Evaluating the Impact of the 65 mph Maximum Speed Limit on Iowa Rural Interstates

Abstract

In response to a change in federal policy, the state of Iowa raised the maximum speed limit on its rural interstate highway system to 65 miles per hour (mph). Higher speeds are generally thought to produce economic benefits, largely due to reductions in travel time. However, higher speeds are also associated with increased accident risk. Our study examines whether a significant change in fatal and major-injury accident rates can be detected following the implementation of the higher speed limit. Our analysis attributes a 20% increase in the number of state-wide fatal accidents to the speed limit change. The impact is largest on rural interstates where the number of fatal accidents increased by 57%, implying two additional fatal accidents each quarter. Our analysis fails to find an impact on the number of major-injury accidents.

Key Words:

• Intervention models
• Time series analysis
• Traffic safety
• Transportation statistics.