Evaluating the Impacts of Speed Limit Policy Alternatives

As of June 2014, Michigan is one of eight states with a differential speed limit in place on its rural freeways, which sets a maximum speed of 70 mph for passenger vehicles and 60 mph for trucks and buses. In select urban environments, these speed limits are both reduced to a uniform 55 mph limit. Legislation was introduced in early 2014 that proposed increases to these speed limits. In order to ascertain the potential impacts of these proposed speed limit increases, this research examined a broad range of traffic safety, operational, environmental, and economic data. This study involved a comprehensive state-of-the-art review of prior research on the relationships between traffic speed, safety, and crash risk. A survey was conducted of state agency practices with respect to speed limit establishment and another survey was conducted to obtain feedback on proposed changes from the trucking industry. National fatality data were collected and analyzed to ascertain the effects of speed limit policies on traffic fatalities, with specific emphasis on maximum speed limits and the effects of uniform vs. differential limits on urban and rural interstates. Further analyses were conducted at the disaggregate level to examine crash trends on Michigan freeways. These results were supplemented by the analysis of field speed data that were collected on freeways in Michigan, Indiana, and Ohio. Differences in mean speeds, 85th percentile speeds, and the standard deviation in speeds among passenger cars, trucks, and buses were examined with respect to the speed limit policies in place at each of 160 roadway locations. Based upon the results of the fatality and speed data analyses, a benefit-cost analysis of the proposed speed limit change was conducted in consideration of both road user and agency costs. Recommendations were presented to aid in decision-making related to speed limit policies.