As speed limits are currently set for the maximum safe speed on roadways, variable speed limits (VSLs) use speed limit signs that change to slow drivers to avoid creating heavy traffic congestion. Sensors along the roadway detect when congestion or weather conditions are dangerous. They then automatically reduce the speed limit (in at least 5-mph increments) to slow traffic evenly to allow smooth traffic flow, delay the onset of congestion, and avoid stop-and-go conditions. VSLs are also called speed harmonization and dynamic speed limits.
Contrary to what one may think, slowing traffic down in heavy traffic actually increases the number of cars that can travel on a road. The slower speeds make drivers more comfortable driving closer together and keep stop-and-go conditions to a minimum.
Depending on the purpose of the system, speed limits can be mandatory or suggested. Changeable message signs can also be used with this system to give drivers travel-time information or other details.
The ideal system monitors data from roadway sensors and automatically adjusts speed limits when congestion reaches a certain amount. The speed limit is lowered to keep an even traffic flow at a speed that is reasonable with the traffic volume. This delays the start of stop-and-go conditions and reduces the number of rear-end collisions.
Two common purposes for using VSLs are for bad weather conditions and congestion management:
- Weather-related VSLs are used on roads where fog, ice, rain, or other factors often affect safety. When weather conditions get close to being unsafe, the operating agency reduces the speed limit to lower the chance of collisions.
- Congestion-related VSLs are used when traffic builds and congestion is likely. When traffic and/or speed exceeds a certain amount, the strategy is deployed. The intent is to handle more traffic volume at a slower—but not stop-and-go—speed.
In both cases, the speed limit decrease is intended to alert drivers of conditions up ahead. Ideally, the speed limit and message alerts are automated and do not require intervention from an operator. VSLs are commonly used with automated traffic warning and lane control signs. They are also regularly combined with temporary shoulder use.
- Freeways or roads with frequent congestion.
- Areas prone to bad weather.
How Will This Help?
- Improves safety by slowing vehicles and reducing crashes due to bad weather and congestion. Furthermore, the lower speeds help reduce the severity of accidents that might occur.
- Improves trip-time reliability by delaying congestion and allowing traffic to flow smoothly and efficiently. Slowing traffic down can actually increase the number of cars that can be handled.
- Benefits the environment by decreasing emissions, noise, and fuel consumption.
Sydney, Australia, implemented a fog warning system that includes suggested speeds. During fog, the suggested speed limit is constantly adjusted to the speed of the previous vehicle (but no higher than the speed limit).1
In Denmark, studies found the following.
The VSL signs also reduced speed variance.2
The Netherlands also uses VSLs for weather conditions. Visibility sensors measure the level of fog, and when visibility drops to 459 or 230 ft (140 or 70 m), the speed limit is dropped to 50 or 37 mph (80 or 60 km/h), respectively. After implementation of the VSL during fog conditions, drivers reduced their speed by 5.0–6.2 mph (8–10 km/h).3
The Netherlands is examining speed limits through a dynamic speed limit project. The project was developed “to gain more insight into the impact of variable, tailor-made speed limits.”8 A pamphlet on VSLs provides the following details on the benefits:
Adjusting speed limits to unexpected and varying situations such as weather conditions, congestion, or an incident can improve traffic flow. It will also enhance the safety and improve local air quality. By applying a flexible and wide range of speed limits (50 to 120 km/h [31 to 75 mph]) operators can influence the actual situation on the road. Thus road users can drive faster if possible and slow down if necessary.8
An evaluation of permanent speed reduction showed excellent compliance (approximately 1 percent violators) due to enforcement. Reductions in emissions and noise level were also noted. Residents along the routes expressed thanks for the change.
The evaluation of VSLs in the Netherlands is currently ongoing. Researchers are exploring how to use data from traffic-monitoring equipment to assist in detecting shockwaves and then to manage upstream speeds to limit the impact of the shockwave. A system called Speed Controlling Algorithm Using Shockwave Theory (Specialist) is being developed. Weather forecasts (e.g., a major rainstorm) are also considered when setting the VSLs.9
Using VSLs during heavy traffic, the United Kingdom saw:
- A decrease in emissions of 2 to 8 percent.
- A noise reduction of close to 7 decibels.
- 20 percent fewer property-damage-only crashes.
- 10 percent fewer injury crashes.
- An improvement in travel time reliability.
- A smoother flow.
- More even distribution of vehicles to travel lanes.
- A calmer driving experience.4,5
In England, the Managed Motorways concept was deployed on the M42 roadway. The figure below shows compliance with the required VSL.10 The graph starts with data from January 2006 and ends with data from May 2009. In late 2007 to early 2009, the required speed limit was set at 50 mph when hard shoulder running was allowed. During this time, compliance was not as good as when the speed limit was set to 60 mph during hard shoulder running (started in early 2009). Drivers did not see the reason for the lower speed and thus drove at the higher speeds. This behavior is also shown in the decline in compliance when the speed limit was set to 40 mph.
On roads with VSLs in Germany:
- The travel times decreased by 5 to 15 percent.
- The number of crashes decreased by 30 percent.
- The maximum volume handled by the road increased by 5 percent.1,6
In Denmark, VSLs resulted in decreased speeds of less than 5 km/h and reduced speed variance. A survey showed that 46 percent of Danish travelers felt safer after the changes.7
The Texas Department of Transportation (TxDOT), with the assistance of the Texas A&M Transportation Institute (TTI), currently operates a VSL pilot program. This program has VSL pilot studies in three locations (the Brownwood, Waco, and San Antonio Districts) to gauge the impacts and potential benefits of VSLs in two instances:
- Under weather conditions within an active work zone.
- Under congestion conditions.
TTI is analyzing the impacts of the VSL on travel speeds, safety, users’ perceptions, and costs and benefits. Researchers provided TxDOT with a report that was made available to the Texas Legislature in December 2014.
Application Techniques and Principles
General conditions for VSL systems include:
- Large amounts of peak-hour congestion on a freeway.
- Available right of way for overhead signs and frequent changeable message signs.
- Significant incidents related to lines of traffic or merging.
- An automated deployment system.
- Strong communication with the traffic management center and support from it.
The following are key factors to consider for successful deployment:
- The success of VSLs is closely linked to driver compliance. Thus, agencies must inform the public of new measures and regulations as they are put in place.
- VSLs should be used in response to an actual situation. If users do not believe the system is needed, compliance will be low. If the reason for the new speed limit is not obvious, it should be explained through signage.
- Speed limit signs have to be visible to all vehicles. The signs should be placed on scaffolds over every lane of traffic. Changeable message signs should be placed regularly to either explain the lower speed limit or warn about extraordinary events.
This strategy has been successful in Europe but is new to the United States. Public acceptance and understanding of the system are crucial to its success. Drivers must understand why the speed limit is being reduced and that the reason is valid. All drivers must understand whether the new speed limit is suggested or required. A system that automatically changes the speed display ensures that changes take place before stop-and-go traffic begins to occur. Also, the speed limit signs have to be visible to all vehicles.
Who Is Responsible?
The local TxDOT office and/or the regional mobility authority bears the responsibility of developing and maintaining VSLs. These agencies should:
- Determine the feasibility of and need for the strategy.
- Confirm the availability of needed right of way so that signs can be installed at regular intervals for best visibility.
- Provide the adequate infrastructure for the traffic management center and other devices such as signs and sensor.
Project Time Frame
The lengths of VSL projects differ based on the available infrastructure and amount of planning prior to starting. The systems should have ample connections to the local traffic center. Other supporting infrastructure and policies should also be in place. Since some additional signage will be required, a typical VSL deployment may take between one and two years to begin.
The cost of installing VSLs on a roadway varies depending on the existing infrastructure and the selection and spacing of overhead signs, changeable message signs, and other related signage.
The following standard traffic information helps determine the need for a VSL system:
- Traffic volumes.
- Travel speeds.
- Climate and weather conditions for the area.
- Incident presence and location.
Variable Speed Limits Best Practices
- Type of location: Freeways.
- Agency practices: Strong program support from administrators and policy makers.
- Frequency of reanalysis: Annually or after substantial changes in traffic demand or road capacity due to construction.
- Supporting policies or actions needed: Possible changes to ordinances and laws.
- Complementary strategies: Temporary shoulder use and queue warning.
For More Information
Jones, J. C., M. C. Knopp, K. Fitzpatrick, M. A. Doctor, C. E. Howard, G. M. Laragan, J. A. Rosenow, B. A. Struve, B. A. Thrasher, and E. G. Young. Freeway Geometric Design for Active Traffic Management in Europe. Report FHWA-PL-11-004, Federal Highway Administration, U.S. Department of Transportation, 2011. http://international.fhwa.dot.gov/pubs/pl11004/pl11004.pdf.
Mirshahi, M., J. Obenberger, C. Fuhs, C. Howard, R. Krammes, B. Kuhn, R. Mayhew, M. Moore, K. Sahebjam, C. Stone, and J. Yung. Active Traffic Management: The Next Step in Congestion Management. Report FHWA-PL-07-012, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C., 2007. http://international.fhwa.dot.gov/pubs/pl07012/atm_eu07.pdf.
- J. Coleman, J. Paniati, R. Cotton, R. Covey, D. Graham, J. McCauley, G. Mortford, J. Paniati, M. Parker, H. Pena, M. Robinson, and W. Taylor. FHWA Study Tour for Speed Management and Enforcement Technology. Report FHWA-PL-96-006, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C., 1995.
- P. Rämä. Effects of Weather-Controlled Variable Message Signing on Driver Behavior. VTT Publications No. 447, Technical Research Centre of Finland, VTT Building and Transport, Helsinki, Finland, 2001.
- B. Kuhn. “PCM International Scan Tour—Netherlands.” Unpublished Personal Notes, Rotterdam, Netherlands, June 2006.
- B. Kuhn. “PCM International Scan Tour—England.” Unpublished Personal Notes, London, England, June 2006.
- Highways Agency, Department for Transport. M25 Controlled Motorways Summary Report, Issue 1. London, England, 2004.
- B. Kuhn. “PCM International Scan Tour—Germany.” Unpublished Personal Notes, Frankfurt, Germany, June 2006.
- B. Kuhn. “PCM International Scan Tour—Denmark.” Unpublished Personal Notes, Copenhagen, Denmark, June 2006.
- Rijkswaterstaat. “Dynamic Speed Limits.” Pamphlet, Undated.
- J. C. Jones, M. C. Knopp, K. Fitzpatrick, M. A. Doctor, C. E. Howard, G. M. Laragan, J. A. Rosenow, B. A. Struve, B. A. Thrasher, and E. G. Young. Freeway Geometric Design for Active Traffic Management in Europe. Report FHWA-PL-11-004, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C., 2011. http://international.fhwa.dot.gov/pubs/pl11004/pl11004.pdf.
- N. Hopcraft. “Managed Motorways Overview and Introduction.” Presentation to FGD Scan Team, June 10, 2010.