The topic of raising maximum statutory speed limits has been a subject of debate for many decades. Supporters of high speed limits cite travel time savings and lower crash risk due to lower variances in speed as reasons to increase a speed limit, while opponents point to increases in crash and fatality rates that have followed speed limit increases in the past. As recently as February 2017, legislators in the state of Iowa have discussed increasing the maximum speed limit on rural interstates from 70 mph to 75 mph, an increase that would make Iowa the 19th state to have a speed limit of at least 75 mph.

The primary goal of this study is to assess the degree to which recent increases in maximum speed limits across the United States have impacted the number of traffic fatalities on rural interstates. To this end, the research includes two separate investigations of state-level fatality trends. These investigations include a state-level aggregate analysis, as well as a disaggregate road segment-level analysis, each of which uses information from federal highway and traffic safety agencies from 2001 to 2016 to examine the effects of rural interstate speed limit on traffic fatalities.

A series of negative binomial models are estimated, with the results showing significantly higher rates of fatal crashes in states with higher maximum limits. In the state-level analysis, the results show that increasing the statewide percentage of rural interstates posted at 70, 75, or 80 mph by one percent is associated with fatality increases of 0.2%, 0.5%, and 0.6% respectively. Likewise, the results from the road-level analysis indicate that raising the speed limit on a road segment from 65 to 70, 70 to 75, or 75 to 80 mph is likely to increase the fatal crashes by 29.7%, 2.9%, and 53.3%, respectively. In addition, the segment-level analysis estimated quantities of fatal crashes related to distractions and speeding. The results of this study provide important insights to inform subsequent policy discussions related to speed limit increases.