Variable Speed Limits

1
Variable Speed Limits

• Making Work Zones Work Better Workshop
• Orlando, Florida 17 Sept. 03
• Davey Warren
• Office of Safety Programs
• Federal Highway Administration

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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

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What is a Variable Speed Limit (VSL)?

• Speed limit changes with changing conditions

1950
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Part-time Speed Limit Work Zone
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Focus of presentation

• Modern systems change the speed limits in real
  time
• Traffic conditions
• Adverse weather conditions
• Road surface conditions
• Work Zones

1960
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Computer controlled limits
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Variable Speed Limit System Components

• traffic and speed detectors
• variable speed signs
• microprocessor
• communication
• environmental sensors
• base station for recording speed limit changes

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Why Use Variable Speed Limits?
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Objectives

• Increase compliance
• Improve safety
• More efficient use of highway
• Less burdened justice system
• Responsive to dynamic conditions
• Provide real time information

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Variable Speed Limit Applications

• General VSL
• Winter Weather and Adverse Road Conditions
• Fog
• Congestion
• Work Zones

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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.)

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I-40 New Mexico

• fully automated
• maximum speed limit
• constrained by NMSL
• minimum speed limit
• downstream hazard warning
• roadside station after each interchange

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Average Speeds and Variable Limit Over 24 hr
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N.M. Automated Speed Control Logic
Smooth Mean Speed /-
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Congestion
1960

• 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

• Examples
• I-90, Washington
• E18, Finland
• I-40, Arizona
• Characteristics
• Cover longer stretches of weather-susceptible
  roadways
• Speeds set to reflect roadway/weather/visibility
  conditions

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Fog

• Examples
• I-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

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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

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VSL in Work Zones

• Examples
• Michigan
• Maryland
• Virginia (on hold)
• Characteristics
• Portable speed trailers
• Typically deployed in longer term work zones

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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

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Michigans 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

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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

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Michigans 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 lt 40 40 40 40
40 v lt 43 45 45 45
43 v lt 48 50 50 50
48 v lt 53 50 55 55
53 v lt 58 50 60 60
58 v lt 63 50 60 65
63 v lt 68 50 60 70
v 68 50 60 70
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Michigans 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

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Marylands 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

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Marylands 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

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Marylands 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

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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
• Dont 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)

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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

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Additional Information

• http//www.itsdocs.fhwa.dot.gov/itsweb/welcome.htm
  
• Examples of Variable Speed Limit Applications
  (EDL12164)
• Safety Applications of ITS in Rural Areas
  (EDL13609 )
• http//gulliver.trb.org/publications/nchrp/nchrp_l
  rd_47.pdf
• NCHRP Legal Research Digest 47 Judicial
  Enforcement of Variable Speed
• http//safety.fhwa.dot.gov/fourthlevel/pdf/poynton
  1.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)

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Questions

• Davey Warren
• FHWA HSA-20
• 400 7th Street, SW
• Washington, DC 20590
• 202-366-4668

davey.warren_at_fhwa.dot.gov