When the speed limit is lowered in an area, it’s usually in an effort to improve safety. Reducing the speed limit too much, however, may actually cause more crashes, according to a group of Penn State researchers.
The study found reducing speed limits by 5 mph decreases the amount of total and fatal crashes.
The catch, however, is reducing speed limits by 10 mph or more has the opposite effect because drivers “stop paying attention,” according to Penn State assistant professor of civil engineering Vikash Gayah.
The group collected the data in Montana, a state that posts speed limits lower than engineers recommend.
“We found there was an increase in fatal and injury crashes at locations with posted speed limits set 10 miles per hour or more below engineering recommendations,” Gayah said in a news release.
Speed limits are set based on results from engineering studies that collect free-flow traffic data and then select an appropriate speed using a statistical model. But factors such as school zones, citizen or political pressure, and perceived safety issues contribute to the fairly common practice of lowering speed limits below engineering guidelines, the researchers found.
And while the study may offer some insight into driving habits, PennDOT press officer Marla Fannin said in an email that speed limits are determined by a standard process.
The exception is that the speed limit may be reduced up to 10 mph under “specific safety-related conditions” as described under Pennsylvania state law, according to Fannin.
“We set speed limits based on engineering analysis and in conformance with the appropriate state regulations,” Fannin wrote.
She also said 2018 crash data is not yet available.