Systematic Evaluation of a Variable Speed Limit System in St. Louis Missouri

This paper presents the systematic evaluation of Variable Speed Limit (VSL) system implemented on I-270 in St. Louis County. The system varies the speed limit in real time during periods of recurring and non-recurring congestion to prevent traffic from breaking down. The system was evaluated objectively and subjectively, and this paper describes the objective evaluation of the system. The objective evaluation was performed by comparing the traffic flow parameters during static speed limit (pre-VSL) with variable speed limit (post-VSL) by using five measures of performance: 1) speed, 2) volume, 3) occupancy, 4) travel time, and 5) congestion measures. Identification of congested segments and the time periods for the analyses were the initial steps for evaluating the system. Traffic data were provided by MoDOT collected using Remote Traffic Microwave Sensors. Average speed, volume and time occupancy profiles indicated that the VSL system change in traffic conditions including congestion. Preventing the highway from reaching its capacity and avoiding the traffic breakdown is one of the main objectives of VSL system initiation. It can be achieved by upstream drivers reducing speed, metering the upstream traffic and preventing traffic from breaking down. Congestion measures evaluation indicates change in Travel Time, Percentage of Congested Travel and reduction in duration, extent and intensity of congestion. The VSL system benefits were determined, which indicated that the system benefits will pay for the initial capital cost of the system over the years. This study can be used by Departments of Transportation for evaluation of active management systems.