Literature Review of the Implications of Differential Speed Limit Implementation
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2017-12-01
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TRIS Online Accession Number:01609742
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NTL Classification:NTL-OPERATIONS AND TRAFFIC CONTROLS-OPERATIONS AND TRAFFIC CONTROLS
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Abstract:This report addresses the matter of differential and uniform speed limits and their application at the state level. It was prepared in response to a request in the 2017 session of the Louisiana House of Representatives where the Louisiana Department of Transportation and Development (DOTD) was asked to “study the safety and operational impacts of differential speed limits on interstate highways” (House Concurrent Resolution 112). Thus, the purpose of this study is to inform the Louisiana Legislature about the documented various benefits and costs of a differential speed limit for passenger cars versus larger vehicles specifically with regards to the topics of Safety, Mobility and Operations, Fuel Consumption and Emissions, and Truck Determination as outlined in HCR 112. The majority of research into this topic has not been able to conclusively determine which speed limit system is safer, a uniform speed limit (USL) or a differential speed limit (DSL). There is uncertainty amongst the literature as to which is better, USL or DSL. The uncertainty is due to several factors that work against each other. Changes in observed speed are usually less than the changes in the posted speed, which dulls the impact of a DSL. If the posted speed limit is too low, many drivers will ignore it leading to low compliance rates. A slower speed reduces the severity of crashes; however, a greater speed variance increases the opportunity for crashes. Almost 75% of the nation’s truck fleet cannot travel above a preset speed because they have speed limiters built in. In other countries, the limiter is mandatory, but no study has attempted to quantify the safety. In America, the current trend has states moving toward a USL. Passenger car drivers tend to prefer a DSL, but less strongly than truck drivers prefer a USL. The time and cost to fully implement a speed limit regime change can be substantial. With the lack of strong evidence in the existing published research in either direction, this paper cannot conclude which method is better than the other. Further research (on a much larger scale than this literature review) would be necessary to conclude which speed limit arrangement is better.
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