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21st Century Operations Using 21st Century Technologies

Synthesis of Variable Speed Limit Signs


Variable speed limit (VSL) systems utilize information on traffic speed, occupancy, volume detection, weather, and road surface conditions to determine the appropriate speeds at which drivers should be traveling, given current roadway and traffic conditions. The use of VSL during less than ideal conditions, such as heavy traffic and adverse weather conditions, can improve safety by decreasing the risks associated with traveling at speeds that are higher than appropriate for the conditions and by reducing speed variance among vehicles. In addition, VSL can be used to dynamically manage speeds during planned (rush hour congestion) and unplanned (incidents) events. Used in conjunction with managed lanes and other active traffic management (ATM) strategies, VSL can respond to downstream congestion to eliminate or delay bottlenecks and mitigate the possibility of crashes.

VSL has been successfully implemented in Europe since the 1960s, with deployments in countries such as the Netherlands and Germany generating significant benefits. While the United States has deployed VSL systems for safety purposes over a few decades, during the past 15 years there has been a renewed interest in expanding VSL use among the States in order to achieve operational benefits.


The purpose of this synthesis report is to provide a comprehensive review of current practice on VSL operations, particularly experiences from deployments in the United States, and to identify successful and best practices from the following perspectives:

  • Planning and policy.
  • Design, deployment, and standards.
  • Operations and maintenance.
  • Outcomes.


The research team conducted a comprehensive literature review along with agency interviews to gather information on existing, deactivated and planned VSL systems. Literature reviewed included published research, policy, and operating documentation from departments of transportation (DOT) and cooperating law enforcement agencies, and public-facing outreach material, such as websites. Thirteen agencies were identified for interview; the research team established a list of questions and conducted the interviews via telephone.


Several VSL systems have been implemented successfully in the United States for congestion-based active traffic management, work zones, and weather. The research team identified the current state of the practice, lessons learned, and best practices from implementations to date. Key findings:

  • VSL planning processes should use the systems engineering process to clearly identify and communicate objectives, requirements, and anticipated costs/benefits.
  • VSL infrastructure requirements generally include changeable speed limit signs, weather/environmental sensors, traffic speed/volume sensors, and communications equipment to transmit data. Infrastructure repair and replacement is a considerable cost that should be recognized early. Durable signing is especially important to maintaining a functional system.
  • Selection of speed control algorithms greatly depends on the primary functional requirements. The success of VSL systems relies to a significant degree on driver compliance, and therefore it is essential that regulatory systems are consistently enforced. However, in real-world deployments, particularly those in the United States, many systems are still advisory or cannot be enforced as intended.
  • Dynamic speed limit setting control algorithms can be difficult to calibrate due to data quality or delays and driver behavior. Agencies should anticipate the need for periodic adjustment and enhancement of algorithms over time based on observed roadway and driver characteristics.
  • In most cases, VSL implementations can generate preferential system benefits in terms of traffic efficiency and safety. Because VSL systems have different deployment goals and corresponding system design, varying system benefits result. Speed homogenization projects usually use simple algorithms in response to real-time traffic, road, and other conditions (e.g., weather, work zone, incidents, visibility, etc.), and they usually focus more on safety. Multi-objective projects, mostly as a part of ATM systems, report positive effects on mobility, safety, and even environmental impacts.
  • State and local statutes and agency policies should ensure that a VSL system is enforceable if a regulatory speed limit is desired. It is also beneficial to begin meeting with law enforcement partners early to discuss concerns and processes for enforcing the VSL system, if enforcement is required.
  • When calculating the system's cost, maintenance, operations, staffing, evaluations, and end-of-life replacement costs must be considered.
  • Future deployments of VSL/speed harmonization could incorporate emerging connected/automated vehicle technologies; real-time collection, storage, processing, and decision-making using emerging big data sources will be necessary for the next generation of VSL/speed harmonization systems.
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