Variable Speed Limits

Making Work Zones Work Better Workshop Orlando, Florida 17 Sept. 03

Davey WarrenOffice of Safety ProgramsFederal Highway Administration

Presentation Overview

• What is a variable speed limit?

• How do variable speed limits work?

• Objectives of variable speed limit system

• Work Zone Tests

• Lessons learned

• Future products and additional information

What is a Variable Speed Limit (VSL)?

• Speed limit changes with changing conditions

19501950

Part-time Speed Limit Work Zone

Focus of presentation

• Modern systems change the speed limits in real time:• Traffic conditions

• Adverse weather conditions

• Road surface conditions

• Work Zones

19601960

Computer controlled limits

Variable Speed Limit System Components

• traffic and speed detectors

• variable speed signs

• microprocessor

• communication

• environmental sensors

• base station for recording speed limit changes

Why Use Variable Speed Limits?

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24TIME

Spe

ed,

mi/h

Objectives

• Increase compliance

• Improve safety

• More efficient use of highway

• Less burdened justice system

• Responsive to dynamic conditions

• Provide real time information

Variable Speed Limit Applications

• General VSL

• Winter Weather and Adverse Road Conditions

• Fog

• Congestion

• Work Zones

General VSL

Examples• NJ Turnpike

• I-40 New Mexico

• Germany

Characteristics

• Typically cover longer stretches of roadway

• Broad range of input criteria for speed limit decision (traffic speed, volume, crashes, congestion, construction, ice, snow, fog, etc.)

I-40 New Mexico

• fully automated

• maximum speed limit• constrained by NMSL

• minimum speed limit

• downstream hazard warning

• roadside station after each interchange

20

30

40

50

60

70

80

0 2 4 6 8 10 12 14 16 18 20 22 24

TIME

Sp

ee

d,

mi/h

Smooth speed Speed Limit Speed Limit With Histeresis

Average Speeds and Variable Limit Over 24 hr

N.M. Automated Speed Control Logic

Smooth Mean Speed +/-

Congestion

• Examples

Lodge Freeway, MI

M25, UK

Netherlands

• Characteristics• Cover stretches of congested roadways

• Speeds set to reflect traffic conditions

• Slow traffic approaching backups

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Winter Weather and Road Conditions

• ExamplesI-90, Washington

E18, Finland

I-40, Arizona

• Characteristics• Cover longer stretches of weather-susceptible

roadways

• Speeds set to reflect roadway/weather/visibility conditions

Fog

• ExamplesI-75, Tennessee

I-80, Nevada

F-6, Australia

A 16, Netherlands

• Characteristics• Typically deployed in areas

that experience highly variable, severe fog

• Speed and visibility sensors

VSL in Work Zones

• Dynamic conditions in work zones make them excellent candidates for VSL

• FHWA is sponsoring 3 field tests of VSL in work zones

• Michigan

• Maryland

• Virginia

Field Test Objectives

• Objectives:• Deploy practical variable speed limit

systems in work zones

• Evaluate the effectiveness of the systems on:

• Speed limit compliance• Credibility of speed limits• Improved Safety• Improved Traffic flow

Michigan’s VSL System

• Four deployments to date

• Where: • 19-mile work zone

• I-96 near Lansing

• ADT 29k – 53k

• (over 10% trucks)

• When: late May – August 2002

• Duration: Short – All less than 1 week

I-96 Michigan

• 6-7 variable speed limit trailers• RTMS Traffic detector

• Solar power

• Controller with RF communication

• LED speed display

• ½-1 mi spacing• line of sight communication

• 70 to 40 mi/h• Based on prevailing speed

• Max based on nature of road work

• Pagers used to transmit speed limit changes to police

Michigan’s Speed Control Logic

Profile 1 Workers Unprotected

2 Workers Present

3 No Work Activity

L.O. Threshold - 0% 50 60 70

H.O. Threshold – 70% 40 40 40

v < 40 40 40 40

40 ≤ v < 43 45 45 45

43 ≤ v < 48 50 50 50

48 ≤ v < 53 50 55 55

53 ≤ v < 58 50 60 60

58 ≤ v < 63 50 60 65

63 ≤ v < 68 50 60 70

v ≥ 68 50 60 70

Michigan’s VSL System

• Preliminary Results:

• Better compliance

• More uniform speeds during off-peak

• Travel times reduced

• Greater speed reduction at crossover

• RTMS accurate for volume but marginal for speed

• Trailers can be set-up within 10 minutes

Maryland’s Work Zone VSL

• VSL Trailers – 4

• Radar unit for speed

• Queue Detector Trailers – 2

• Variable Message Signs on Trailers – 2

• Base Unit – Central Control (CHIPS System Trailer) – 1

Maryland’s VSL Algorithms

• Logic:• User selectable percentile speed

• Percentile can vary depending on whether workers present or not

• User can specify times to switch percentile

• Speed limit will be updated no more frequently than every 2 minutes.

• 2 other candidate algorithms programmed• Tweener

• Oz

Maryland’s VSL System Status

• Status:

• Completed pre-deployment testing and accepted equipment

• First deployment will occur in the next month

• The second deployment has yet to be scheduled

Lessons Learned

• Engineering• Perform “off-site” testing on a real road

• Eliminate unnecessary fluctuations in limit

• May need to change as often as once a minute

• Need signs on both sides of road with 3 or more lanes

• Implementation• Make sure that all stakeholders are on board and working

towards common goals

• Don’t try to do everything all at once – start with something easy

• Work closely with the media; have only 1 or 2 PR people for the media to contact

• Be flexible (expect the unexpected)

Summary

• VSL being used around the world to help manage speed and improve safety

• VSL seems to be particularly appropriate for work zones

• Enforcement remains an issue

• More and better data needed on effectiveness

Additional Informationhttp://www.itsdocs.fhwa.dot.gov/itsweb/welcome.htm• Examples of Variable Speed Limit Applications (EDL#12164)

• Safety Applications of ITS in Rural Areas (EDL#13609 )

http://gulliver.trb.org/publications/nchrp/nchrp_lrd_47.pdf• NCHRP Legal Research Digest 47: Judicial Enforcement of Variable

Speed

http://safety.fhwa.dot.gov/fourthlevel/pdf/poynton1.pdf• Controlled Motorways: Variable Speed Limits on the M-25, UK

http://safety.fhwa.dot.gov/fourthlevel/ppt/Warren_Vsl.ppt• Variable Speed Limit slide show

Future Products• NCHRP 3-59 Variable Speed Limit Implementation Issues (2004)

• Field Test Evaluation Cross-Cutting Study Report (2004)

Questions

Davey Warren

FHWA HSA-20

400 7th Street, SW

Washington, DC 20590

202-366-4668

davey.warren@fhwa.dot.gov