VARIABLE SPEED LIMIT SYSTEMS ON URBAN FREEWAYS : REVIEW AND RECOMMENDATIONS

Variable speed limits systems use dynamic message signs to post speed limits based on real-time traffic conditions. They are an alternative traffic control technique designed to reduce speed variance, reduce speed-related accidents, and encourage stable traffic flow by providing drivers advanced information about travel speeds. Experimental installations in Britain, Germany and the Netherlands have resulted in reduced speed variance, more uniform headways between vehicles and more efficient use of all lanes of the highway. Drivers in these countries are satisfied with the systems and recognize the benefits of more stable traffic flow. A review of installations in urban areas in these countries and the United States is used to make recommendations for future installations. Future installations should include a flexible display for regulatory or advisory speed messages and a dynamic message sign for supplementary messages. Although special signs have been designed for variable speed limit systems existing dynamic message signs may be used. The calculated speed for display should be based on shock wave prediction, average speed and environmental factors. This proposed system is expected to achieve significant reductions in speed variance, however, applications in the United States are not expected to be as successful with driver compliance as some European systems which maintain 98 percent compliance. Variable speed limit systems are recommended to be installed on sections of highway with an existing speed-related problem. Systems should only operate when traffic flow conditions merit additional control. The potential benefits of the proposed speed management system are evident in a cost-benefit analysis of a section of the Capital Beltway in Maryland with frequent speed-related accidents. The projected cost savings in the first year, from accident costs alone, is expected to be approximately 3.25 times the cost of installing the system