NEVADA SPEED MANAGEMENT WORKSHOP

Davey L. WarrenOffice of Safety and Traffic Operations R&D

Federal Highway Administration

Reno, NV

April 4, 1996

Realistic Speed LimitsWhy & How

Speed limits and the law Speed limits, speed and accident risk Speed limits and driver acceptance Design speed vs posted speed Methods for setting speed limits Recommended approach Implementation & federal assistance

SPEEDLIMIT

85PERCENTILE

Speeding Problem

Rational for government regulation of speed Speed a factor in 10% of accidents & 30% of

fatal crashes Severity increases with speed Speed limits widely disobeyed Conflict between access, safety, and mobility What is a safe speed & who decides?

Why Do People Speed

Speed set too low Road design conducive to high speeds Perceived risk of accident or citation from

speeding is low (everyone is doing it) Fast is consistent with social values General conflict between mobility & safety

Basic Tenets of Law Laws protect the public by regulating

unreasonable or unsafe actions Actions of a reasonable person should be

legal Most people drive in a safe and reasonable

manner Law cannot be effectively enforced without

the public consent & voluntary compliance

Speed Limit Laws

BASIC SPEED RULE -Driver must travel at a speed that is reasonable and proper for conditions and not endanger others or crash.

SPEED ZONING -Gives highway agency or localities authority to post limits higher or lower based on an engineering study, and to set them to indicate the maximum reasonable and safe speed.

Speed Laws

Illegal to exceed Reasonable and safe

speed not relevant Easier to enforce Not fair unless speed

limit set for optimal conditions

Exceeding limit is evidence of traveling at unsafe speed

Recognizes no one speed is safe for all conditions

Post for prevailing conditions

Check against traps

PRIMA FACIE LIMIT ABSOLUTE LIMIT

Methods Used To Set Speed Limits

Arbitrary ....lack funds & personnel

Political Pressure ....based on complaints and reactions to accidents

Traffic engineering ....based on traffic and road characteristics...but

still somewhat subjective and arbitrarily applied

Current Practice

States Locals85th percentile speed 100 86Roadside development 85 77Accident experience 79 81Adjacent limits 71 4510 mph pace 67 34Roadway geometrics 67 57Average test run speed 40 50Pedestrian activity 40 50

SPEEDLIMIT

85PERCENTILE

Requirements of Speed Limits

Be fair Be related to risk Be credible and

accepted by drivers Enforce control over

unreasonable behavior

SPEEDLIMIT

85PERCENTILE

Speed and Accident Risk

Safest near mean speed of traffic

Roads with highest speeds are safest

Limit should be set at upper end of low risk band

0

2

4

6

8

10

12

ACCIDENTS

Freeway

Main Rural

Two Lane

OVERTAKINGS

0 5 10 15 20-5-10-15-20

Relative Risk

Deviation from mean speed, mi/h

Accident Risk in Built-up Areas

0

2

4

6

8

10

12

14

Percentile Speed

5 15 50 85 95 100

Speed of traffic best indicator of reasonable and safe speed

0

2

4

6

8

0 5 10 15 20-5-10-15-20

Relative Risk

Deviation from mean speed, mph

OVERTAKINGS

ACCIDENTINVOLVEMENT

SPEEDLIMIT

85PERCENTILE

Driver Acceptance 30% nationwide Range from 3-100% Worse with low limit Better in west/midwest Better w/prima facie Only 1 speed zone in

10 better than 50 percent compliance

Group Mean %

%Compliance

Most Limits Set Way Below Prevailing Speed

Prevailing speeds 8-12 mph above limit

Trucks 3 mph slower Speeds vary 4-8 mph

over a 24-hr day No difference between

weekday & weekend

0

10

20

30

40

50

60

25 30 35 40 45 50

Speed Limit, mi/h

85 %tile Average

Operating Speed

Factors Influencing Speeds

Number of access points along road

Commercial development

Severe curvature Level of accidents Adverse weather

Minimum Design Speed All geometric elements must meet or exceed Only when constrained by terrain or

development is minimum used Maximum safe speed for worst driver under

bad road conditions Reaction time of drunk drivers, stopping

with bald tires on slippery road, average comfort level of blind folded passengers in 1930 vintage car

Raising limit has little effect on prevailing speed of traffic

Lowering limit doesn’t slow traffic

Little or no change in behavior

-5

-4

-3

-2

-1

0

1

2

3

4

5 C

hang

e in

85t

h P

erce

ntile

Spe

ed, m

i/h

-15 & -20 -10 -5 +5 +10 & +15

Change in Posted Speed Limit, mi/h

AverageChange

MaximumIncrease

MaximumDecrease

Compliance before and after change

0

10

20

30

40

50

60

70

80

90

100

Per

cent

age

of D

river

Com

plia

nce

-15 & -20 -10 -5 +5 +10 & +15

Change in Posted Speed Limit, mi/h

93

82

68

25

15

26

36 39

55

66

Before After

Accidents Before & After

Per

cent

Cha

nge

in C

rash

es

-50

-40

-30

-20

-10

0

10

20

30

40

50

Lowering Speed Limit Raising Speed Limit

-10.4

-25.2

+6.9

-11.3

+27.5

+5.3

Upper Estimate

Lower Estimate

Best Estimate

Most speed limits set unreasonably low

Make technical violators out of large percent of motorists driving at safe speed

Current speed limits do not reflect accident risk Inconsistent with traffic law system Misallocates enforcement resources

Most motorists drive at a speed road and traffic permits regardless of posted speed Don’t automatically drive 5 mph over limit

Speed limits have lost their informational value Engineering of credible limits which are appropriate and enforceable the

key to managing risk

What We’ve Learned

Recommended Procedure

85PERCENTILE

SPEEDLIMIT

24hr free flow speed Round up 1/2 mile interval 500ft from jct. & curves Dry roads, typical traffic No other adjustments

Design Speed vs

Maximum Curve Speed

Source: FHWA-RD-89-226

Design Min. Tolerable Min. Impending Impending Rollover

Speed E Lateral Gs Radius Skid (wet) Skid (dry) Car

40 mi/h 0.06 0.15 509’ 63.3 mi/h 87.5 mi/h 98.1 mi/h

70 mi/h 0.06 0.10 2,083’ 120.7 mi/h 177.0 mi/h 198.4 mi/h

Car: f(dry) = 0.65

Special Zones

School zone - • Base on average speeds when children present

• Restrict to 15 -30 minutes periods when kids present

Work zones• Base on average speed during good conditions

• Use warning sign for hidden hazards & post advisory speed at average speed of traffic when hazard exits

• Vary limit based on speeds when work in progress

Residential Areas

Post at 85% rounded to nearest

If lower limit desired, must change the road environment

Road humps, traffic circles, etc.

Confine engineering measures to small area

General Approach Engineer rational speed limits Educate and inform public

Hazards of traveling too fast or too slow Limits engineered to be safe & reasonable

Strict enforcement and adjudication Eliminate tolerances & inform judiciary

Evaluate Effects on drivers, safety, police, courts

SPEEDLIMIT

85PERCENTILE

Key Benefits Respect for law and enforcement Effective allocation of resources Deterrent to excessive speed by clearly separating

occasional violator from majority Reduce conflicts caused by large variations in

travel speeds Improved road safety and better use of roads

SPEEDLIMIT

85PERCENTILE

Federal Fund$ Available

Engineering Education

Enforcement Evaluation

$ Mileage of roads to be rezoned by class Frequency and duration of speed studies Public information and outreach plans Enforcement and judicial cooperation Evaluation Estimated total cost and cost share

Next Step-Application to practice

Where to get more information

Greg NovakFederal Highway Administration705 N. Plaza Street, Suite 220Carson City, NV 89701 775.687.1204fax 775.687.3803Email: greg.novak@fhwa.dot.gov SPEED

LIMIT

85PERCENTILE