Nov. 29, 2022
MSU study shows increased traffic deaths linked to state speed limit
Story courtesy of Bridge Michigan.
The move by Michigan lawmakers in 2017 to allow 75 mph speed limits on over 600 miles of rural freeways came with a cost: more crashes and more deaths, new research shows.
The number of total crashes rose nearly 17 percent, from an average of 3,624 per year in the three years before the change to an average of 4,241 in 2018 and 2019, according to a study conducted in part by Peter Savolainen, a professor of civil and environmental engineering at Michigan State University.
Fatal crashes of those stretches of highway rose from about 11 per year to 18, a 63 percent increase, he said.
That’s the equivalent of an extra fatality every year for every 85 miles of highway with the higher limit, according to the study published in the journal Traffic Safety Research.
A 2020 analysis by Bridge Michigan found a similar increase in crashes on stretches where the speed limit was increased from 70 mph to 75 mph, such as Interstate 75 north of Bay City, U.S. 131 north of Grand Rapids and Interstate 69 east of Flint.
Savolainen examined Michigan State Police crash statistics and acknowledged that some of the increase in crashes is attributed to overall rise in traffic volume.
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Even so, the jump in crashes and fatal crashes on 75 mph freeways was statistically significant, he said.
Savolainen said he and others are still researching the impact on 900 miles of rural roads where the limit rose to 65 mph, but said preliminary estimates show similar increases in crashes.
Overall, speed limits were increased in about 16 percent of the state’s freeway miles, 1,557 miles in total.
Michigan and Maine are the only states east of the Mississippi River where speed limits are over 70 mph. It is far more common in the vast expanses of western states.
Research has shown that crashes increase as speed limits rise, as do the severity of those crashes.
“When the crashes do occur,” Savolainen said, “they do tend to be more severe because of the underlying physics.”
One of Savolainen’s peers, MSU civil engineering professor Timothy Gates, predicted in 2016 that a bump in speed limits would add three to 16 fatalities per year. Other consequences included increased gas consumption, air pollution and crashes, Gates said in 2016.
It’s for politicians, who make decisions on speed limits, to balance the costs with the benefits, Savolainen said. “Is that increase in fatalities worth the travel time savings?”
Some had feared that motorists would take the extra 5 mph and add it to their already above-the-limit speed. But the latest data shows that largely did not happen.
Average speeds increased just over 1 mph, from 68.9 mph to 70 mph on stretches of Interstate 75, Interstate 69 and the other areas where limits increased.