Effects of the 65 mph speed limit on injury morbidity and mortality

Effective December 1987 and January 1988, the maximum speed limit on rural limited access highways in Michigan was raised from 55 mph to 65 mph. This study examined the effects of the raised limit on injury morbidity and mortality. A multiple time-series design was used, comparing roads where the speed limit was raised with roads where the limit remained unchanged. Data were collected on numbers and rates of automobile crashes, injuries, and deaths from January 1978 through December 1988. Time-series intervention analyses were conducted to estimate effects associated with the speed limit change while controlling for long-term trends, seasonal cycles, and other patterns. Statistical controls were also included for major factors known to influence crash and injury rates. Results revealed significant increases in casualties on roads where the speed limit was raised, including a 19.2% increase in fatalities, a 39.8% increase in serious injuries, and a 25.4% increase in moderate injuries. Fatalities also increased on 55 mph limited access freeways, suggesting that the 65 mph limit may have spillover effects on segments of freeways where the limit was not changed. No significant changes in fatalities or injuries were found on other types of roads. The increased convenience of reduced travel time with the higher speed limit is obtained at a significant cost in terms of injury morbidity and mortality.