1. Variable Speed Limits Speed Management Workshop Dallas, TX
March 6, 2000
Davey Warren
Office of Safety R&D
Federal Highway Administration

2. This Presentation background objectives examples lessons learned
foreign
domestic
lessons learned

3. Part-time Speed Limits
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 minutes at the beginning and ending of the school day. Yet most of the activity takes place with a minute window.
1950
2000

4. I will draw heavily from my experience with the development of a prototype variable speed limits system in the late 1980’s and tested on a section of I-40 in Albuquerque, NM

5. First variable speed system in US Michigan (1960)
Lodge Freeway
21 speed signs over 3 mi
60-20 mi/h
video surveillance
gradually slow approaching backups

6. Variable Speed Limit Systems
traffic and speed detectors
environmental sensors
variable message signs
display safe speeds for different road conditions

7. Deviation from mean speed, mph
Objectives 2 4 6 8 10 20
-10
-20
Relative Risk
Deviation from mean speed, mph
OVERTAKINGS
ACCIDENT
INVOLVEMENT
Increased compliance
greater safety
efficient use of highway facilities
less burned justice system

8. European Examples
Netherlands
Germany
Finland
England

9. Germany fully automated rural autobahns
variable speed signs 1 mi spacing
km/h per Road Traffic Code
monitor traffic, speed, surface, wind, fog
20-30% reduction in accident rates

10. German Automated Speed Control Algorithm
*Unrestricted at very light traffic flow

11. Finland 15 mi rural freeway 120-100-80 km/h variable speed signs
1 mi spacing
both sides of road
hazard warning
monitor surface and weather conditions
unmanned
Over 60 variable speed signs ; fibre optic technology
Highest limit when dry, lowest speed when snow and ice on road
Visibility km/h below 280m; 80 km/h below180m
wind velocity 100 m/s 80
95% of drivers endorsed the used of variable speed limits
on average 80 percent thought that the prevailing speed limit was appropriate
Speeds were about 2 mi/h slower and an extra 1 mi/h when the slippery road warning was displayed.
There was a reduction in the standard deviation of speed
However B/C less than 1 at current volume levels
Issues- reliability of road surface information
If drivers come to rely on variable signs to warn of danger, when signs are blank, driver assume there are no problems. If sign fails to operate, driver will proceed unaware of the danger. Need security plan to ensure operation

12. M25 Motorway (UK) automated 70-60-50 mph fog,, , Ø
fibre optic technology
“failsafe”
The concept was to control traffic speeds in peak periods and thus smooth traffic flow. The goal was to reduce disruptive speed differentials and need for lane changing, reduce incidence of stop and go driving, reduce accidents and increase driver comfort.
The speed limits were automatically controlled based on traffic flow and speed and automated enforced.
If there is any fault in the system, even if it is not apparent to drivers, or there is any doubt about the speed limit set, then there will be no enforcement

13. M25 Enforcement System radar speed measurement automatic camera
rear of gantries
fixed signing
police control (civilian staff)
fine & penalty points
Variable
speed
limit
System consists of radar,flash, and 35mm wet film camera to record offending vehicles. (time, data, location, speed, speed limit and speed limit change) A removable memory card within the sign interface provides a record of the offence and operational data to assist in analysis.
Cameras are mounted on back of signs gantries and over specific lanes. They may be moved between housings and run on dummy mode, with flashing only when film runs out.
Two photos taken 0.5 sec apart. The distance traveled by the vehicle in this time is used as a secondary method of speed measurement.
System in Netherlands measures speed over 1 km.

14. Traffic Monitoring speed limits obeyed more even headways
more even lane use
less lane changing
more comfortable
minor improvements in throughput, travel times, safety, emission
Only influence shoulders of the peak
average speed is just below the limit
more even distribution of traffic across lanes
2/3 would like to see system extended
Only marginal improvements in throughput. Police records show initial large improvement but data distorted by poor before monitoring data. Other factors such as road works, different seasons, etc. confound short term evaluations.

15. US Examples
NJ Turnpike
Tennessee
Washington
New Mexico

16. New Jersey Turnpike 120 signs over 148 miles 65-30 mi/h
based on mean speed
north section automated
hazard warning signs
Installed in late 1960’s. Sign display technology changed over time.
120 sign over the 148 miles. Loops in the pavement used to collect speed and volume data. Addition of weather sensing equipment is planned in the near future.
The variable speed limits on the northern section near NYC are automated and based on the average speed of traffic. Manual override is used for lane closure and construction zones.
The speed limits on the other sections can be reduced manually for six reasons:
fog, ice, snow, construction, accidents, and congestion. There are hazard warning signs that display “Reduce Speed Ahead” and the reason for the speed restriction.

17. Tennessee (I-75) Fog warning Speed and visibility sensors
Microwave link to police center
Semi-automated
Photo courtesy of PB Farradyne

18. Washington (I-90) 17 mi over mountain pass
monitor speed, road surface and weather
65-35 mi/h during winter
hazard warning
semi-automated
Designed to improve safety by minimizing speed differentials within traffic stream during adverse winter weather and provide motorists with advanced warning of weather conditions, road conditions, traction device requirements, snowplow activity, avalanche control and road closures.
Six weather stations along the pass provide information on precipitation, pavement surface condition, visibility and wind speed. Two radar sensors(one each direction) are used to monitor speed at 15 traffic stations.
Speed limit displayed on fully configurable LED matrix sign that can also display 3 line hazard warning message.
Speed data updated every minute and weather data every 3 minutes. The recommended speed is based on a smooth average speed plus an adjustment for road surface and weather conditions. The recommended speed is transmitted to an maintenance operations center to be confirmed by an operator. If not rejected within one minute, recommendation will automatically be implemented. The decision to reduce the speed limit is also based on feedback from plow operators and State Patrol.
200 in-vehicle units for receiving the speed and hazard information.

19. Photo courtesy of PB Farradyne

20. New Mexico (I-40) fully automated maximum speed
minimum speed or hazard warning

21. Prototype test constrained by NMSL

22. Maximum and Minimum Limit

23. Downstream Hazard Warning

25. Roadside station downstream of each interchange

26. Central station (unmanned)
communication hub
permanent log of speed limits
monitor system status remotely
manual over-ride

27. New Mexico Automated Speed Control Algorithm

28. Smooth Mean Speed

29. System Performance

30. Lessons Sign location, spacing and visibility Power failure
Control algorithm
Smooth speed
Hysteresis
max & min
Environmental sensing
Low cost & fully autonomous

31. Questions Davey Warren FHWA HRDS-05 6300 Georgetown Pike
McLean, VA 