Do Wildlife Bridges Work?

Terrapin crossing sign and cutouts for turtles to safely cross under road in St. Simon’s, Georgia, where female turtles make the treacherous journey to higher ground to lay their eggs.

Photo credit: Bubba73 (Jud McCranie) [CC BY-SA 4.0], via Wikimedia Commons

> Do Wildlife Bridges Work?

cWildlife crossing effectiveness is proven—here’s how animals learn to use them

Once you start talking wildlife bridges with people who aren’t familiar with them, you often hear in jest, “Oh right, so the animals can read signs and understand where they’re supposed to cross now?!”

Yes, exactly! Well, the understanding where they’re supposed to cross thing—they figure that out, sometimes quickly, sometimes a little more slowly, but yes, they absolutely do. In many cases, a wildlife bridge or crossing structure is put in place where there’s already a natural migration pattern or where there is heavy wildlife crossing. Sometimes this is near a waterway so perhaps around a bridge, or in other cases a particular forested area or habitat that has been broken up by roadways is a natural place to reconnect habitat with a wildlife bridge or underpass.

Oregon mule deer caught on to using wildlife underpass right away

South of Bend, Oregon, Highway 97 saw hundreds of wildlife-vehicle collisions (WVCs) every year so the Oregon Department of Transportation (ODOT) decided to build an underpass along with tall fencing along the road. Animals trying to cross the road wander the fence line until they find the underpass, then they cross under the highway safely, unbeknownst to most drivers who don’t even notice that they’re driving overtop the animal corridor. Of course there are always animals that make their way outside the fencing and don’t find the underpass directly, so the ODOT made additional adjustments to the wildlife bridge design. They added “jump-outs,” which are kind of like emergency exit ramps for those animals that somehow get beyond the fencing and are heading towards the highway. The jump-outs slope up and over the fence, away from the highway and back to safety.

On Highway 93 that runs through the Flathead Indian Reservation in Montana, cameras have recorded the movement of more than 30 different species, from bears, bobcats and badgers, to white-tailed deer, raccoons, wild turkeys and more.

Wildlife bridges work—but it can take time

For some, like the grizzly bears of Banff National Park in Alberta, Canada, it took upwards of five years for the bears to consistently use the wildlife crossings, even though many other species such as elk, moose, coyotes and wolverines, were already using the wildlife crossings regularly. With such lag, people (understandably) start to wonder whether the cost of such wildlife crossings, usually to the tune of about $4 million, is really worth it. But researchers continued to study strands of bear hair DNA on barbed wire to observe bear activity and whether or not bear were mating with bears on the other side of the highway. Ultimately, the wildlife crossings were encouraging the grizzlies to expand their breeding ground—which had been disrupted by road development—and the animals were indeed mating successfully. One studly black male sired 11 offspring with five different females, just by crossing back and forth over the wildlife crossing!

Which animals prefer which types of wildlife crossings

While the turtles prefer some reflective light, other animals like cougars and black bears prefer dark and narrow wildlife passages. Ungulates, like elk, deer and moose, would prefer something a little roomier and with more light. Amphibians, like those using the salamander tunnel in Massachusetts or reptiles crossing via the Alligator Alley underpass in Florida, generally gravitate to watery areas and obviously don’t need as much of an opening to pass through. Many studies have already been done in different parts of the United States on different wildlife crossing projects to determine the best size, shape, lighting and placement for wildlife bridges to be effective. Scientists and traffic engineers will continue to make structural design improvements to further enhance usage and to benefit individual species.

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