Book Description

"The National Highway Traffic Safety Administration (NHTSA) reported that 37,461 people died in 2016 on US roads, marking the second year in a row that the number has increased [1]. Many transportation-related agencies have adopted a “Vision Zero” or “Towards Zero Deaths” campaign aiming to eliminate the need for that statistic. Speed can often play a role in fatalities, so setting the proper speed limit for roadways is important in saving lives. American speed limits have a complicated history over the past 50 years, which hinders researchers’ ability to look at much longer stretches of data and draw conclusions about determining the best speed limit strategy. From 1973 to 1987, the maximum speed limit was 55 MPH. After that, Congress raised the allowable limit to 65 MPH. It was not until 1995 that states had full discretion over setting an appropriate limit. Some states raised the speed limit for all vehicles equally – a uniform speed limit (USL). Others raised speeds for trucks less than that of cars – a differential speed limit (DSL). On the surface, the benefit of a DSL seems intuitive, lowering the speeds for big trucks would reduce the severity of accidents. However, it also has the effect of increasing total variance in speed which increases the number of accidents. This report aims to explore the literature published about these issues and to inform decision makers about the factors to be weighed when considering DSL versus USL. 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 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."--Introduction.