Not So Fast: Lowering Speed Limits Isn’t Stopping Wildlife-Vehicle Collisions

A new study shows drivers aren’t obeying the speed limits meant to reduce accidents

A herd of elk running across a road in a sagebrush landscape. An SUV nears the herd with brake lights on.
Wildlife-Vehicle Collisions Lowering speed limits isn't preventing vehicles from hitting animals. © Mark Gocke

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Common wisdom may suggest that lowering speed limits on highways would reduce the number of collisions between wildlife and vehicles. But a newly published study shows that isn’t so. Researchers from The Nature Conservancy in Wyoming, the Northern Rockies Conservation Cooperative, and the Western Transportation Institute tested the idea that reducing night-time speed limits during high-risk times would cause motorists to drive more slowly, giving them more time to avoid hitting animals on the road. The problem is, the average driver didn’t obey the lower posted speed limit, and there was no consistent reduction in the number of wildlife-vehicle collisions.

“Given the small reduction in vehicle speeds, it is not surprising that there was little benefit of reduced speed limit for deer or people. Our conclusion is that this simply isn’t an effective way to reduce these accidents since people don’t comply with the posted speed limit, “ says Dr. Corinna Riginos, who led the research. “Since changing human behavior isn’t easy, the only sure way to significantly address this problem is to build over- or underpasses that allow animals to cross roads without touching the pavement.”

The Approach

Prior to this study, almost no data existed on whether reducing speed limits could reduce wildlife-vehicle collisions. To test the practice, the researchers partnered with the Wyoming Department of Transportation to change the speed limit at six sites in Wyoming known to be important winter range and migration areas for mule deer. The speed limit was reduced from 70 to 55 miles per hour at night since most wildlife-vehicle collisions occur in the dark. The researchers then compared the number of collisions at the test sites with and without the speed limit change. They also tested for differences in vehicle speed in the reduced speed limit zones and in adjacent locations where the speed limit was not reduced. On average, drivers slowed down by only 3-5 mph rather than the required 15 mph.

At winter range sites, the number of deer vehicle collisions was not affected at all by the reduced speed limit. At migration sites, they were modestly lower under the reduced speed limit – for unclear reasons because driver speeds were similar at all sites.

Read the full study here.

The Nature Conservancy is a global conservation organization dedicated to conserving the lands and waters on which all life depends. Guided by science, we create innovative, on-the-ground solutions to our world’s toughest challenges so that nature and people can thrive together. We are tackling climate change, conserving lands, waters and oceans at an unprecedented scale, providing food and water sustainably and helping make cities more sustainable. Working in more than 70 countries and territories, we use a collaborative approach that engages local communities, governments, the private sector, and other partners. To learn more, visit or follow @nature_press on Twitter.