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.
PURPOSE OF REPORT
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:
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:
United States Department of Transportation - Federal Highway Administration