A young elk near a wildlife bridge in Banff Springs, Canada. George Rose/Getty Images

Highways are dangerous barriers for all sorts of wildlife. Around the world, bridges and tunnels just for animals make it easier for them to migrate, mate, eat, and survive.

Sometimes the only thing that keeps a slithering salamander from becoming a squashed one is a little attention to infrastructure.

As cars take over paths that used to belong to the wild, moving through space as a rodent, a lizard, or something higher up the food chain can turn deadly, fast. Experts estimate cars in the U.S. collide with large animals over 1 million times per year, costing over $8 billion in repairs and injuries. To help animals navigate our fast-paced highway systems (and avoid turning into roadkill in the process), nature conservationists have started building wildlife bridges and tunnels, designed especially for critters. These designs aren’t only intended to help animals evade death. They’re used to facilitate gene flow (read: migration) that could be stunted by a highway.

The earliest recorded man-made animal bridge was erected in France in the 1950s to help hunters guide deer. Since then, wildlife-crossing designs have spread worldwide, especially across Europe. Over 600 of the crossings are in the Netherlands, where they carry badgers, bison, and elk. The longest bridge in the world is located there, spanning 800 meters. Other Dutch eco-innovators partnered with gas companies to install circular pipes beneath roadways, which serve as underground paths for foxes and mice.

While animal crossings are more often located above or below highways that traverse rural landscapes full of endangered species, there are plenty of urban or suburban examples. In Oregon and Washington, culverts built under roads that traverse bodies of water have been widened to divert more than just fish. In Brisbane, Australia, above the infamously dangerous Compton Road, three canopy bridges woven from heavy rope allow gliders and possums to swing through the sky.

Others, like Oslo’s “bee highway,” are more conceptual. There, to save the endangered bee population, locals started planting flowers atop rooftops and within parks along a pre-determined route. Bees could then travel through the city at ease, fed and happy.

In the 1980s, salamanders in Massachusetts were some of the first to get this treatment in the urban U.S., when citizens grew concerned that the spotted amphibians wouldn’t be able to cross an increasingly bustling street to find their mates. Local samaritans formed “bucket brigades” each spring, banding together to carry salamanders from their forest lairs on one side of Henry Street to the breeding ponds on the other. By 1987, preservationists and public works officers started to build an official underpass. Now, 100 to 200 salamanders (and teeny frogs) pass through the tunnel each year.

(It is perhaps not surprising that Massachusetts was also the site of a famously adorable story of ducks stopping traffic, Make Way for Ducklings. People there are good at making way!)

These ducklings could have used an animal bridge. (Wilfredo Lee/AP)

On Christmas Island, off the coast of Australia, rangers have tried another approach to facilitate mating, erecting a 5-meter-high plastic bridge to direct the thousands of red crabs who once scurried down busy thoroughfares to procreate and spawn.

Thirty underpasses and one big crab bridge allow thousands of crustaceans to travel to the sea each year, where they release billions of larvae. (Parks Australia Media)

In addition to helping crabs fulfill their natural life cycles, the bridge is great publicity. “Sydney can have its Harbour Bridge and San Francisco its Golden Gate Bridge, but it’s our crab bridge which is currently wooing tourists from all over the globe,” Linda Cash of the Christmas Island Tourism Association told the Telegraph.

And elsewhere, passages are just meant to keep vulnerable animals alive long enough to mate at all. Take turtles, for example. Their relationship with urban life is fraught: They’re lured away from the beach by headlights, only to be crushed by errant cars or stumble into gutters. They’re tricked into following artificial light instead of the moon, and left stranded, vulnerable to prey. And sometimes, they fall while crossing railroad tracks, crushing them and disrupting train travel.

To save them from the latter fate, engineers from the West Japan Railway Company have created a special U-shaped turtle track, layered underneath existing railroad lines.

Measuring the efficacy of any crossing—under, over, or otherwise—is difficult. If the goal is to get animals across safely, road accident statistics can tell one story (after Arizona installed more than a dozen underpasses and overpasses near Flagstaff and Payson, built especially for roaming elk, the number of traffic accidents went down 90 percent), but ecologists can also monitor how many animals are using them by installing video cameras, or putting a layer of sand on the underpass that is later combed for foot prints.

Even before Banff National Park in Canada finished installing its 38 underpasses and six overpasses in 2014, it had recorded more than 150,000 large mammal crossings since the first started going up in 1996. Eleven species of wildlife were tracked, including “grizzly and black bears, wolves, coyotes, cougars, moose, elk, deer, bighorn sheep, and more recently wolverine and lynx,” according to the National Park’s website.  

A deer caught on video using the Snoqualmie Pass, in Washington. (Washington State Department of Transportation/Reuters)

Fences are often erected in tandem with other crossing infrastructure, because they’re another tool that can separate animals from the highways that lurk at the edge of forests. In Banff, the combined forces of fences and under/over passes has reportedly reduced the rate of traffic accidents by 80 percent.

What fences don’t solve, though, is the other issue roads pose: isolating members of the same species in different pockets, close but disconnected. Allowing woodland creatures to actually access other animal neighborhoods, filled with procreation potential and food sources, should be the priority, says Richard T. T. Forman, an ecology professor in the Harvard School of Design. Basically, the move is to build bridges, not walls.

“There also tends to be a pretty high roadkill rate at the ends of the fences,” he said. “The animals either go one way into the overpass or go to other end and escape around it, and tend to get hit there.”

A Maryland park official repairs a turtle fence in 2006. (Timothy Jacobsen/AP)

And even among connectors, not all wildlife crossings are created equal. Overpasses are much better than underpasses, says Forman. In a 17-year-long study on Banff National Park’s wildlife infrastructure conducted by Tony Clevenger, all but two of the large mammals used the overpasses more than the underpasses: Black bears used both equally, and mountain lions preferred to slink through the narrow underpasses.

But overpasses are also much more expensive to build than underpasses, meaning transportation agencies much prefer building the latter. It’s this reality that Forman says drives the large discrepancy between the number of bridges (in the dozens) and the number of tunnels (in the hundreds) in the U.S., especially.

To make these projects cheaper, Forman suggests incorporating wildlife crossings into the design process earlier. “Whenever there’s a road construction project, like upgrading a bridge or widening a highway, that’s a strategic moment to include a wildlife structure,” he said.

Maryland’s $2 billion Intercounty Connector (ICC) included a plan to build 10 animal underpasses from the get-go. And even before construction finished, animals were ducking underneath. “The deer, the raccoons and opossums, they’ve been going through these culverts long before they had to,” Robert E. Shreeve, who worked on the project with the Maryland Department of Environment at the time, told the Baltimore Sun. “Suburban animals are not shy.”

When an underpass wasn’t doing enough to protect native animals like deer, bighorn sheep, javelinas, and tortoises in Arizona, the state invested $9.5 million in a six lane-wide wildlife overpass over a Tucson highway. Already, video has captured thousands of animals traversing the paths. “Once a week, I get an old codger saying these wildlife crossings don’t work,” Arizona Game and Fish statewide research biologist Jeff Gagnon told The Arizona Republic. “Well, they do.”

All of these crossing strategies have been criticized by some as a Band-Aid for—and distraction from—the larger problem, which is that we’re only reclaiming space for animals because too much of it has been taken away by cars. Preventing accidents doesn’t have to be a matter of diverting animals from land that’s rightfully theirs; it could be a matter of reducing vehicle miles traveled, and slowing the spread of highways into the wild.

“I resonate with people who want to maintain roadless areas in remote areas,” Forman said. But in rapidly urbanizing locations, that’s not always an option. “In that context, we can’t wait forever. This is getting one segment of society to do something about it: namely, transportation planners and engineers.”

Correction: A previous version of this article misspelled Tony Clevenger’s name.

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