The effects of higher speed limits on traffic fatalities in the United States, 1993-2017

Farmer, Charles M.
Insurance Institute for Highway Safety
April 2019

Objective: The objective of this study was to estimate the effects of increased speed limits on U.S. traffic fatality rates during the period 1993-2017.
Methods: State-by-state annual traffic fatality rates per mile of travel were modeled as a function of time, the unemployment rate, the percentage of the driving age population that was younger than 25, the safety belt use rate, and the maximum posted speed limit. Separate analyses were conducted for interstates/freeways and all other roads.
Results: A 5-mph increase in the maximum state speed limit was associated with an 8.5% increase in fatality rates on interstates/freeways and a 2.8% increase on other roads. In total during the 25-year study period, there were an estimated 36,760 more traffic fatalities than would have been expected if maximum speed limits had not increased—13,638 on interstates/freeways and 23,122 on other roads.
Conclusions: Higher speed limits can yield societal benefits through reduced travel time, but there is a price to pay in terms of additional lives lost. Those responsible for managing the roadway system must recognize and carefully consider this trade-off before deciding to increase speed limits.

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