A yellow moose crossing sign next to a road through a forest.

Why Didn’t the Grizzly Bear Cross the Road?

It's not because it didn't want to; it might be because the overpass wasn't wide enough. Those design flaws can be fatal for wildlife.


Many of Canada’s highways run through forests and other natural habitats, and while this has allowed us to explore our country’s diverse environments, it can be bad news for animals in the area. To help these animals safely traverse their habitats, many roads now contain bridges known as wildlife overpasses. Yet a new study has found that these overpasses are often narrower than recommended, leaving the largest creatures vulnerable.

The study was led by researchers at the University of British Columbia and published in the Biodiversity and Conservation section of PeerJ.

For years, wildlife vehicle collisions across the country have been a concern for both humans and animals alike. In Canada, there are roughly four to eight large animals collisions every hour — and a total of about 11,000 animal-related collisions every year in British Columbia alone.

To help reduce these accidents and keep our wildlife safe, many countries have now constructed wildlife crossings such as underpasses, overpasses, and viaducts. These structures offer animals a safe way to move through their habitats, and are often covered in local plants and vegetation in order to make them feel natural for the animals.

Previous research has suggested that wildlife overpasses — bridges that cross over highways — should be at least 50 metres wide, and have a width-to-length ratio of 0.8, so that even the largest animals feel comfortable crossing. But do wildlife overpasses around the world actually meet these standards? To learn more, the researchers behind the study carried out a census of 120 of these structures located in North America, Europe, Asia, and Oceania.

The researchers used Google Earth observations to measure the dimensions of the 120 overpasses in their sample. They found that on average, the overpasses were 34 metres wide, falling short of the recommended 50 metres. In North America, only 20 out of 28 (or 29%) of overpasses included in the study were wide enough for large animals.

The researchers went on to study 12 of these North American overpasses in more detail. They set up cameras at overpasses in B.C., Alberta, Montana, and Washington state, and monitored the crossing rates of 10 large mammals commonly found in these regions: black bears, grizzly bears, wolves, coyote, cougars, deer, elk, moose, and bighorn sheep.

Qualitatively, they found that wider overpasses that adhered to the recommended guidelines had both a higher crossing rate and a greater diversity of species present than narrow overpasses. In the future, the researchers hope to expand this work to a greater number of overpasses in order to draw additional conclusions.

These findings can help guide policymakers in setting clear rules for wildlife overpass construction in the future. While many countries don’t mandate that overpasses adhere to these recommended sizes, the results of this study demonstrate how important wide overpasses can be when it comes to large mammal species.

“Overpasses are a win-win,” said Liam Brennan, an undergraduate student at the University of British Columbia and author on the paper, in a press release.

“[T]hey promote biodiversity and, with other measures like fencing, save animal and human lives.”

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Emily Deibert is a PhD student in the Department of Astronomy & Astrophysics at the University of Toronto with a passion for science outreach and communication. She earned her HBSc (Astronomy, English, and Mathematics) at the University of Toronto. She is excited about turning scientific research into stories and sharing these stories with the public.