Michigan's 5 mph increase in speed limits led to 5% increase in crashes, study finds

In the four-and-a-half years since Michigan raised some of its freeway speed limits, the state has seen a jump in crashes.

Maximum speed limits were raised for some 600 miles of Michigan freeways from 70 mph to 75 mph beginning in May 2017. The speed limit for trucks were also increased by 5 mph to 65 mph on all freeways at that time as well. 

Since then, the state has seen a 5% increase in crashes.

The findings come from Michigan State University researchers who published their work in the Journal for Traffic Safety Research. 

"Research has consistently shown speeds (do) increase following speed limit increases. These increases have also generally coincided with increases in both the frequency and severity of crashes," the paper said. "The results show that the locations where the speed limits were increased experienced a 5% increase in crashes, while a marginal reduction in crashes was observed where speed limits did not increase."

Michigan raised its speed limits over a two-month period after passing a bill in 2016. The legislation ordered that approximately 614 miles of freeways increase their posted limit to 75 mph. State road speed limits for trucks were also increased to 65 mph on state roads.

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To research if the raised speeds led to more crashes, researchers collected data from crashes in sites with the same speeds and with increased speeds. They looked at road, speed, and traffic characteristics of both sites before and after the law took effect.

"Crashes of all types increased by approximately 5.0% at those sites where the speed limits were increased," the report said, emphasizing the findings are consistent with other speed changing scenarios.