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The numbers tell the story.

Mule deer and pronghorn used new over and underpasses on one of Wyoming’s most dangerous highways for animal collisions more than 60,000 times between 2012 and 2015.

At the same time, traffic accidents between vehicles and wildlife fell from about 85 per year to 16, addressing one of Wyoming’s perennial problems.

The reason is simple: manmade tunnels and bridges.

“The whole point of these structures is to mitigate or minimize roadway impacts,” said Hall Sawyer, a wildlife researcher with Western Ecosystems Technology Inc. “The two obvious impacts everyone is aware of is deer mortality and driver motorist safety.”

Preliminary numbers have been collected for years showing the increase in animals using over and underpasses in western Wyoming and how they correlate to declines in vehicle collisions. But on Monday, the scientific journal The Wildlife Society Bulletin published a complete report from Sawyer and other researchers detailing exactly what over and underpasses can do for wildlife and people.

The paper focused on one set of Wyoming highway crossings near a place outside Pinedale called Trapper’s Point.

Deer and pronghorn have, likely for millennia, traveled between winter and summer ranges over an area now split by Highway 191. Fences used to line the highway to prevent cattle from crossing, but wildlife inevitably moved over or under the wires and onto the highways.

Now they are funneled by tall fencing to one of two overpasses or six underpasses.

The benefit isn’t just for the safety of passing motorists, Sawyer said. Those animals that moved one way or another 60,000 times were trying to find better food or suitable mates. The over and underpasses improved their ability to cope with Wyoming’s harsh climate the way they had before asphalt and vehicles.

“If those animals weren’t able to access that winter range, over time if they couldn’t get to these important winter habitats, we would lose that segment of the population,” said Dean Clause, wildlife biologist for the Wyoming Game and Fish Department in Pinedale. “We manage for a certain number of animals in both of the herd units and we wouldn’t be able to maintain the numbers we have today.”

Clause worked with Sawyer and others to find the best places for the passes, as well as determine sizes. The overpasses are about twice as wide as some in other areas and include features such as dirt berms, encouraging even more passage by Wyoming’s pronghorn population, he said.

The paper also detailed the need for both types of structures depending on the migrating populations. Underpasses – the ones that look like big tunnels under a highway – are used almost exclusively by deer. Overpasses, on the other hand, are used most often by pronghorn.

The different species evolved in different environments, Sawyer said. Pronghorn aren’t comfortable if they can’t see long distances.

“Animals habituated to it right away,” Clause said. “They figured it out and they don’t even hesitate anymore.”

While the results are clear, they come with a cost. An underpass is about $400,000, and an overpass is about $2 million for two-lane highways.

On the other hand, the average pronghorn-vehicle accident costs about $8,300, Sawyer said, which means the overpasses led to a cost savings of almost $579,000 a year.

Several other sets of passes exist already in the Cowboy State. And the Wyoming Department of Transportation would like to see more, said Tom Hart, wildlife specialist with WYDOT.

The department is considering underpasses on Highway 189, a dangerous stretch of road in southwest Wyoming near LaBarge. The vast majority of the crashes are due to mule deer, which means underpasses may be enough.

Sawyer and Hart both talked about underpasses or overpasses on Interstate 80 in southwest Wyoming. Migration maps that use GPS data from deer and pronghorn show animals bunching at the interstate before turning around. A way to connect either side could restore historic migrations and give wildlife better access to food on either side of the interstate.

But those ideas are further down the line, Hart said. WYDOT, like all other state agencies, is facing budget cuts during Wyoming’s economic downturn.

“In a lot of cases, on those landscapes we have the nickel-and-dime effect, a new road here, a new housing development here, drought there,” Sawyer said. “But providing animals access to both sides of the highway can help offset some of those negative things happening on the landscape.

“It’s one of those positive things that seems to be working really well.”

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Follow Managing Editor Christine Peterson on Twitter @PetersonOutside.

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