Wildlife Crossings an Ill-Suited Solution to Animals Killed by Vehicle Collisions

How preventing wildlife death means a societal shift in how we transport ourselves

Eight mule deer cross the overpass along Highway 93, Elko County, Nevada, in this Oct. 14, 2014 image - photo: Nevada Department of Wildlife

If you’ve ever been in a car, chances are you’ve either seen or been part of a vehicle collision with an animal. It’s common, with over one million animals dying on roads every year in the United States, according to the U.S. Department of Transportation. This loss can be devastating for an ecosystem, and has led some states to build wildlife crossings under and over roads. However, some scientists fear these crossings are not enough to address an issue symptomatic of a larger problem – too many cars on the road. 

Much of the wildlife hit by cars each year are ubiquitous prey animals like deer, squirrels, and rabbits. These animals are adapted to avoid predators by dashing in random directions so predators can’t predict their movement, according to professor Fraser Shilling, director of the Road Ecology Center at UC Davis. While these movements are effective in avoiding natural predators, when faced with a fast-moving automobile, this unpredictability can be lethal if they jump in the direction of the car. And, with roads a relatively recent development in animals’ habitats, there’s little sign that they’ve been able to adapt to these changes. 

“Busy roads have really only been around since the last fifty years,” said Shilling. “Evolution takes thousands of years, so to talk about changes in animal behavior in the last fifty years is completely outside evolution.”

Being able to cross these roads is important for wildlife. Highways that stretch through their habitats effectively cut animals off from large sections of their home, leading to increased genetic isolation. This means there are fewer possible mates for these segregated species, which causes inbreeding. Birth defects have been seen in mountain lion populations in southern California, where reports of kinked tails and missing testicles in male mountain lions have been documented. These issues can lead to infertility and other debilitating genetic flaws. 

If animals do try to cross roads, especially busy ones, they risk being killed. While this loss is difficult for the health of any species, it can be particularly detrimental for those that are or have been at risk of endangerment, such as the case of the recently delisted grey wolf. In April of 2021, a grey wolf from the Cornucopia Pack in Oregon was killed in a vehicle collision. 

“Animals have not adjusted to the roads and we have not adjusted to the animals,” said Shilling. “Instead, we’re going faster, there’s more cars on the roads, and we’re doing a better job at killing them.” 

Some states like California have opted to build wildlife crossings to try to mitigate this loss. An example of this is the Mammoth Lakes 395 Wildlife Crossing, proposed by Eastern Sierra Land Trust. The project proposes the construction of both underpasses and overpasses on Highway 395 and Highway 203 in California’s Mono County, between Crowley Lake and Mammoth Lakes. This area has the highest reported frequency of wildlife vehicle collisions in CalTrans District 9, a region that borders the central and southern part of the Sierra before ending just north of Los Angeles. 

“There are migratory paths that cross 395 and get really concentrated there outside of Mammoth,” said CalTrans District 9 senior biologist Katie Rodriguez during a meeting hosted by Eastern Sierra Land Trust in February of 2021. Many mule deer pass through the area and are hit, causing damage to drivers and death to themselves. 

“One of the difficulties with funding this project is that it unfortunately does not meet the qualifications for our state transportation safety funding,” said Rodriguez. “Even though we have so many collisions, it’s compared to the rest of the state, so when we have other locations in the state where there’s lots of fatality accidents, those are more of a prioritization for safety funding.” Because of this lack of funding for wildlife crossings, the ones that do manage to get built tend to have little impact in decreasing the overall number of animals hit.

It’s also difficult to build a crossing that every species is going to want to use. For rare animals like snowshoe hares, ringtails, and wolverines, they often never find the wildlife crossing because they avoid the roads. This means that they continue to be isolated from large parts of their ecosystem. 

“You can’t build a single wildlife crossing that all species love,” said Shilling. “The roads are a wall, and if you build a wildlife structure that only half the wildlife species want to go across, then you’ve built a filter across the highway. It’s better than a wall, but you still haven’t solved the problem.” 

For a more widespread decrease in wildlife vehicle collisions, a change in driving behavior needs to occur, according to Shilling. One solution includes driving at slower speeds, meaning 25 miles per hour or less – anything faster makes it much more difficult to stop at a short distance if an animal is on the road. 

However, the most certain way to avoid a collision is by not driving, which brings to light a larger conversation about the United States’ reliance on automobiles as a primary form of transportation. 

The positive impact of driving less on the rate of wildlife vehicle collisions was illustrated during the height of the pandemic stay-at-home-orders in March and April of 2020. The number of collisions in states like California, Idaho, and Maine declined by 21-58% in those two months, according to research by the Road Ecology Center. If this reduction were to have sustained through the year, it would have meant that between 5,000-13,000 large animals in these states would have avoided death by vehicle collision. 

Now that driving is back to comparable rates to the pre-pandemic levels, this research has given scientists and the general public alike reason to reconsider the role of driving in America. 

“Individually, don’t speed, but as a society, we have to find a way to drive less, like massively drive less,” said Shilling. “The best thing to do for wildlife is to find ways to support a society where driving is not essential.” 


Claire Carlson writes about the environment for the Ally. Support her work.