1 Variable Speed Limits Speed Management Workshop Dallas, TX March 6, 2000Davey WarrenOffice of Safety R&DFederal Highway Administration
2 This Presentation background objectives examples lessons learned foreigndomesticlessons 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.19502000
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 Freeway21 speed signs over 3 mi60-20 mi/hvideo surveillancegradually slow approaching backups
6 Variable Speed Limit Systems traffic and speed detectorsenvironmental sensorsvariable message signsdisplay safe speeds for different road conditions
7 Deviation from mean speed, mph Objectives24681020-10-20Relative RiskDeviation from mean speed, mphOVERTAKINGSACCIDENTINVOLVEMENTIncreased compliancegreater safetyefficient use of highway facilitiesless burned justice system
8 European ExamplesNetherlandsGermanyFinlandEngland
9 Germany fully automated rural autobahns variable speed signs 1 mi spacingkm/h per Road Traffic Codemonitor traffic, speed, surface, wind, fog20-30% 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 spacingboth sides of roadhazard warningmonitor surface and weather conditionsunmannedOver 60 variable speed signs ; fibre optic technologyHighest limit when dry, lowest speed when snow and ice on roadVisibility km/h below 280m; 80 km/h below180mwind velocity 100 m/s 8095% of drivers endorsed the used of variable speed limitson average 80 percent thought that the prevailing speed limit was appropriateSpeeds 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 speedHowever B/C less than 1 at current volume levelsIssues- reliability of road surface informationIf 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 gantriesfixed signingpolice control (civilian staff)fine & penalty pointsVariablespeedlimitSystem 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 useless lane changingmore comfortableminor improvements in throughput, travel times, safety, emissionOnly influence shoulders of the peakaverage speed is just below the limitmore even distribution of traffic across lanes2/3 would like to see system extendedOnly 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 ExamplesNJ TurnpikeTennesseeWashingtonNew Mexico
16 New Jersey Turnpike 120 signs over 148 miles 65-30 mi/h based on mean speednorth section automatedhazard warning signsInstalled 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 centerSemi-automatedPhoto courtesy of PB Farradyne
18 Washington (I-90) 17 mi over mountain pass monitor speed, road surface and weather65-35 mi/h during winterhazard warningsemi-automatedDesigned 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.