ',1\.'(,Y ,L._," ,..-,

- 128 -

213. Baerwald, John E., "Needs and Methods of Speed Control,"IJovember 1964.

214. Smith, Wilbur, and Charles S. LeCraw, Jr., Traffic SpeedEnforcement Policies, The Eno Foundation for HighwayTraffic Control, Inc.: Saugatuck, Connecticut, p. 16,1948.

215. Marcellis, Jack C., An Economic Evaluation of TrafficMovement at Various Speeds, Vehicular Speed RegulationResearch Project, Dept. Civil Engineering, Universityof Illinois, pp. 1, 39, 49, 56, October 1963.

216. "Speed Limits May Be Outmoded," Traffic Digest and Review,Vol. 13, No. 11, p. 14, November 1965.

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/3[

APPENDICES

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APPENDIX A

GLOSSARY OF TERMS

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Sources for this glossary were:

I. TRAFFIC ENGINEERING HANDBOOK, Third Edition,Institute of Traffic· Engineers: Washington,D.C., 1965.

II. DICTIONARY OF HIGHWAY TRAFFIC, J. StannardBaker and william R. ··Stebt;irls, Jr., 'l'rafficInstitute, Northwestern Univ0rsity: Evanston,Illinois, 1960.

III. TRAFFIC ENGINEERING, T.M. Matson, W.S. Smith,and F.w-.-nurd~-·McGia\v-HillBook Co.: New York,1955.

IV. MATHEMATICS DICTIONARY, Second Edition, GlennJames and R~obert C. ~(ames, editors, D. VanNostrand Company, Inc.: Princeton, New Jersey,1959.

absolute speed limi t: "a speed above vvhich it is always ille­. gal to drive." Also known as a maximum lawful

limit. (I, p. 538)

advisory speed limit: the maximum safe speed that is postedbelow a warning sign. "In most states, the ad­visory speeds are not legally enforceable, butin some courts violation of the advisory speedsis admissible as evidence that the driver wasoperating in a reckless manner." (I, p. 541)

average overall speed: "The average of the overall speedsof all vehicles on a given roadway during aspecified period of time." (II, p. 12)

average overall travel speed: "The sum of distances dividedby the sum of overall travel times (a space­mean speed)." (I, p. 159)

average spot speed: "The arithmetic mean of the speeds ofall traffic, or component thereof, at a speci­fie d po i nt. " (I, p. 159 )

critical approach speed: "At an intersection, that speed abovewhfChial-vehicle does not have sufficient distanceto stop in time to avoid collision with another

- 135 -Preceding page blank

vehicle approaching the lntersection on tnlstreet " (T'" p. 39)

cri tical speed (on cllrve): "The speed above wh ich ,::; veh io]will slide off the curve rather than followaround it." (II, p. 40)

design speed: (of highway) ~ "A speed selected for J=)urpose',of design and correlation of those features fa highway, such as curvature, superelevaticn,and sight distance, upon which the safe opera­tion of vehicles is dependent. It. is the h j=;1:­est cc)rlotir}llOUS speed. at v.l}-·!::_ch incl"i'\/ .~l 'J;'~j­

hicles can trave' with saFety upon a highwaywhen weather conditions are favorahle, traf~ic

density is low, and the design features of thhighway are t~e governing condItions forsafet_y." (II, p. 413)

85th-percentile speer:!: "That speed di~ or belovJ ,,/hich b'; pE;'"

- -'---cen-t 6:F vc~hicles traveJ." (T J I C 68) ;'l'hr:th c>"""~clt-l- 111i '0(-']...,-', -,.. ,.yo,... r'<;rr-x pc~ \._,,,,1 _l e ,~OU C d..L,).' l1a\. e Cl ,_.' __ ospon,.L,.",.-J

definition.]

free-mov ing vehicle: "One in ",hich the dr i V('C i.co; llDt. re­stricted in selecting his speed by other v~­

hicles ... Some observers classify a free-movingvehicle as one which has not less than 6 - 9sec. headway from the vehicle ahead of it andis making no apparent effort to overtake andpass thE? vehicle ahead of it." (I, p. 539)

b.eadway: "The time intE?rval betwE!en passages of consecut]vehicles measured from head to head, moving lD

the same direction as they pass a given poiY-it,"(II, p. 91) or "The distance, measured frontto front, between consecutive vehicles."(II, p. 92)

maximum law~u~ ~imit: I!! a speed above which it is always iIIgal to drive. II Also known as an abscl 1Jte SDC'

limit. (I, p. 538)

median spe~d (of t~raffic): "That speed belo"v Whlcn 50 pCIC(;r

and above which 50 percent of the speeds 0 ­

curred." (II, p. 128) [the 50th percentile]

modal average: II t.hat. speed at which the g;:-eatest DllIT1Der c)f~­

vehicles t.raveL" (III, p. 51)

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!"l9m~~~~__~..p~~i (of traffic)' "i\ :nJn:-!inq sp0(::cl at which driver'.:;operate on a glven section of highw~y in the ab­sence of traffic:: interference." (II, p. 140)

?perating speed: "The highest overall speed exclusive ofstops at which a driver can travel on a givenhighway under prevailing conditions without atany time exceeding thl3 design speed."(II, p. 151)

optimum. sp~ed: "The average speed at. wh ich traff ic mUE: t movewhen the volume is at a maximum on a given road­way. An average speed either appreciably higheror lower than the optimum will result in a re­duction in volume." (II, p. 152)

overall travel speed: "The speed over a specified section ofhighway, being the distance divided by overalltravel time ... " (I, p. 159)

overall travel time: "The total time of travel, includingstops and delays, except those off the traveledway ... " (I, p. 159)

pace of traffic: "the range of spee:d which includes thegreatest number of vehicles for some nominalincrement in speed, usually 10 mph." (III, p. 51)

prima facie speed limit~ "a speed above which the driver ispresumed to be driving unlawfully but if chargedwith exceeding it, a driver may show cause toprove that his speed was safe for conditionsand, therefore, that he was not guilty of aspeed violation." (I, p. 538)

running speed: "The speed over a specified section of high­way, being the distance divided by running time

" (I, pp. 159, 161)

running time: "The time the vehicle is in motion ... " (I, p. 159)

skew distribution: "A non-symmetrical distribution. A dis-.. tribution is skewed to the left (right) if the

longer tail is on the left (right) - also callednegative (positive) skewness ... " (Iv, p. 126)

space-mean speed: "The speed corresponding to the averageo( overall travel times or running times over aspec if ied section of highway." (I, p. 161)

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"The speed of a 'Ichicle as it passes a spot orpoint on a s tr(~et or high"Jay." (I, p. 1') 9)

ten-mile-per-hour pace: "The lO-mph speed range containingthe -Tarcrest percentage of vehicles in a sampleof spot speeds." (I, p. 159)

time-mean speed: "ThE~ average of spot speeds of individualoverall travel speed values." (T, p. 161)

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APPENDIX B

SENSOR SYSTEM DESCRIPTION

One of the difficult ies wi til the traffic measuring devices in conunon use I

t.ill' influence th" devIce exerts on till' very tlling it i~, mea~-;lIring. Obtrusive

llll,tl"'ds such as radar ,'1;~d speed tapes tend to interject bias into the data bCGlU';('

of tlieir visibility to the driver.

As an integral portion of the present study, the Institute tur Rc:e8rch in

Public Safety undertook the development of an unobtrusive traffic measuring system

in which loop detectors were connected via telephone lines to a process control

computer.

Eight locations along State Route 37, NClrth and South of the City ot BloomingLull,

Indiana, were monitored 24 hours a day during Phase I of the study. This number

was expanded to fourteen during Phase II.

The unobtrusive instrumentation at each site relayed two signals which were

Interpreted by the IBM 1800 computer to yield the vehicle's speed, length, location,

lane of .travel, direction, headway (the time differential between the preceeding

vehicle and the vehicle being monitored), and time of transit.

One of the chief attributes of the computer-'sensor system is its ability to

collect, array, and store data without outside intervention. No human judgment

(or error) became a factor in the data assimilated by the systt,m. Other than the

weekly calibration and a replacement of magnetic tape on a five day cycle, no

personnel are involved in the system operation.

1.0 The Computer

The computer system developed for the present project has as its core an

IBH 1800 system. An IBH traffic control system program was modified bv project

personnel to the special needs called for when measuring vehicle velocities data.

The requirements of the project contemplated the use of the computer as a portion

of an information sys tern which would provide data for deci~;ion-making purposes.

- 141 - Preceding page blank

'~'rl~ (iecisi~Jn-rnaking process contemplated i~ t:~lese projectf; was mc)re analyt[ci1J

in nilture; however. the system could also be adapted for tactical decision-making.

Hardware

During Phase I. the IBM 1800 system was composed of the following component';:

(' " 1802 Central Processing Unit (16K, 4mics.), .l..J

(2) 2401 Magnetic Tape [Jnit

(3) 1442 Card Read Punch

(4) 1810 Disk Storage (250,,000 words)

(5 ) 1826 Data Adaptor

(6) 1816 Printer Keyboard

(7) 1802 Process Contro lIe r.

Twelve interrupt levels aTe l1eu',;;ary for a r~ini:11aJ :-;ystPfn. I'he systr;~1.1 a}(.1

has :1 1053 character printer ~\,o\:rhich is used for data retrieval, while t;--;f' t:h~r-

printf-'l- (1816) maintains a printed record of system statLi:,. in urder to ,-(",ivl::

the vehicle information from the highway, four digital input ~;trips with 5i:;:_(·.-n

points each are mounted in the computer interfaciny.

Sys tern Software

The basic system software used in the 5yste!11 design is IBN' f; 1800 .;). -:'i "''''

Sharing Executive (TSX). Version 3. Modification Level 7. Through 3 series of

prugrammed interrupts. queued programs and non-process programs, as ,,'pll as optir.-,lzi:c

alterations in the systems director. the facility has been adapted to the specific

,-~qllirements of traffic study. Non·-process rrograms can nt' run during LOI.' traffiC

v111uru' periods wi~'hout disturbing data collection.

Program PILOT', an incore subroutine, scans t',o ,j; '.') t"

ill LSt'conds to check the status of each vehicle detect,:f. A "!·:.'L :,':nlta;,o

caused by a vd:1cle passing over a magnetic loop det~ct"r v'i l' hp recr'rd"" aloll

with the time AS indicated on a 30.000 second r.lock e This~ (Hid the Tetu',-;-. to

nonr.;ilcv which accompanies the passage of the vehicle, ,ire recorded for t\",C 'iire

- 142

lit' 111 k unit I;1;ly continue to colLect and store vphicle data for a period of theet'

Ll' tOllr hours (depending on traffic dt'nsLty) before requiring a tape backup. During

tili,; timt·, prevj':llsly completed tapt's may be re-examined for retrieval of old data

I,J'rOl~r:llll \':;012), :malvsis or any other task which may require the assistance of a

(,' I ~~:nc' tic tape uni t .

Program NCOll replaces the tape removed by NCOO]. To regain the computer for

dny aIlal \',;is requi rinl~ till: tape uni t, tbe' :;COO]-,\(O" 1 sequence may be rerun as often

as neCl'ssary, up until midnlgbt when Program LNDDAY compute'; t H' daily total.s and

c(:lnintidlizt:'s the system with thl~ ne\.J date.

['roc; the o;ignals rcoceived from each loop set, vehicle sp.:>ed, ],'n"th, d.l rectton

~md .lane of travel are calculate'll. AlsD rlc'corded is the time of and th~: time

bet\>.'cel1 the vehicle in the set and the previous vehicle. In this manner, the system

vicle1:; lane usage information as well as speed relationship data.

2.0 Sensor System and Interface Equipment

I110 sensor consists of an RCA Vehicle Detector Uni 1: h'll Lcll is conrwcted to a

1"CJp of Hire placed into the roadway. The vehicle detector senses the DreSE,nce of

3 vebicle in the loop and closes a relay in the detE,ctor amplifier. The relay is

cOI1r:e,~t:'l to ,] flhone line V>!hich terminates in an interface system specially developed

£0:- t'1Js project bv Indiana University Institute for Research in Public Safety

personnel. The interface system interrogates the relay bv means of an electrical

signal and reports the relav state to t\\e computer in digital input form.

By combining the sensors in groups of four, it if; possible to 11]or;.itor tr:1ffi l'

in live <1Lrcctions and to ckt:crmine the spe",d ,md length Ol~ ",acn vch:ic 1,p ;'lassil1S

throug;l the loop ';c' t. Inasmuch a:; the events are U me relatl'd, traffic densi ties

as \';011 as information on trafftc flo\v composition lTlay also "e determined.

- 143

l()()p.~:, as hoth are necessary to c;llcu~ate ,he vf'hic]p's velocity. length. direct,<)',

headway and time of passage. PILOP senses these timps, and stores them in the

appropriate buffer in INSKEL COM~roN.

Program IROOI, an interrupt: core load, checks the INSKEL COMMON buffers for

each loop set and dumps any Iull buffers to the appropriate disk files. Two 25

vehicle buffers are allotted for each loop set, so that one may be storing data wh i.!

the other is being written to disk. If IROQi :;enses that a disk file is full, it

queues up PC002.

Program PCOOZ, a queued program, checks th., disk fi les and dumps the full

files to magnetic tape. The disk files are then reset, and the magnetic tape is

checked to see if it is almost full. In this fashion, the system monitors traffic

24 hours a day without operator intervention.

Many non-process operations may be conducted utilizing the collected data,

and other computer analysis functions may be performed while the ;;ystem is monitor Lng

traffic. A series of non-process programs have been developed to allow the users

to care for the collection <lcU.vj ty while engaged in other activity. Many of these

programs provide for the maintf'nan(~e of t}w comput"r-sc'l1sor syntem files and its

caLibratio;:. The ::ime-shil'-jllg cCljJability (,I" the :B'l I80!) ',";',X rr"\'ldes tor seeming ..

coincidental data collectlOlJ dr:d analys~,; <1('ll\f11).

Program :"COOJ establishes the disk file,·:, i.nit laJizes tiw tape and allows fo,

parameter entry. When the loop sets are calibrateCl (by radar), adjustments in

velocity and length are accomplished by means of modjfication in one of the conver';!'·:

factors within the program whi ch calcuI.3tpd these items from the four time0 provide j

by PILOT'.

Programs NC004, NC005, and NC006 allow the operator to turn the loop sets on

and off, dnd to enter the Data [nput (interface) time ,;ense base, day of "'-'eeK, \.-"fed i."

and special ccnditions. The traffic at ;1 givpn'~it(' P1°\J be' eX'ln:irkd 'm a real

time basis if desired.

- 144

Description of Loop Installation

At each site, four loop detectors are installed in the configuration shown in

Figure B-1. The cuts are 1/8 inch wide, and originally 2 inches deep for the loop

wires. The wire depth, and the loop dimensions have been subject to some minor

variance in order to optimize response. After the cutting operation, the wire

(19 strand, 14 gauge TIl) is placed in the cuts, making three turns per loop, and

all leads are brought out to the edge of the road. When the \"i re installation is

completed, the cuts are filled with a quick-settlng hydraulic cement.

At the edge of the roadway, each pair 0;: loop wires is spliced to a pre·-twistc·(j

shielded cable, which is buried up to a service pole.

The Service Pole

Mounted on each telephone pole is an equipment cabinet cUld an electric power

meter. The cables from the loops come up the pole in a piece of condui t and into

the equipment cabinet, where they tenninate on a U~rminal strip. On this strip the

loops are connected to the detector amplifiers and the detector output (relay

closure) is connected to the telephone lines, which in turn run to the computer

room of the Institute (FigureB- 2). IISV servi ce is provided to the pole by Public

Service of Indiana for the operation of the Vehicle Detectors.

The Vehicle Detector

"The PCA Multi-Pak Vehicle Detector was designed primarily as an intersection

traffic control device. Its solid state circuitry detects a phase shift in the

loop impedance whenever a metallic vehicle crosses the wire loops embedded in the

roadway. Since each loop at the site requires its own circuitry and relay, the

standard package contains a power supply and four detector modules, grouped together

in a 4-Pak configuration.

To adapt the Ve-Det to the special usages demanded by the research act Lvi ty,

several modifications were required. The original reli\\'s were replaced by fast,:r

mercury wetted relays, and at some sites, the tuning board gain has been increased

by changing two resistors. This may become necessary where the cuts are deep or

- 145 -

th,' r03dway has steel reinforcing. At some installation';, lhe loop induct:mce was

,,0 gn'.lt as to require the addition of external capacitance (0.012 mf, in paralle1

'wi th the loop) to achieve a tuning peak within the range of the Ve-Det' s variable

capacitance. Occasional tuning is required, as the loop sensitivity experiences

minor fluctuations with time and \-leather.

The Computer Interface Circuit

An IBM 1800 Computer with digital voltage input has two input condLtions:

Voltage at input

-1 to +30 volts

-6 to -30 volts

-1 to _. 6 volts

o

The Detector returns a -25 volts signal (sent to the si te via one rvlephcJI1'2 ",;Ln'

to the computer digital input point when no car is in the lOOp. When ~ vehicle

enters the loops, the relay change at the Detector cause" the digital input poi.TIt

to be connected to the computer ground (zero volts). With this information the

computer is able to compute vehicle speed, length, l1I.unber of vehi cles per hour, and

so forth.

The interface connections syster::l useri is of nomina] cost ($200) aQri replacE'S a

svstem DelW in general use, which requires mechanical relays and costs In excess

01' $2,500.

Sensor Sites: Phase I

Each sensor site :la,.; two loop "'iets" ... so called 1.·,eUJ\lH' .'1 :".,·or l,f c,,: ·in·

Inops is required in each lane to gather the nece~:oar:' ; .. t:,rmat; 1.1;L' , :l r ~

rlUlli tared at each sIte, and thIs configuration is narK'd :1 lelop "p.li. ~-", iJun" <

Phast: l, ,iixteen loop se ts were being monitored on Norti. and SO:lt':, StaLe kOUL~' j,',

I'll' ;j total of eight locations, i.e., two lanes of trave] at eight: spu:s. For

I'fdSOnS of programming, these sets are numbered from zero throl1g!l ;'i ftt'ell. ThE

addltional :3ites created for Phase II will be discnssec in :.he tOLlowin section.

146

Loop sets 0-7 are located on State Route 37 South, with sets zero through

three monitoring southbound traffi,', .Illd :;et', four through seven monitoring nortb--

hound t rat fi c. The ~;cquenc(' of l1umheri ng is such that ascending numbers move away

from Bloomington.

Pa~r. 0-4 is located in a 45 mph zone, on a blacktop section of Highway 37. It

is approximately 75 feet North of an intersection with a stop street. For southbound

traff.ic, there is a negative slope of -5.2%. Thie; is a ilJain artery leading into

BLoomington, and some rather high rush hour traffic may he experienced (rate of

900+/hour). Loop set zero monitors southbound traffic, and loop set four monitors

lhll"thbound traffic.

Pair 1-5 is located in a 55 mph zone, enough South of the speed change from

45 mph to 55 mph to be unaffected by the former speed limi to The si te is in the

middle of a short straight stretch which acts as the connector for two curves in a

general "s" configuration. The road is 22' blacktop, and the site is in a no passing

zone. Set one monitors southbound; five monitors northbound.

Pair 2-6 is located in a 55 mph zone, in a 22' blacktop section of Highway 37.

Traffic from the North has good visibility to the site, and its path is straight and

~ffectjvely level. 130' South of the site, traffic experiences a gradual curve to

Ihe right. Set two monitors southbound traffic; six monitors northbound traffic.

Pair 3-7, the southernmost site, is located just beyond the bottom of a long

hill, with good straight visibility in both directions. The road is 22' blacktop,

and the speed limit is 55 mph. Set three monitors southbound and set. seven monitors

northbound traffic.

Loop sets eight through fifteen were installed on State Route 37 North, with

sets of eight through eleven monitoring southbound traffic, and sets twelve through

fifteen monitoring northbound traffic.

Pair 8-12 is located in a 65 mph zone, approximately 0.2 miles North of a

flat curve with a speed limit of 55 mph. The geometry is [ouch that speed would be

- 147 -

Suuthhound Vt';.iclf.:; ,a\'t ~\L'en in ll. f:;S mpll zone tor (Y\lCf l~) m1,ii.·~.

iI,· rO,ld j s concrete, 24' wide. 't'l eight [1l0l1itnrs soutJILH,'Jnrl tr.1ffil., dnd set

twelvp monitors northbound traffic.

r'ili]" 9-13 is located at the nort~]ern end of tIl(' straight-away from Pair 8-12,

,,, ,) Lmg flat curve.

.~lbl~ ~it this site.

The full expanse of the South straight·-;rvJay is not fu1 IV

The speed l;.rr;~t on tbi,', r:oncrete, 2f,' wide "ortjon of the

rOJdwav is 65 mph. Seven h'..ll1dred fe,~t to th,:' l,uTth lie,; OJ ;;;;:;,11 bridge on the' saml'

curve. Set nine mllnitors southbound traffic; ~;E,t thirtt'~'n i!ionitorc; northbol111J

i'ai " 10-14 bon~ers the northern end ~ . .. uphill Jectlon (for

,j'1rt):bound traffic) which contain~; a p;lssing zone fur non:]l: iJu;ld traLii It lie,

mldway through a long curve which terminated in the hiLL secti"n just m('ntiunt'd.

;he road is 24' concrete, and the speed .Limit is 65 :nph.

bound traffic; set 14 monitors northbound traffic.

,'air 11-15 borders the northeDl end of a long level c,Lraight-away, and the

,c:outhern end nf 3 long level curve. The road 1S concri'te, and the ';peed limit is

65 mph.

traffic.

Set eleven monitors southbound traffic and set fifteen monitors l1ortl:bound

Fi i',uTes

~,rofiJes ,.L.''j('l curvature infornatioll as indic3led.

Phase 11

In late July, 1969, a meeting cf project officials, including th6c 'n)~ the

\3tiona1 Highway Safety Bureau, the' l-lesear,:h Triangle Institute, and the inc;tituCi lor

Research in Public Safety, reviewed the system's capahilit~ .,s an instrument of

traffLc data collection. At that ti.me, the system had a dc'cj()n'itrau'd capability of

"peed measurements to within +1 (J mph and length measurements t"wi tn, n +::.0 fee t.

148

Figure B-1

25'

-+i

!I

I

-rI6'

1

10' --1Top View of CutsIII Roadway

r- -+~4w:~--~~·_......,r--~----rII

Roadway II

Cuts II

~, :r '- - - -f--..J_ -+'r--, _...a.-~"lr")-~I--,tJ'---~

Telephone TrenchPole

....._---~".

I

Cross-sectional Viewof Roadway

c _

Concrete

-- EARTH ---

- 149 -

At the southernmost location wherethe wires cross the roadway, there1"Ust be ;l minimum of 18' of clear­~nce between the wires and the road.

\()TE:

/

! I

Figure B-2

IiII

] (]' min. ground

I\i

iIi

t

4' ~1 in. de p t h

clearance

3'

... -:;..........- ........ 1

.- -.. .".------.--- -- - -To Vehicle Detector Loops ...--=:.::-:: •.

GRADE ~

-----------------__- __-+H+l:-L----t--

(0) ~eter Trough and Meter.

:D 24" x 16" x 12" Equipment Cabinet,Supplied by IoU.

(1) Grounding Stake, or Butt Groundat Bottom of Pole May be Used.

- 150 -

S H 48

Figure B-3h I'l1WaV 37Sites un ,.., t;lte '" .Location of Sensor

~, r

RD

151

j;;j

Fi9urc B-4

Plan View of Roadway Section for Site O-~

,(-,~.

\\

\\

\

A­I\ (~

o_..-- 0

Ie'

;I

')~----~~II

01:::'1

SITE 0-4

(~TI~I 1\1011"/\ ~lJ

~--.--- r---r--~

.+- -- --- t

~. Ie.- 15 i~

D" 2' 30'

1494 2

IIIIi

I Ii

! IId~r,:'

o IIen I'" II I

I ,

I ,I 1

.' IS, I---- +'! Ii

[--0 ~ -/1 f j-- 4

1. -L 1. 1 r-;;;,,0f ~ I ",'''''':.:-ra 1_". .----Ii!

. - II'

- 152 -

NOT REPRODUCIBLE

Figure B-5

Plan View of Roadway Section for Site 1-5

Gn-:::-,

()

+--0

I) CJ()-,

___ -1 __

I-I

j!

I :_ ~_ _ 1 _ _ ~ .,._ ......-...+--..:.,

SITE 1-5

,- 1 -

I-, - ,

I- t

I

F"RAILROAD

OVE::. APt.. ~si- +_ ..... _---l-

A 19- 0 i..

D' 4

T ' 239 "

-= 4 75,0' ~----e.-:..:.:::__

E = 19,9"

- 153

Fiqure 8--6Plan Viel. of Roadway SectJ on for Sl te 2-(,

TO ,- ~

~~

if>W,

,1D3

\\

__ L._

r 0O'""(,RADE

'00N

+ ~jOT

- 1.0'1. TOGRADE. '')( A L~

GRADE

/

t. 8/'... "iD(>Nt LJ ,?~'

(P<~)UC~i <:J,() r

T -. 141 5

55 p....1f'H

- 154 -

Figure B-7Plan View of Roadway Section for Site 3-7

I

~ ­,

,)

o

.- ()

"

co.+ 0

"'

-..---I

SITE 3-7'

!

--1!

I

f~~f- ---

f- --- f·-

f--..----- -- -----

.-

1--- --

I~I

I

oo

'"

TO 2-G

4'"w

N

-~---3

- 155 -

Figure B--8Plan View of Roadway Section for Site 8-12

fa 21 - n

1I,

I-----+------ 6

+tTO 20-26

~

'l!

.-Iz ,

'2

!I

I--I

I-~

IIl.- 24'

t-- 12 '

iI

P, T FOR CURVE CONTt.:NIW;

20 - 26

lS6

N

t

Figure B-IoPlan View of Roadway SecUon for Site 10-14

TO 11-15

t6 = 29' - 4 7' l T

D=I'-24'

T = 1088.4'

l= 2127.4'

E = 1422'

-+-10-

C\i

tTO 9-13

SUPER[lE.VATiO~J

'OR I" C liRv£

- 14 -------r- +0.32 0'.I GRADE

I-/lilN£]

I

tTO PC,

24'

12'

N

t

- 157 -

N

\

Fiqure B-9Plan Vie'" of Road\oJay Sectiun for S1 cc Q-13

...\1° () t' () r

t_

ra 2') - 31

_ 158 -

Figure B-llPlan View of Roadway Section for Site 11-15

I' '2'1088 '0'

2140,]'

Pc.

----15 -~- +0,26·..

tTO 10-14

- 159 -

Phase 11 called for post-accident follow-up and the use of the system as the accident

data collection approach. To aid in this data collection and system evaluation,

the svstem was expanded. Six additional sensor sites were located on State Route

37 :Jurth, with the computer software being rewritten to monitor these new sites. The

installation and the computer software were completed November 4, 1969. These change;.'

<ire ,:i scussed below.

Six loop pairs were added; five were placed in between Loops Pairs 8-J2, ,mel

y- L\ and one (Set 20-26) was placl~d 0.2 miles South of 8-12. Set~, 20 through 2~,

are monitoring southbound traffic, and Sets 26-31 are mc;r;itu'Jl1g :lcrthbound traffic,

(See diagrams at the end of this section for spacing and locations of these new ~;ites.

The same basic computer hardware which monitored 16 loop sets was ut i lizl'J for

the l'xpanded system. The only addition came in the interfacing, witb four digit;j]

Input strips peing utilized, instead of two. The 16K Ll[ core did prove to be "ufdc,

after modification of the system software. Also, while the overall logi c remaincd

the same, in terrupt levels and core allocations were reduced to accomodate the load

levied on the system by the additional input. IBM-furnished TSX up-dates arrived

out of sequence, causing some delay until the system could be built up under Vers ior:

3, Modification level 7, but the expanded r,yc;tem came on-line c)n November 4, 1909.

In order that the 1800 could monitor the almost twice as many loops as before,

core work areas were preserved. Whereas in Phase I eacb loop set had two 50-veh Jele

in-rare huffers to store vehic1e data until It could he written to disk, in PhiL-I, TI

Pdch was reassigned to two 25-vehicle buffers. Addresses, previouslv stored in orp,

In Phase II were calculated by progrmn PILul', AJso, ,,,ithout requiring a n>wlrin~;

operation, all unnecessary interrupt levels were stripped of their in-core ,'Jork .'lrea'i,

.saving 100 words for each of the four deleted levels. The cetained -Interrupt levels

we re pared down to their absolute minimum necessary work areas. Some of these core­

saving measures resulted in longer execution times, but the basJ_c five millist'('(.1nd

interrupt was undisturbed. All fifty-six digital input points w(~re sri] 1 scam;Ld

every five milliseconds.

- 160 -

-

Figure B-12Location of Additional Sensor Sites on State Highway 37 North

rI

'0oN

I

N<I!<0

WHISNAND W)AD

ell

""-

'.D~

'" I

::J I~ iI\2 t23 :t . 29

III

IN<I!r"

f22 , 28

r,"t i<0

lJ~ t'" E1llLl' S ~Ol>D

1O...T

21 . 27

0

'"9

8 , 12 t<f)

"Q

20 ){ 26 +q:1O

~OLD HiGHWt.Y 37

ICASCADE:lDRIVE - IN - THEt.TRE

- 161

.....

Figure B-13

PIa. View of Roadway Sect'o. fOt

S't. 20-26

----"

N

t

TO .. 0.04G~4/)£

1]06'

2/'CU/:iV€

P.l.

TO 8-12

1

j

.\ ~ 18'13'

D • 0'-56c::-'-- r " ~64.2'

> I. ~ 1951.8', ~ le.4)'

- 164

Figure B-16Plan View of Roadway Section for Site 23-29

TO 24 - 30

t,M'IfIf)

- .14"

+TO 22-28

23--

- 165 -

--29

F'i STure B-15Plan View of Roadway Section for Site 22-28

---t- ---- -- -I

III

22 ----- 28

ro 2: ?7

- 166-

Figure B-18

Plan View of Roadway Section for Site 25-31

(', 41" 02 FIT

o 0" :,b'r 22'J7.3"

43'1'" 4'

E 0 415,82'

SUPERl c f VArlON

FOR I' CURVl

II

oo t ,20'1.N

TO ') -13

JT

"-~ -.14 .;.

tTO 24 - 30

I----]1--1 -0 ,0

II I

peL 125 -- --.r --I

I

- 167 -

Fiqure B-17Plan View of Roadway Section for Silt, 24--30

TO 2'='-31

•J..-T:n I'z

Ii

I

ll---- '_, rop Sl(-~I

30-14°.'0---- ~ ----- ;~4 -----

i

+TO 23 -;~9

--. I__ ,",'H:::' NA~I" ""\,~ ~",,---,_J.L-

I ()i __:tI

I

- 168 -

APPENDIX C

SPEED LIMIT PRACTICES

SURVEY QUESTIONNAIRES

S PEE 0 LIM I T

SGRV2Y

P RAe T 1 C E S

04-S69047

PLEASE CHECK THE ANSWER OR ANSWERS TO THE FOLLOWING QUESTIONSWHICH ARE APPROPRIATE TO YOUR STATE.

I. Which of the following applies to your state regardingspeed limits?

(1) The state statute sets numerical speed limitsbut makes no provision for the setting or changingof limits by other agencies or jurisdictions.

(2) The state statute does not set numerical speedlimits but makes provisloIls for the setting ofspeed limits by other agencies.

(3) The state statute sets numerical speed limits andmakes provisions for changing or setting of speedlimits by other agencies or jurisdictions.

II. To which of the following agencies is authority to set orchange speed limits delegated? (You may check more than one.;

(1) No delegation of authority(2) State Highway Commission(3) State Traffic Engineer(4) State Police, Highway Patrol, or other state

enforcement agency(5) Other state administrative agency(6) County or municipal administrative agencies(7) County or municipal law enforcement agencies(8) County or municipal traffic engineers(9) Others (specify:

(--------------------

III. If in your state numerical limits are set by statute andlocal jurisdictions are allowed to set or change limits,which of the following constraints must they follow? (Youmay check more than one.)

(1) Subj ect to no constraints(2) Must be above a specified minimum speed(3) Must not exceed a specified maximum speed(4) Must be reasonable and proper(5) Must be supported by engineering or traffic study(6) Others (specify: )

( )

- 171 -preceding ~age b\an~

IV. Using answer choice~'; (1:, (2), ,:3), (4), and (5) I WhIChimmediately follow th;::: questJO;l, in0.icate hew speed limLtsare set In your state at each of the locations (a through m)listed below. (You may use more than one answer choice.)

-----------_.__._----_._-~ ..

ANSWER CHOICES:

a.b.c.d.e.f.g.h.i.j .k.l.m.

(1) BY LEGISLATIVE OR ADMINI STRA'l'IVB DECISIONWITHOUT A TRAFFIC OR ENGINEERINC-; SURVEY

(2) BY TRAFFIC SURVEYS AND ENGINEERING METHODS(3) BY CITIZENS' PETITION(4) BY STATE OFFICIALS OR AGENCY REPRESENTING

THE STATE(5) BY LOCAL OFFICIALS OR AGENCY REPRESENTING

A LOCAL JURISDICTION

expres sw,ay S

rural two- and four-lane highwaysrural country roadsurban expresswaysresidential streetsbusiness districtsschool zonescurvesbridgesexits and cloverleafsconstruction zonesintersectionsOthers (specify:

(

V. If engineering or traffic studies are used in setting oraltering speed limits, check any of the following factors whichare included in the stu~.

(1 )( 2 )( 3 )(4 )(5 )( 6)(7 )(8 )(9 )

(10 )(11)(12 )(13)(14 )(15)(16)(17 )(18)(19)(20)(21)

85th percentilepaceaverage test run speeddesign speedmaximum comfortable speed on curvesball-bank indicatorspacing of intersections and drivewaysnumber of roadside businesses per mileslipperiness of pavementroughness of pavementpresence of transverse dips and bumpspresence and condition of shoulderspresence and width of medianaccident experience at that or similat locationstraffic volumeparking and loading of vehiclespercentage of commercial vehic1estraffic signals and other traffic controlspresence of pedestrianslength of zone and effect of adjacent zonesOthers (specify:

(

- 172 -

VI. Is the numerical spc,-:;Q limit tor trucks in your st<.1te differentfrom the speed limit for other motor vehicles?

(1) YES(2) NO

VII. Check any of the following reasons which explain the differenceor lack of difference between speed limits for trucks and othertraffic in your state.

(1) Safety factors (e.g., truck size or maneuverability)(2) Prevention of road wear(3) Influence of trucking or other industries(4) Facilitation of traffic flow(5) Others (specify:

(------_._-----------

VIII a. Which of the following devices and instruments are available toyou for measuring vehicle speed, traffic characteristics, androadway features?

(1)(2 )(3 )(4 )(5)

Radar (6)Vascar (7)Road sensors (8)Aircraft (9)Speed timer

Ball-bank indicatorComputerVehicle countersOthers (specify:

(-----------

b. List any of the above or any other devices which are presentlyunavailable to you but which you feel would aid you in your work.

IX a. After a speed limit is set or changed in your state, is anattempt made to determine the effect of the new limit?

(1) YES(2) NO

b. I f so, how?

X. Rank the following (1 = most important) according to what youthink is the function of speed limits in your state. DRAW ALINE THROUGH ANY OF THE FUNCTIONS SPEED LIMITS DO rtOT SERVE.

(1) To reduce accidents(2) To slow traffic down(3) To make traffic flow more uniformly(4) To increase street and road capacities(5) To make streets safer for pedestrians(6) To decrease wear on streets and highways(7) Others (specify:

(--------------------

- 173 -

S PEE D LIM I T P R A ~ TIC S S

04-S6JC47

PLEASE CHECK THE ANS~I/ER OR ANSh'ER;; TO THE FOLLOWING OUES'1'TONSWHICH ARE APPROPRIA~E T(5~·YOtTR-~·~:JlTRI SDl~(~TION~-·-···~~-'--·- _ _.__._-_ _--

I. Has the state delegated any of its authority to establish oralter speed limits in your jUTlsdiction?

(1) YES(2) NO

II. If so, what agency grants this authority?

(1) State legislature through statute(2) State legislature by other means(3) State administrative agency(4) Don't know

III. Which of the following agencies or persons have authority toset speed limits in your jurisdiction? (You may check morethan one.)

--_._----------

(1 )(2)(3)

( 4 )( 5 )( 6 )(7 )(8 )

State Highway CommissionState Traffic EngineerState PolicE~, Highway Patrol, or other stateenforcement agencyOther state administrative agencyCounty or municipal administrative agenciesCounty or municipal law enforcement agenciesCounty or municipal traffic engineersOthers {specify:

(

--------------

IV. Of the choices given in question III above, who actually setsmost of the speed limits in your jurisdiction? (Use morethan one only if necessary.)

V a. Are minimum speed limits used .in your- jurisdiction?

(1) YES(2) NO

b. If so, check the types of roads they are used on.

(I) express'i,]ays(2) rural two-lane roads(3) rural four-lane roads(4) urban expressways(5) Others (spec ify: .. __ .. __...._. . _

{

"--_.._-----.- 174 -

VI. Are engineering studIes useG to determine speed limits 1nyour jurisdiction when they are set or altered?

(1) YES(2) NO

VII. If engineering or traffic studies are used in settinq oraltering speed limits, cheek any of the following factorswhich are in~luded in the study.

(1) 85th percentile(2) pace(3) average test run speed(4) design speed(5) maximum comfortable speed on curves(6) ball-bank indicator(7) spacing of intersections and driveways(8) number of roadside businesses per mile(9) slipperiness of pavement

(10) roughness of pavement(11) presence of transverse dips and bumps(12) presence and condition of shoulders(13) presence and width of median(14) accident experience at that or similar locations(15) traffic volume(16) parking and loading of vehicles(17) percentage of commercial vehicles(18) traffic signals and other traffic controls(19) presence of pedestrians(20) length of zone and effect of adjacent zones(21) Others (specify:

(------------------

VIII. Using answer choices (1), (2), (3), (4), and (5), whichimmediately follow this question, indicate how speed limitsare set in your jurisdiction at each of the locations (athrough m) listed below. (You may use more than one choice.)

ANSWER CHOICES: (1) BY LEGISLATIVE OR ADMINISTRATIVE DECISIONWITHOUT A TRAFFIC OR ENGINEERING SURVEY

(2) BY TRAFFIC SUR~~YS AND ENGINEERING METHODS(3) BY CITIZENS' PETITION(4) BY STATE OFFICIALS OR AGENCY REPRESENTING

THE STATE(5) BY LOCAL OFFICIALS OR AGENCY REPRESENTING

A LOCAL JURISDICTION

a.b.c.d.e.f.g.h.~.

j .k.1.m.

expresswaysrural two- and four·-lane highwaysrural country roadsurban expresswaysresidential streetsbusiness districtsschool zonescurvesbridgesexits and cloverleafsconstruction zonesintersectionsOthers (specify:

- 175 --

IX a. Which of the following devices and instruments are availableto you for measuring vehicle speeds, traffIc characteristics,and roadway features?

(1)(2 )(3)(4)( 5)(6 )(7 )(8)(9 )

RadarVascarRoad sensorsAircraftSpeed timerBall-bank indicatorComputerVehicle count:erOthers (specify:

(----------)

-j

b. List any of the above or any other devices which are presentlyunavailable to you but which you feel would aid you in your work.

---- -------------

X a. After a speed limit is set or changed in your jurisdiction, l~

an attempt made to dE~termine the effect of the new limit?

(1) YES(2) NO

b. If so, how?

XI. Rank the following (1 = most important) according to what youthink is the function of speed limits in your jurisdiction.DRAW A LINE THROUGH ANY OF THE FUNCTIONS SPEED LIMITS DO NOTSERVE.

(1) To reduce accidents(2) To slow traffic down(3) To make traffic flow more uniformly(4) To increase street and road capacities(5) To make streets safer for pedestrians(6) To decrease wear on streets and highways(7) Others (specify:

(

- 176 -

APPENDIX D

DATA TABLES FOR

SPEED LIMIT SURVEY RESPONSES

TABLE I: TOTAL NUHBER OF STATE RESPONSES (48 STATES)

I. 1 ) 02) 33 ) 46

nLl) 0na 11' ) answer

II. 1) D2 ) 383) 84 ) 65) 36) 347 ) 18 ) 49 ) 13

na) 0

III. 1) 22 ) 83 ) 174) 65) 376 ) 13

na) 2

IV d. 1 ) 23 b. 1) 22 c. 1) 15 d. 1) 20 e. 1) 17 f. 1 ) 162) 42 2) 42 2 ) 34 2 ) 44 2) 29 2) 283) 0 3 ) 1 3) 0 3) 0 3) 1 3 ) 04) 18 4) 19 4 ) 11 4) 17 4) 10 4 ) 115) 5 5) 7 5) 18 5) 10 5) 29 5) 30

na) 0 na) 0 na) 1 na) 0 na) 1 na) 1

g. 1 ) 18 h. 1) 2 i- 1) 3 j. 1) 3 k. 1) 6 l. 1) J.2 ) 28 2) 40 ')' 35 2) 3:3 2) 32 2 ) 14~ )

3 ) L 3) 1 3) 0 3 ) I) 3 ) 0 3 ) 11

4) 10 4 ) 10 4 ) 9 4) 11 4) 14 4 ) 115) 20 5) 7 5) 8 5) 6 5) 9 5) 8

na) ] na) 5 na) 8 na) 6 na) 5 na) 9

m. 1) 12) 43 ) 04) 15) 1

na) 44

v. 1) 47 6) 37 11) 20 16) 22 21) 102 ) 30 7) :'9 12 ) 25 17) 9 na) 13 ) 23 8) 30 13) 18 18) 224 ) 31 9) 17 14 ) 39 19) 235) 20 10) 15 15) 27 20) 33

- 179 - Preceding page blank

• ,I

TABLE I cont'd.

VI. 1) 102 ) 17

na) ')u

cal 1

"JT I . ans,"vcrcd "I"on VI

..-

1) 20/ \ 2~ I

J) 04 ) 95) 10

na) 3cal 1

VIII a. 1) 48 b.2) 143 ) 84 ) 155 ) 46 ) 477) 188) 449) 4

n;\ ) 0

TV ,1 • 1) l f)"

2 ) 11na) 0cal 1

'" _iii _"._~IiI.

on VI

oo

102

o

J) I)

2) .23) ]4) 15) :26) 17) 38) 19) :2

na)

- 180 -

to

TABLE II

I . 1. 20

II. 2.22

III. 1. 72

IV. u.. 1.83b. 1. 89c. 1. 62d. 1. 89c. 1. 79f- lo 77g. 1. 60h. 1. 25i. 1.14j. 1. 20k. 1. 271. 1.16m. .14

V. 10.31

TOTAL STATE AVERAGE NUMBER UF RESPONSES

PER QUESTION

\111. for those who responded "1" in VIfor those who responded "2" in VI

1. 36.76

VIII. a.,b.

4.20.27

All averages are based on a denominator of 48,unlc~~

otherwise indicated.

- 181 -

TABLE III: 'roTAL NU]\1BER OF CITY RESPONSES (l13 CITIES)

I . 1 ) 902) 1 r- )

na) 1;

C;l,\ 5

T T 1 ) 7'j...L ,.....

;; ) ~)

] ) 114 ) 1

nn. ) 13Cd: 5

II I . 1) 46:2 ) 2 rj

3 ) 6it ) fJCj) 26 ) ",

~'~

7 ) ,19ti) 41

na) )

TV. 1) 102) 9J) 14 ) 0') ) 8G) 17) 498) 46

na) r-J

V .:1. 1 ) ':J7 b. 1) 312) 48 2 ) 2

na) 0 3 ) 4cal 5 4 ) 30

5) 15na) 48cal 5

V I. 1 ) JI02) 0

na) 2cal 1

- 182 -

·-r',:). !.;': all' , W,l

VII. 1 ) 105 6) 48 11 ) 4U J 6) ')] 2 J ) 1 "12) 45 7 ) 50 12) 35 17) 17 na) 13 ) 33 8) 44 13) ~n 18) 634 ) 56 9) 29 14) 76 19) 665 ) 46 10) 28 15 ) 57 20) 67

VIII. a. 1) 11 h. 1) :I2) 68 2) () )

c) ~

3) 0 1\ 2- I

4 ) 53 4 ) 125) 12 :':' ) 31

na) 16 na', 16

b. 1) 7 l. 1) G2) 60 ~' ) 723 ) 1

"

) ) ,-4) 37 ,1 ) 175) 23 S) 28

na) 33 '!\;:l 23

C. 1) 8 j . 1 , 4I I

2) 54 " ) 713 ) 1 3 ) ()

4 ) 20 4 ) 4CJ5) 29 '.' ) J1

na) 36 na) 13

d. 1) 9 k. 1) 132 ) 74 :) ) 62

n r; '1) ]

4) 49 ,1 ) 26C,\ 22 r~ ) 37- I

na) 10 11,1 ) 14

c. ] ) 31 1- 1. ) 102) 73 2 ) 683 ) 6 3 ) 14 ) 12 'f ) 165 ) 45

c' , 25._-~ ,;

na) 6 na) 2l.'

f. 1 ) 27 In. 1 )2) 81 2) 123 ) 1 3) 0

4) 17 /1 'i 45) 45

r' \ 5') }

ni'1) 5 nd) 95

g. 1 ) 352) 553 ) 24 ) 13:, ) 29

na) 10- 183 --

IX u. • 1 ) 112 b. } )2 ) 9 2 ) )

L.

3 ) 24 3 ) 101 ) 7 4 ) 45) 20 5) 16 ) ~) 8 6) 67) 14 7) 148) 91 8) J9) 14 q\ Q

.. I

na) 0 nd) 81

X a. 1 ) 882 ) 16

n21) 8cal

,.L

na == no ,lnswer

C',~ - CC)1-1tr~-1;~lictor-'C. _ s,,' v"

TABLE IV: TOTAL CITY AVERAGe NUMBER or RESPONSES

PER QUFc)TION

II. for those who responded "1" in I 1. 02

1 11 • 1. 76

IV. 1. 09

V.b. for those who responded Ii 1" in V.a.. 149

VII. 8.91

VIII.a. 1. 26b. 1.12c. .99d. 1. 35e. l. 47f. 1. :)0g. 1.18h. 1.14i- l.10j . 1.18k. 1. 201. 1. 06m. .14

I )~ . a. 3.08b. .41

All averages are based on a denominatorwise indicated.

- 185 -

tJ Tl 1C ~-~ ~; otl,,~r--

STATE AND CITY RANKHrC BY FREQUENCY OF SELECTION Of' FACTORS

IN sV AND eVIII

FACTOR._--------

roughness of pavcmcncprc:;cncc of trdn:;V0, L)C dLP~; dnu "),L,

presence und coneJition uf :~hc,uldc;

pr'~~,('n(~c: ,In']1icPh of rn,-(] '111

acciaent experience at that or similarlocations

traffic volumeparking and 1oac1in<; of vf:hicle:-;['c'rccntagc of con;mercj ,11 vc:hj (' i C'S

traffic sion,ds and other tr,lffic ('illlt"

presence of ?cdestrians1en0th of zones and effpct of adjacent

zonesot her~;

17

11'"

1 .- *, )

86

j,8

] 9

LO17

2

')

14*20*14*1 1 '"

,~

20*

CITY RANK

11217

71110

C)

121819141618

2

68

20543

21

I-i 85th percentile

paceavcraqc b-::;t run ";"c,-:,d(los icm speednaximum com for tid) 1 c :;1'(" ',l onl)iJ 11-bank j :ldjr'Ll1 ('> l"

~~,r)~1(~in(J of in t.f',:-::{1Ci.._-. '~)il.. ,:.lfl(l

III 1mlJor () f r () a (i ~:) i (1 c' ; I t I ~-~ i Ci t.~ S ~:; ( ~ ~ ~

~::. ]L I) [)(~ l~ i n.c; s:~ () f r) d '/(" rk.!- 11 1..

curve";

d :·.l"'l:1(~·.·,'(i·1

pc'r mil-,·

* two factors with the :C;dmc r<l.nk.

- 186 -

APPENDIX E

SELECTED SPEED DISTRIBUTIONS

FROi·l THE IRPS SENSOR SYSTEH

I.n INTPODUCTION

The purpose of this ~ppendix is to nlve the reader some

insight into traffic flow Lehavior. As described in Appendix

B, the computer sensor system records the speed, headway,

length, and direction of every vehicle passing over a sensor

loop. It is thus possible to obtain histograms showing the

entire population of vehicle speeds for a given time interval.

~ach histogram in this appendix gives the speeds of the entire

population of vehicles passing over one loop durinq one time

interval; the loop and the time interval are shown at the

top of the histogram.

The data are presented ln three groupings. Section 2.0

shows how the speed distributions vary with time at a given

sensor site. Section 3.0 demonstrates how the distributions

;il.rv with sensor site location over a single time period.

Curves showing various other effects of interest are included

_:_" section 4. O.

). 0 TEJ\1PORAL DISTRIBUTIONS

This section shows how speed distributions vary with

time, first hy time of day, then by day of week.

2.1 Distribution by time of day

The speed distributions of Figures £-1 through E-12

;how the speeds in a series of sequential bvo-hour time inter­

vals covering the day of November 6 I' 1969, at loop 5i te 10.

Loop 10 measures traffic ln the southbound lane of Indlana

- 189 -Preceding page blank

S t2tC I!lCfhway No. 37, north of Bloomington.

th(~ top of a lonq straiqht downhill scotton (for southbound

trdffic) 'tIhich contain:; a pa~;:;ing zone for northbound trdffic.

It lies 11I1eJ'lldy through a lonq curve which tenninates in the h

section just mentioned. The road is 24' concrete, and the> ,;p,

i.rni t is 65 mph G FiqureE-l

others are sequentially !'!umbC'rC:?d thereaft~er.

:\lovember 6, the road scLeface \las dry.

'1'11 rc) 1.1 ut

Tr1e effects of rll~~:h hOLl~- traffic ~;ho',,'\:'"uF;' c:: (~(J:L-~

also noteworthy that a L::LJ~crer percentaol: of' traffic i~; 1;J1

five mph of the mean speed at the high volume traffic conol

than at conditions of 10\',/ volume traffic.

Rain occurred throuqhout the day on November 11 1'i en: rc:::',

£-13 and E-14 taken on November 13 I in contrast to FiJf\lrcc;

E-5 and £-6 (the same time periods but on November 6) shu,,!

a distinct drop (Smph) in mean speed as well as a distinct

drun in volume (about 60 vehicles durinq this time period ., -

The speed distributions of Figures r:-15throuqh - 21

how the s~eeds vary 1n an Indiana winter week.

{.-lutions, aq~in taken from sensor ~~ite 10, S110W "Lnc sne rjc:

between 0930 and 1130, starting with Sunday, Noverrher ),

and con c 1 udin q \'li th Saturday, ~10\TCrlbF;rl''j, ") i.

during this week was \raried - the roac1~3 were ury !JCp :~unda~'

- 190 -

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mornlng until Monday niyllt. when rain was recorded. Tuesdav

and Wednesday, the roads were d~v; Thursday it rained all day.

About 1300 on Friday, Ute rain turned to snm·l,. and Saturday

night some snow was still present.

IT· ;fl\'Jay 37 is heavily travc,llc:,c], and the roads dry

rapidly once precipitation was stopped. Precipitotion dGe~

not seem to have: h2':c) Flue'l c:E f:< during c:his peri \j()te r

also that the volume oj: \.'c' lC.iC~: during t-Lj3 time pc'ciod

shows surprisingly little var ation from 541 on Sunday to 4'

on Friday.

3.0 SPATIAL DISTRIBUTIONS

Figures E-22 through E-34 demonstrate how speed distribu-

tions vary with location. The locations chosen are those in

the northbound lanes of Indiana Highway 27 and include loops

7, 6, 5, 4, 26, 12, 27, 28, 29, 30, 13, 14, and 15. Loop 7

is the southernITlost loop, and the rest are in their spatial

order north of loop 7. These 10005 are described in more

detail in Appendix B. The date chosen was November 6, 1969,

a dry day, between 1600 and 1800.

The volume increases noticably from loop 7 through loop 4,

as Bloomington is apprcY:Lc'ned fn)l1l the south, and decreases

from loop 26 through loop 15, as Bloomington is left by

vehicles going north. Loop 4 shows a distinct skew to the

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left, due in part to an intersection fifty feet from loop 5.

With the exception of loop 4, the distribution of traffic

speeds conforms remarkably well to a normal distribution.

4.0 ADDITIONAL EFFECTS

The computer sensor system enables a detailed examina­

tion of the effects of many factors on the speed distribu-

tions. The specific effects demonstrated in Figures E-35 through

E-40 are those of an intersection, some bridge repair, and a

fatal accident.

4.1 Effect of an Intersection

Figures E-35 and E-36 show the speed distributions on

February 26, 1970 for loop sites 7 and 8. These loops are

both located in a 45 mph zone, on a blacktop section of

Highway 37 immediately south of Bloomington, with loop 7

measuring the southbound traffic and loop 8, the northbound

traffic. This loop pair is approximately 50 feet north of an

intersection with a heavily travelled stop street. For

southbound traffic, there is a negative slope of -5.2o/~

This is a main artery leading into Bloomington, and some

rather high rush hour traffic may be experienced (900+/hour)

Both figures show a bimodal speed distribution, the lower

mode presumably corresponding to the speeds of those drivers

who are turning or are behind those who are turning, the upper

mode corresponding to the speeds of the other drivers.

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4.2 Effect of an Accident

Figures E-39 and E-40 demonstrate the effect of a fatal

accident on speed distributions. Figure E-39 shows the speed

distribution at loop 12 on March 20, 1970, a rather normal

Friday afternoon, for the time interval 1430-1530. At 1345 on

March 27, 1970 an automobile accident occurred five miles

north of this loop. One person was killed and nine injured,

and there was a considerable traffic jam along the road.

The effects of this accident are shown in Figure E-40. The

time covered in Figure E-40 includes the time when traffic

was stopped as well as that when the traffic was beginning

to move more normally. The traffic jam and the subsequent

speed increase both show up clearly.

4.3 Effect of Highway Maintenance

Figures E-37 and E-38 show the effects of a road ob­

struction - in this case painting on a bridge with a flagman

present. Figure E-37 shows the distribution of speeds of

southbound traffic immediately south of the bridge; figure

E-38 shows the distribution of speeds of northbound traffic

immediately south of the bridge.

As expected, the traffic corning off the bridge is going

slowly (the speed limit here is 65mph) and somewhat irregularly,

whereas that going northward shows two distinct peaks, one

peak presumably corresponding to those vehicles who were slowed

appreciably by the flagman, the other corresponding to those

who were waved on through without much hesitation.

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