Most Wildlife Bridges Are Too Narrow, According to New Study

An estimated 1 million animals are killed on American roads each day. Throughout the year, there are also more than 1 million deer fatalities stemming from collisions, with these crashes killing around 200 people, as well. Wildlife bridges aim to address issues like this, as well as to help connect animals to their fragmented habitat. However, a new study finds that most of these structures may be too small.

Researchers at the University of British Columbia (UBC) recently examined wildlife overpasses throughout the world to better understand their dimensions. The team says recommendations call for a minimum width of 50 meters, along with a width-to-length ratio of 0.8 to make larger animals comfortable traversing the bridges. The team says when the structures are used by the animals for which they’re designed, they can make a huge difference.

Liam Brennan, a UBC undergraduate student and the study’s first author, explains, “Overpasses are a win-win: they promote biodiversity and, with other measures like fencing, save animal and human lives.”

However, his team’s findings, published in the journal PeerJ, show that nearly three-quarters of bridges in North America are narrower than the recommendations. In all, the researchers counted 120 overpasses using Google Earth, with the majority not as wide as recommended by local guidelines. Among those in North America specifically, 20 out of 28 were narrower than 50 meters. This may impact usage, as camera data from 12 North American bridges showed that wider bridges were utilized by more animals, as well as a broader range of species.

To help correct the issue and ensure the structures are used optimally, the researchers say there should be both localized guidelines on dimensions, as well as a means to monitor these bridges to ensure the animals for which they were built are getting the best use out of them.

However, they also write that more research could be done on the benefits of narrower bridges.

The paper’s abstract reads, “Future studies, however, are encouraged to further explore the specific instances when underpasses and narrower overpasses present more cost-effective ecological solutions, or how these structures can complement wide overpasses in successful wildlife crossing systems.”

You can find the whole paper here.