Motor vehicle crashes are a serious social problem in the United States. Each year a large number of motor vehicle crashes occur and many people are killed or injured, resulting in substantial economic costs. To minimize economic costs, it is necessary to determine optimal speed limits on roadways because of the strong relationship among posted speed limit, crash frequency, and crash injury severity.

A comprehensive literature review about the relationship among posted speed limit, crash frequency, and crash injury severity level was conducted. Crash frequency prediction models and crash injury severity models are developed to obtain crash frequency and injury severity of victims in motor vehicle crashes at different posted speed limits. Model tests were also performed to verify the model fitness of data. Crash costs were then calculated based on crash frequency, injury severity level, and unit cost of each severity level. In addition, CORSIM simulation was used under various posted speed limits to obtain parameters related to operational cost. Total cost curves were then built to show the relationship between posted speed limit and total economic cost.

Using the developed crash frequency models, injury severity models and CORSIM simulation results, case studies were conducted to determine optimal speed limits on selected roadways. The results determined optimal speed limits on specific roadways on the basis of total cost.