Raising speed limits on highways could lead to more crashes on nearby roads, according to recently released research by the AAA Foundation for Traffic Safety.

This “spillover effect” creates unintended safety hazards for local communities typically not involved in the decision-making process to raise the posted speed limit on a nearby highway.

According to NHTSA, speeding contributes to nearly one-third of road fatalities in the past 20 years.

AAA Foundation researchers investigated whether changes to posted speed limits on interstate highways could cause drivers to adopt risky speeding behaviors on local streets.

The study looked at crash data before and after speed limit increases on interstates in Georgia, Michigan and Oregon.

Hot spot analyses were conducted on roads within a 1-mile radius of interstates.

Researchers found many instances where speed-related crashes increased on nearby roads following the interstate speed limit change, suggesting that drivers exiting the highway continued their faster tendencies.

“Increasing posted speed limits on freeways may improve traffic flow but could also lead to safety concerns in adjacent communities once speeding drivers proceed to neighboring roadways,” said Dr. David Yang, president and executive director of the AAA Foundation. Local streets around our homes are designed for slower speeds and diverse road users such as pedestrians and cyclists—not for speeding drivers.”

The AAA Foundation’s research reinforces the importance of the Safe System Approach (SSA).

Credit: AAA Foundation for Traffic Safety

The SSA is a strategic way of leveraging the engineering and behavioral countermeasures proven effective at preventing traffic crashes and the injuries that can result from them.

This research has uncovered an unintended but potentially lethal consequence of raising highway speed limits,” said Jake Nelson, AAA’s director of traffic safety advocacy. “With the U.S. near a record-high traffic death toll, road authorities can use these findings and an effective tool to pinpoint lifesaving solutions to help ensure that we all get home safely each day.”

Previous speeding research by the AAA Foundation found that raising posted highway speed limits is associated with increased crash frequencies in some situations.

At the same time, changes in travel times were small in response to both raised and lowered speed limits.