On July 1, 2005, the State of Iowa implemented a 70 mph speed limit on most rural Interstates. This document reports on a 4 year update of a study of the safety effect of this change. Daytime and nighttime serious crashes were studied for a period of 14 years prior to the change and 4 years afterwards. Cross median crashes were studied for 4 years before and 4 years after. Simple descriptive statistics reveal increases in most crash severity categories for the 4 year period following the speed limit increase when compared to the 4year period prior to the increase. When compared to longer term trends, the increases were less pronounced in some severity levels and types, and for a few severity levels, the average crash frequencies were observed to decrease. Few of the changes in crash frequency were larger than the normal year to year variation is these statistics. Three types of crash were found to have increased by amounts larger than might be expected by normal variation: nighttime fatal crashes (52%), serious cross median crashes (25%) and all (total) crashes (25%). Only the increase in all (total) crashes was found to be statistically significant at the 90% confidence level. While not statistically significant at a high level of confidence, the results suggest that further study should be undertaken to understand and consider actions to mitigate nighttime and cross-median crashes. (Author/publisher)
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