Measurement and Assessment of Driver Compliance with Variable Speed Limit Systems: Comparison of the United States and Germany

Active traffic management (ATM) systems are becoming increasingly important, particularly in urban areas aiming to combat recurrent and nonrecurrent congestion. ATM systems apply proactive strategies to improve safety and mobility, as well as to reduce emissions, noise, and fuel consumption. As one of the key elements in the ATM toolbox, variable speed limit (VSL) systems are being increasingly deployed around the United States. In the past, most VSL systems in Europe have included automated enforcement as part of the system, while systems in the United States have relied on traditional speed enforcement strategies. In the Portland, Oregon, metropolitan area, a new traffic- and weather-responsive VSL system has been deployed. It uses variable advisory speeds (VAS), allowing law enforcement to rely on the basic rule for enforcement. Thanks to the availability of high-resolution intelligent transportation systems data, it is possible to conduct post hoc analysis after VSL deployments. This paper focuses on driver compliance with displayed VAS messages. “Compliance” is defined as the difference between the displayed speed and the measured speed of traffic. In addition to analyzing more than 35 days at the Portland site, for comparison purposes, some additional analysis of a VSL site in Munich, Germany, is also included. The analysis of compliance for VSL–VAS deployments is useful for understanding the benefits of these important ATM strategies, assisting with targeting enforcement actions, and better understanding where and when to employ VSL–VAS systems.