Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

This report provides guidance on the use of variable speed limit (VSL) systems in wet weather at locations where the operating speed exceeds the design speed and the stopping distance exceeds the available sight distance. The use of VSLs during inclement weather or other less than ideal conditions can improve safety by decreasing the risks associated with traveling at speeds that are higher than appropriate for the conditions. By using VSLs, agencies can take into account traffic volume, operating speeds, weather information, sight distance, and roadway surface condition when posting speed limits. This report provides guidelines for the design, installation, operation, maintenance, and enforcement of wet weather VSL systems. The guidelines presented in this report are intended for a broad range of audiences, from the transportation policy professionals who are considering whether or not their agency should use VSL systems to the engineers who are actually designing VSL systems for their jurisdictions. The information within this guidebook should be useful to anyone considering the implementation or development of a VSL system. This document also provides examples of the challenges, obstacles and issues that organizations have encountered when implementing VSL systems so that future implementers can develop practices that will increase their likelihood of success.