Speed kills? Not always: the New York State thruway experience

The New York State (NYS) speed limit increased from 55 mph to 65 mph in August 1995. This study examines the change in motor vehicle fatality rates on the NYS thruway and NYS Interstates before and after the speed limit change. We also compare the fatality rates pre- and postincreased speed limit between the NYS thruway and other major NYS roadways, where speed limit remained unchanged at 55 mph.

The number of fatalities and vehicle miles traveled (VMT) on individual roadways were extracted from NYS Department of Transportation reports. The data were divided into groups by prespeed limit change, August 1992 to August 1995, and postspeed limit change, August 1995 to August 1998, and roads where the speed limit remained unchanged at 55 mph. Data were also analyzed to assess the effect of variability of speed and general topography on changes in fatality rates on roads where the speed limit was increased.

There was a 28.3% decrease in absolute mortality and a 42.6% decrease in mortality adjusted for VMT, on the NYS thruway after the speed limit increase from 55 mph to 65 mph. When compared with roadways, where the speed limit remained unchanged, there was a 52.4% reduction in fatality rates below the predicted rates after the speed limit increase (p = 0.009). On the NYS interstate system, there was a 13.5% decrease in absolute mortality and a 29.2% decrease in mortality adjusted for VMT, after the speed limit increase. The percentage of traffic traveling >10 mph faster than the speed limit dropped from 39% to 8% on roads, where the speed limit was increased from 55 mph to 65 mph. In the NYS interstate system, "flat" roads (elevation <200 m) had a 30.2% reduction in absolute mortality, whereas "mountainous" roads (elevations >200 m) had a 17.6% increase in absolute mortality in response to the speed limit increase (p < 0.001).

Motor vehicle speed is a key determinate of the severity of injury in an individual crash; however, speed variance and road topography have also been found to impact the total number of motor vehicle fatalities in the NY State highway system.