Impact of 65-mph Speed Limit on Iowa’s Rural Interstate Highways: Integrated Bayesian Forecasting and Dynamic Modeling Approach

Abstract

In 1987, a provision of the Surface Transportation and Uniform Relocation Assistance Act allowed states to raise speed limits on rural Interstates to 65 mph (104.6 km/h). By 1995, nearly all states had done so. Several studies have investigated the safety impacts of these increases. Methodologies varied from analysis of variance to simple before and after comparison and have included time series procedures, with and without intervention variables. In general, these studies have produced contradictory findings. An integrated Bayesian forecasting and dynamic modeling approach used to determine the impact of the increased speed limit on rural Interstates of Iowa is reported. The approach is used to verify that raising the speed limit to 65 mph (104.6 km/h) led to an increase in fatal accidents on rural Interstates of Iowa. Comparison of fatal accident data on rural Interstates of Iowa and New Jersey yields similar results. Although this conclusion was anticipated, the study further demonstrates that a Bayesian/dynamic approach is more robust than the standard time series model.