Work Zone Mobility and Safety Program
Photo collage: temporary lane closure, road marking installation, cone with mounted warning light, and drum separated work zones.
Office of Operations 21st Century Operations Using 21st Century Technologies

Text from 'Variable Speed Limits' PowerPoint Presentation

Slide 1

Variable Speed Limits

Making Work Zones Work Better Workshop

Davey Warren

Office of Safety Programs

Federal Highway Administration

Slide 2

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

Logo: U.S. Department of Transportation, Federal Highway Administration

Slide 3

What is a Variable Speed Limit (VSL)?

  • Speed limit changes with changing conditions


Black and white photo from 1950 of a man in a snow storm changing a speed limit sign from 65 to 35 mph.

A recent photo of a school speed limit sign where motorists must obey the 25 mph speed limit when the lights are flashing.

Speaker Notes:

Variable speed limits are not new.

Over a half century ago on the Pennsylvania Turnpike, a revised speed limit of 35mi/h was superimposed on the usual 65 to mandate caution in inclement weather. Source: Smithsonian Vol. 21 No. 7The Pennsylvania Turnpike: a dream of a road.

Reduced speed limits in school zones during certain times of the day are another form of variable speed limits. In some cases flashing lights are used to indicate when the lower limit and in other the time is specified on the sign. A similar type flashing sign has been used in work zones.

These are what I refer to as poor boy variable speed limits.

One issue affecting the credibility of these part-time limits is the duration the reduced speed limit is in force. In some zones the reduced speed is in effect the entire school day. In others for 45-60 minutes at the beginning and ending of the school day. Yet most of the activity takes place with a 15-20 minute window.

Slide 4

Part-time Speed Limit Work Zone

Photos: At a part-time speed limit work zone, speed limit signs are placed on both sides of a street (as shown in the first photo) and are outlined in orange, as shown in the second photo of a part-time 55 mph speed limit sign.

Slide 5

Focus of presentation

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

Photo: Black and white photoof highway traffic in 1960.

Speaker Notes:

Lodge Freeway

21 speed signs over 3 mi

60-20 mi/h

video surveillance

gradually slow approaching backups

Slide 6

Computer controlled limits

Cartoon: This cartoon portrays an older couple in disbelief as they pass a computerized message sign that reads 'Speed Limit 50, Idiot Drivers Next 8 Mi'. The caption on the cartoon reads, "These new computerized message signs don't miss much, do they?".

Slide 7

Variable Speed Limit System Components

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

Photos: Various photos of a traffic and speed detector, environmental sensors, and a base station for recording speed limit changes.

Slide 8

Why Use Variable Speed Limits?

Line Graph: This line graph represents a speed profile from I-40 in Albuquerque, New Mexico in a given 24 hour period. The workzone speed limit is 45 mph, but the results show that the speed limit is too low for normal conditions and too high for adverse conditions. For the majority of the day, when conditions are normal, the average speed is considerably higher, approximately 10 to 20 mph above the 45 mph speed limit. However, midday and into evening when traffic increases and conditions become adverse, the average speed drops significantly lower than the speed limit. Between about 1 and 5 pm, the average speed drops to approximately 10-25 mph less than the speed limit.

Photo: A variable speed limit sign that can change the displayed speed limit according to conditions.

Speaker Notes:

Work zones are dynamic with highly variable speed and road conditions

Safe speed varies with road and traffic conditions

  • speed profile from I-40 in Albuquerque
  • 45 work zone speed limit sign in background not credible

Static speed limits are only good for the condition it was set for

  • Too low for normal conditions
  • Too high for adverse conditions

Slide 9


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

Slide 10

Variable Speed Limit Applications

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

Slide 11

General VSL


  • NJ Turnpike
  • I-40 New Mexico
  • Germany


  • 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.)

Photos: A photo of a toll booth in London where VSL signs are used. A photo of a VSL sign that reads, "SPEED LIMIT 70, MIN 55".

Slide 12

I-40 New Mexico

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

Photo: VSL sign on I-40 in New Mexico that reads, "SPEED LIMIT 50, SLOW AHEAD".

Slide 13

Average Speeds and Variable Limit Over 24 hr

Line Graph: The graph shows average collected speeds and the variable speed limit setting over a 24 hour period. The smooth average speeds are generally lower than the variable speed limit setting. The speed limit with Hysteresis shows the lagging value of the speed limit setting, which is also generally higher that average collected speeds during the 24 hour period.

Slide 14

N.M. Automated Speed Control Logic

Smooth Mean Speed +/-

empty cell Max (mi/h) Min (mi/h)
Dry Day + 6.5 - 6.0
Dry Dark + 5.0 -5.0
Wet Day + 2.5 N/A
Wet Dark + 0 N/A

Slide 15



  • Lodge Freeway, MI
  • M25, UK
  • Netherlands


  • Cover stretches of congested roadways
  • Speeds set to reflect traffic conditions
  • Slow traffic approaching backups

Photos: Black and white photo of highway traffic in 1960. Photo of highway in Europe that utilizes VSL signs.

Speaker Notes:

First variable speed system in US Lodge Freeway in Detroit Michigan (1960):

  • 21 speed signs over 3 mi
  • 60-20 mi/h
  • video surveillance
  • gradually slow approaching backups

Slide 16

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

Photo: VSL sign that reads,"SPEED LIMIT 45", on a snowy, mountainous highway.

Slide 17


  • 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

Photo: VSL sign that reads, "SPEED LIMIT 65".

Speaker Notes:

Microwave link to police center


Slide 18

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

Photo: VSL sign that reads, "SPEED LIMIT 45".

Slide 19

VSL in Work Zones

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

Photo: VSL sign that reads, "SPEED LIMIT 45".

Speaker Notes:

Dynamic conditions in work zones make them excellent candidates

FHWA sponsoring three field tests of VSL in work zones

  • Michigan
  • Maryland
  • Virginia (on hold due to Patent Infringement Law suit)

Slide 20

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

Slide 21

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

Map: Map of Lansing, Michigan and surrounding areas. Major highways include: 496, I-96, and I-69.

Speaker Notes:

ADT= 30-50,000 veh per day

Deployed in one direction only

One was 8- mile section; thin new wearing surface and minor bridge work

  • barrel separated workers
  • 1 of 2 lanes closed

Another 5 mi section was being totally reconstructed-lanes closed on one side and crossed over to other direction

  • day and night
  • jersey barriers separated traffic

A third section involved concrete patching

  • barrels separated workers
  • 1-2 of 3 lanes closed at times

Normal speed limit was 70; static work zone speed limit was 50

Slide 22

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

Photo: VSL sign that reads, "WORK ZONE SPEED LIMIT 50".

Speaker notes:

  • Distributed system
    • each trailer can determine speed limt
    • each trailer communicates with upstream and downstream trailer
    • last trailer serves as master and only one that can be monitored and controlled remotely
  • Vendor- International Road Dynamics (IRD) Marketed as Speed Ranger
  • Units basically display prevailing speed of vehicles passing downstream sign (first and last units display prevailing speed at that location)
  • Speed limits could range from 70 mph to 40 mph (limited for some deployments based on work activities)
  • Maximum 10 mph speed limit differential between signs
  • Spaced 0.5-1 mile apart which was closer than desired 1-2.5 mi because curves and overpasses block line of sight for RF communication
  • Had to put numbers on the back of VSL signs so police would know which speed limit applied to which trailer.
  • Recommend trailers be numbered and deployed in order that motorists see them

Slide 23

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 is less than 40 40 40 40
40 is less than or equal to v, v is less than 43 45 45 45
43 is less than or equal to v, v is less than 48 50 50 50
48 is less than or equal to v, v is less than 53 50 55 55
53 is less than or equal to v, v is less than 58 50 60 60
58 is less than or equal to v, v is less than 63 50 60 65
63 is less than or equal to v, v is less than 68 50 60 70
v is greater than or equal to 68 50 60 70
  • Units basically display prevailing speed of vehicles passing downstream sign (first and last units display prevailing speed at that location)

Average speed updated every 6 min; user selectable between 5 and 15.

  • Speed limits could range from 70 mph to 40 mph (limited for some deployments based on work activities)
  • Each trailer could have a different profile depending on nature of road work activity
  • Maximum 10 mph speed limit differential between signs

Slide 24

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

Speaker Notes:

  • Rapidly changing work zone made evaluation difficult
    • Only preliminary results are available
  • Displayed speed and operating speeds generally increased
  • Average speed generally below displayed speed for VSL but not static 50
  • More consistent speeds during non-peak periods, especially at night
  • RTMS radar generally 2-4 mi/h below speed monitoring equipment using road tubes

Slide 25

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

Photos: Various photos of Maryland work zone equipment including: variable message signs and radar units for speed.

Speaker Notes:

Sign mimics standard speed limit sign—super bright WHITE LED's

Based on user specified percentile speed based on work activity

Example: if work is taking place between 8:30 and 3:30, post at 50 percentile speed; when not working post at 70 percentile speed

Speed limit based on 5-10 minute samples updated no more frequently than every 2 minutes.

Maryland team also has 2 other candidate algorithms programmed- Tweener & Oz

Slide 26

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

Speaker Notes:

Tweener incorporates information from the downstream speed trailer in the logic.

Oz is intended to maximize headways and harmonize speeds during onset of congestion

Slide 27

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

Slide 28

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)

Slide 29


  • 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

Slide 30

Additional Information

  • Examples of Variable Speed Limit Applications (EDL#12164)
  • Safety Applications of ITS in Rural Areas (EDL#13609 )

  • NCHRP Legal Research Digest 47: Judicial Enforcement of Variable Speed

  • Controlled Motorways: Variable Speed Limits on the M-25, UK

  • Variable Speed Limit slide show

Future Products

  • NCHRP 3-59 Variable Speed Limit Implementation Issues (2004)
  • Field Test Evaluation Cross-Cutting Study Report (2004)

Speaker Notes:

NCHRP Project is focusing on enforcement issues. Will be testing Technology Assisted Speed Enforcement in work zone in conjunction with Penn Turnpike Authority Photo radar image to be transmitted to police vehicle station downstream who would then apprehend offender and issue citation.

In addition to cross-cutting report in 2004, will delivier two brochures:

  • One for decision makers: "Why should I use VSL in Work Zones?"
  • One for engineers: "How do I include VSL in Work Zones?"

Slide 31


Davey Warren


400 7th Street, SW

Washington, DC 20590


Office of Operations