Variable speed limit (VSL) systems utilize information on traffic speed, occupancy, and 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 traffic and 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 in traffic. 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.
Based on a comprehensive literature review along with agency interviews to gather information on existing, deactivated and planned VSL systems, this synthesis provides a comprehensive review of current practices on VSL operations, particularly experiences from deployments in the U.S., and to identify successful and best practices from the following perspectives: planning and policy, design, deployment, and standards, operations and maintenance, and outcomes.