The Surface Transportation and Uniform Relocation and Assistance Act of 1987 allowed states to raise the maximum speed limit on rural interstate highways from 55 mph to 65 mph. By the end of 1987, 38 states, including the state of Iowa (on May 12, 1987), had raised the maximum speed limit on designated rural portions of their interstate highway system. A lingering issue is what impact this increase in speed limit has had on safety. In this research, changes in traffic accidents on rural interstate highways are examined using the state of Iowa as a case study. A statistical model is developed that is appropriate for the small count data on rural interstate highways. It is applied to quarterly figures of fatal accidents on rural and urban interstate highways. The objective of this research, then, is to examine fatal accident rates on Iowa rural interstates prior to and after the institution of the 65 mph speed limit in 1987. Controlling for seasonal variations in fatality rates, an assessment is made of the extent to which these rates have changed following this change in speed limit. The report begins with a review of the conclusions of other studies of fatal accident rate change when speed limits were changed. The specific circumstances present in Iowa during the period of analysis are described, as well as the data used in the analysis. A statistical analysis of the Iowa data is performed, followed by an interpretation of what reasonably can be concluded from this analysis. (Author/publisher)
Samenvatting