The 70-mph speed limit: Speed adaptation, spillover and surrogate measures of safety

On July 1, 2005, the speed limit on the rural interstates in Iowa was increased from 65-mph to 70-mph. This research first conducted a before and after study on the rural interstate and other facilities to study the effects on safety performance in Iowa due to this speed limit change. It explored the impact of the speed limit change on two effects known as the "speed adaptation" and "spillover effect." Research was also conducted on traffic citations issued on the rural interstate because citations may be a surrogate measure for highway safety. Finally, research was conducted on the recent increase in the retail price of gasoline and its effect on driver behavior. The rural interstates reported an increase in fatal crashes by 37.9 percent. No spillover effect in terms of crashes, speeds and volume were observed on other road types. Finally, no speed adaptation effect was observed in rural Iowa.