Background: In 1987 individual states in the USA were allowed to raise speed limits on rural freeways from 55 to 65 mph. Analyses of the impact of the increased speed limits on highway safety have produced conflicting results.
Objective: To determine if the 1987 speed limit increase on Washington State's rural freeways affected the incidence of fatal crashes or all crashes on rural freeways, or affected average vehicle speeds or speed variance.
Design: An ecological study of crashes and vehicle speeds on Washington State freeways from 1974 through 1994.
Results: The incidence of fatal crashes more than doubled after 1987, compared with what would have been expected if there had been no speed limit increase, rate ratio 2.1 (95% confidence interval (CI), 1.6-2.7). This resulted in an excess of 26.4 deaths per year on rural freeways in Washington State. The total crash rate did not change substantially, rate ratio 1.1 (95% CI, 1.0-1.3). Average vehicle speed increased by 5.5 mph. Speed variance was not affected by the speed limit increase.
Conclusions: The speed limit increase was associated with a higher fatal crash rate and more deaths on freeways in Washington State.