How wildlife bridges over highways make animals — and people — safer

Bridges for bears and tunnels for tortoises have significantly reduced the number of wildlife-car collisions worldwide.

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Roaring traffic doesn’t stop big mammals like moose and bears from crossing highways—nor does it keep myriad smaller creatures from being squished by car tires. In just two years along one stretch of highway in Utah, 98 deer, three moose, two elk, multiple raccoons, and a cougar were killed in car collisions—a total of 106 animals. In the United States, there are 21 threatened and endangered species whose very survival is threatened by road mortalities, including Key deer in Florida, bighorn sheep in California, and red-bellied turtles in Alabama.

People are also hurt—about 200 die every year in the more than one million car collisions in the U.S., according to the National Highway Traffic Safety Administration. These crashes are expensive, too: Deer-car collisions cost an average of $8,190, an elk-vehicle collision is about $25,319, and a moose-vehicle collision is $44,546, taking into consideration human injuries and death, towing, vehicle repair, investigation of the accident by local authorities, and carcass disposal, according to a paper from the Western Transportation Institute (WTI) at Montana State University.

And the number of these deadly accidents is growing. “Over the most recently reported 15-year period, wildlife-vehicle collisions have increased by 50 percent, with an estimated one to two million large animals killed by motorists every year,” says Rob Ament, the road ecology program manager at WTI.

Fencing, an important component of successful wildlife crossings, directs migrating pronghorn antelope in Wyoming over the highway instead of across it at Trapper's Point.

There’s one solution, however, that’s been remarkably effective around the world in decreasing collisions between cars and animals crossing the road: wildlife under- and overpasses. Studies that looked at a cross-section of native species' deaths on highways in Florida, bandicoots and wallabies in Australia, and jaguars in Mexico, just to name a few, all show that wildlife crossings save money and lives, both human and animal.

“You can get reductions of 85 to 95 percent with crossings and fencing that guide animals under or over highways,” Ament says.

That’s why these traffic-spanning bridges and tunnels—which have been popular in Europe since the 1950s when the first was built in France—are showing up worldwide. Usually looking much like a regular overpass for cars, but decked out with native flora, they aren’t particularly noticeable unless you know what you’re looking for.

And undercrossings, which pass beneath highways to assist shyer and smaller animals, may be invisible to drivers. But they’re helping countless species, from gold monkeys and pumas in Brazil to water voles in London. Ament is even working on wildlife crossings for highways that haven’t been built yet in developing nations like Bhutan, which need safe places for Asian elephants to traverse their territories. As he points out, it’s a lot easier—and cheaper—to build in these mitigations during road construction than it is to retrofit, as has been done in the U.S. and Canada.


Snoqualmie Pass

Washington State is one of the latest to join in. Construction on its first wildlife bridge, east of the Snoqualmie Pass in the Cascades, began in 2015 over Interstate 90, which runs from Seattle to Boston. Though it’s just a bare set of arches now, with native plants to be planted in fall of 2020, deer and coyotes are already using it.

Along with six underpasses built since 2013, these crossings are the first in a set of 20 along a 15-mile stretch of highway on I-90. These passes will allow elk, black bears, mountain lions, pika, and even trout to traverse what was once a near-impenetrable barrier of the road.

A view of I-90 through Washintgon's Cascade mountains from the top of the new wildlife bridge just east of Snoqualmie Pass in October 2018. When complete, only animals will be able to enjoy this view, as the bridge will be closed to humans.

These wildlife crossings aren’t just about saving individual animals—they’re about species survival, too. I-90 is an economically important east-west lifeline in the state, crossing high mountain passes in the Cascades where few other roads are available. But many of the animals mostly want to move north to south. Those south of the highway got trapped in an island of sorts. Bound by the highway to the north and the Columbia River to the south, inbreeding posed a potential problem.

“Localized extinction happens when populations can’t find each other, and if they don’t have genetic variability, they will blink out—especially low-mobility species in old-growth [forest],” says Patty Garvey-Darda, a wildlife biologist with the U.S. Forest Service and the liaison to the state’s transportation department for the I-90-project.

Wildlife crossings are needed for large mammals to avert deadly and expensive car crashes—but smaller animals, like this threatened gopher tortoise in Wiggins, Mississippi, can use them too.

The Snoqualmie overpass helps reconnect these isolated populations. I-90’s underpasses, which connect wetlands and streams back to the Yakima River, are also critical, especially in connecting waterways and aquatic species, says Jen Watkins, with Conservation Northwest, a nonprofit that conserves wild lands and native wildlife. “We have bull trout responding to four new tributaries they haven’t used in recent history. We’re not surprised that the underpasses are working, but the immediacy of the response is thrilling,” says Watkins. Bull trout are a threatened species, and they, along with salamanders and reptiles that are “species of conservation concern,” all benefit from underpasses.


What makes a crossing successful?

One of the most looked-to examples of successful wildlife overpasses is in Banff, over the Trans-Canada Highway. A study there shows that in just one two-mile stretch, wildlife-vehicle crashes reduced from an average of 12 a year to 2.5, reducing costs of crashes by 90 percent—over $100,000. It’s statistics like these that have led to the addition of crossings there over the last two decades.

Tony Clevenger, a senior research wildlife biologist at WTI , has been monitoring wildlife at Banff’s six overpasses (and 38 underpasses) for more than 17 years. He found that animals have different preferences when it comes to feeling safe on an over- or underpass. Doing that monitoring before building is critical, he says.

“Grizzly bears, elk, deer, and moose prefer big structures that are open,” he says. “Cougars and black bears prefer smaller, more constricted crossings, with less light and more cover.” These inclinations are based on how each animal evolved—black bears’ and cougars’ natural home is in the forest, not a meadow, so they won’t feel comfortable on a large, open crossing structure, for example.

Even when animals don’t have safe ways to cross highways, many species will try anyway, including this mother black bear and her cubs in Alberta, Canada.

This informed the landscaping on the Banff crossings. “On one side we would plant trees and shrubs, and on the other side have areas that are open, planted with grass,” Clevenger says.

Long-term monitoring of local wildlife to find where animals were crossing the road—or trying to cross and then turning back—was key to the success of the Snoqualmie project. It started five years before construction began and continues to this day. Washington Department of Transportation roadkill statistics were combined with on-the-ground tracking from citizen scientists and U.S. Forest Service data.

Choice of crossing can even come down to the sex of an animal. A 2014 study in Banff looking at DNA obtained from black bear hair samples and camera data showed that breeding females preferred overpasses, but males liked underpasses. “So now we know if we want to maintain connectivity we need to build overpasses so breeding females can cross,” Clevenger says.

While those animals that are already accustomed to human structures, like coyotes and deer, were using the Washington I-90 crossings almost immediately—even skirting around construction equipment—there’s a longer learning curve for other species. It might take elk, grizzly bears, and cougars a couple years to feel comfortable using the crossings, and wolverines, lynx, wolves, and fishers (recently reintroduced to Washington), five years or more. But once routes over or under the road are established, it becomes intergenerational knowledge: “Cougars with kittens and black bears with cubs will use the crossings,” Clevenger says.Those early adapters are important. Not only are they keeping themselves off the road and reducing car-animal collisions right off the bat, but they also create paths that more reticent animals will follow, says Clevenger. Fences also help—they guide animals away from dangerous highway crossings and toward overpasses and underpasses.

“We don’t want to just connect animals—we want to create ecosystems,” Garvey-Darda says. That means considering the smaller animals, such as shrews, voles, jumping mice, and pika, too. Rocks and brush piles are set up specifically to encourage these more diminutive species to use the crossings.

“We are lucky. In Washington state, we still have all of our native wildlife species—even grizzlies and lynx,” she says. “That’s not true of a lot of states. Wildlife crossings will help make sure that’s true into the future

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