Effective December 1987, the speed limit on rural limited-access highways in Michigan was raised from 55 mph to 65 mph. A multiple time-series design was used to examine effects of the raised limit on traffic crashes and crash casualties, comparing roads where the speed limit was raised with roads where the limit remained unchanged.

Results revealed significant increases in fatalities (28.4%), serious (A-level) injuries (38.8%), and moderate (B-level) injuries (24.0%) on roads where the speed limit was raised. No change in crash casualty figures was found for control road segments. There was no change in the number of traffic units involved in crashes on 65-mph roads suggesting the major influence of the higher speed limit is increased injury severity for crash-involved persons