University of California Transportation Center

In 1987, most states raised the speed limit from 55 to 65 mph on portions of their rural interstate highways. There was intense debate about the increase, and numerous evaluations were conducted afterwards. These evaluations share a common problem: they only measure the local effects of the change. But the change must be judged by its system-wide effects. In particular, the new 65 mph limit allowed the state highway patrols to shift their resources from speed enforcement on the interstates to other safety activities and other highways -- a shift many highway patrol chiefs had argued for. If the chiefs were correct, the new allocation of patrol resources should lead to a reduction in statewide fatality rates. Similarly, the change to drive faster on the interstates should attract drivers away from other, more dangerous roads, again generating system-wide consequences. This study measures these changes and obtains surprising results. We find that the 65 mph limit reduced statewide fatality rates by 3.4% to 5.1%, holding constant the effects of long-term trend, driving exposure, seat belt laws, and economic factors.