United States Department of Transportation
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Abstract:The variable speed limit (VSL) system is designed to display both the maximum and minimum speed boundaries based on measured traffic and environmental conditions. In addition, short driver information messages can be displayed to warn of down stream conditions. The VSL can be installed to operate independently (Isolated), in a series of stations (Linked), as a component of a larger management system (Integrated), and in the future as part of an "In Vehicle" component to provide all types of driver information (In Vehicle). This analysis compares the four types of VSL systems to the existing "fixed" speed limit system which can display only one speed for all conditions. The costs of the VSL equipment include its installation and maintenance, the costs of accidents include accident induced delay costs, and time costs include time to traverse the section. A number of assumptions are made relating to projected accident rates, traverse time costs, and drivers' responses to the system. The model, using Lotus 1-2-3, calculates the costs and benefits for the set of roadway, traffic, and weather conditions entered by the user. Four scenarios were run, one for each of the VSL types using actual site information. The benefit cost ratios computed were 37 to 1 for the isolated system, 22 to 1 for the linked system, 55 to 1 for the integrated system, and 53 to 1 for the in vehicle. These ratios are very sensitive to the assumptions made where actual data were not available.
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