About the research
Iowa’s maximum speed limit for rural interstates has been 70 mph since 2005, and the Iowa legislature has recently discussed the possibility of further increasing the maximum speed limit. This research aims to inform this discussion by examining how traffic fatality rates have changed over time as maximum speed limits have been increased in Iowa and other states, with emphasis on the changes resulting from the more recent increases to 75 mph and above in other states.
The study included state-level and road-level analyses using nationwide data sets and an Iowa-specific analysis using data from within the state. The nationwide analyses confirm prior research showing that states with higher rural interstate speed limits experience a higher number of traffic fatalities. The state-level analysis shows that this effect is even larger when accounting for the proportion of rural interstate mileage in each state posted at the maximum speed limit. However, this increase in traffic fatalities may begin to taper off at the highest speed limits. The road-level analysis indicates that speed limit more strongly affects fatal crashes involving driver distraction than total fatalities or fatal crashes. Additionally, fatal crashes involving speeding are more strongly affected by speed limit on roads posted at 70 or 75 mph than on roads posted at 80 mph.
A simple before-and-after comparison of fatal and serious crash rates on Iowa interstates from 1991 to 2017 shows that crashes increased in the few years after the 2005 speed limit increase but have generally declined since then. Further analyses showed that average and 85th percentile speeds were influenced by roadway geometric characteristics and that speed variance was the primary factor affecting crash rate. The impacts of speed variance are most pronounced for the most severe crashes.