A 1992 study of 100 different sites in 22 states showed that when the speed limit was raised the number of accidents decreased by 6.7%. When the-speed limits were lowered the number of accidents increased by 5.4%
“The objectives of this research was to determine the effects of raising and lowering posted limits on driver behavior and accidents for non-limited access rural and urban highways. Speed and accident data were collected in 22 States at 100 sites before and after limits were altered. Before and after data were also collected simultaneously at comparison sites where limits were not changed to control for the time trends. Repeated measurements were made at 14 sites to examine short – and long-term effects of speed-limit changes.
The results of the study indicated that lowering posted limits by as much as 20 mi/h (32 km/h), or raising limits by as much as 15 mi/h (24 km/h) had little effect on motorist’ speed. The majority of motorist did not drive 5 mi/h (8 km/h) above the posted limits when limits were raised, nor did they reduce by 5 or 10 mi/h (8 or 16 km/h) when limits are lowered. Data collected at the study sites indicated that the majority of limits are posed below the average speed-of traffic. Lowering limits below the 50th percentile does not reduce accidents, but does significantly increase driver violations of the speed limit. Conversely, raising the posted speed-limits did not increase-speeds or accidents.”