Long-Term Speed Compliance and Safety Impacts of Rational Speed Limits

Abstract

In 2006, speeding-related crashes accounted for over 13,500 fatalities in the United States and many studies also indicate that speed and/or speed dispersion are a major cause of crashes on U.S. highways. This paper summarizes the results of a deployment of the rational speed limits (RSLs) approach on two limited access highways in Virginia. The RSL approach involves a coordinated campaign to post sound and credible speed limits, increase speed enforcement, and implement public information and education (PI&E). The goal of the RSL approach is to better regulate travel speeds, thereby improving the safety of the traveling public. While it is common to use enforcement and PI&E in attempt to improve speed limit compliance, there is not much information on whether improvements generated by these campaigns can be sustained following the end of the formal campaign. This paper presents the results of a four-year project that examined speeds, driver attitudes, and safety before, during, and after the RSL approach was implemented. Two limited access highways where speed limits were increased from 55 to 65 mph were examined. Public perception survey results showed that the new 65 mph speed limits were well supported with over 80% agreeing with the new speed limits. The speed analysis showed that the average speed increased by a statistically significant margin immediately after the speed limit was increased, but compliance with the new posted speed limit was also significantly higher than what was observed with the old, 55 mph limit. There were practically no significant changes in speed during the increased enforcement and PI&E campaign or when speeds were examined one year after the end of the campaign. In addition, the standard deviation of speeds was fairly consistent throughout the before and after periods, which suggests that crash likelihood was not increased due to the increased posted speed limit. In fact, crash data showed that safety at the two demonstration sites was actually better than anticipated based on trends at similar comparison sites where speed limits were increased but no additional enforcement or PI&E were present. The results of the study imply that compliance and safety improvements can be sustained for at least a year after a coordinated enforcement and education campaign is completed, provided that speed limits are set appropriately.