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Relationship of traffic fatality rates to maximum state speed limits

Farmer, Charles M.
Traffic Injury Prevention (TIP)

Objectives: The objective of this study was to examine the safety effects of increases in U.S. state maximum speed limits during the period 1993-2013.
Methods: Poisson regression was used to model state-by-state annual traffic fatality rates per mile of travel as a function of time, the unemployment rate, the percentage of the driving age population that was younger than 25, per capita alcohol consumption, and the maximum posted speed limit on any road in the state. Separate analyses were conducted for all roads, interstates and freeways, and all other roads.
Results: A 5 mph increase in the maximum state speed limit was associated with an 8% increase in fatality rates on interstates and freeways and a 4% increase on other roads. In total, there were an estimated 33,000 more traffic fatalities during the years 1995-2013 than would have been expected if maximum speed limits had not increased. In 2013 alone, there were approximately 1,900 additional deaths — 500 on interstates/freeways and 1,400 on other roads.
Conclusions: There is a definite trend of increased fatality risk when speed limits are raised. As roadway sections with higher speed limits have become more ubiquitous, the increase in fatality risk has extended beyond these roadways. The increase in risk has been so great that it has now largely offset the beneficial effects of some other traffic safety strategies. State policymakers should keep this trade-off in mind when considering proposals to raise speed limits.