The Utah Project Saving Wildlife—and Humans—En Masse

A few miles east of Salt Lake City, Utah, deep in Parley’s Canyon, a mule deer finds itself faced with a modern dilemma for wildlife: To cross or not to cross. Unlike their moose cousins, which migrated into Utah only in recent centuries, mule deer hold an ancient lineage in this canyon. Indigenous rock art scattered across the state is amongst some of the evidence that they’ve inhabited Parley’s Canyon and others like it for thousands of years. 

Every springtime, they cross the canyon summit to reach lower elevations in search of the woody shrubs and riparian vegetation along the Weber River. The canyon has long served as a critical migration corridor for native wildlife, with moose and elk following similar paths. But in 1986, a new corridor cut through the landscape: Interstate 80.

When two high-traffic highways like this intersect without any form of stop signs or lights, crashes are bound to happen—especially when one has a speed limit of 80 miles per hour. This section of the coast to coast highway is one of the top twenty five hot spots for wild animal collisions with vehicles, according to a report by The Utah Department of Transportation (UDOT) and the Utah Division of Wildlife Resources (UDWR), averaging around one fatal crash per week alone due to collisions with wildlife back in 2016. Such devastating and frequent accidents have taken their toll on the humans who rely on this roadway. But one could argue that highways like I-80 pose a far more expansive threat to the four-legged victims. 

An Ancient Highway Meets I-80

“We have this road infrastructure that is impeding the wild animals' ability to migrate, to expand their populations,” says Dr. Nicki Frey, extension professor of wildlife biology at Utah State University, “to disperse away from their natal home ranges, and it really can shut down wildlife's natural ability to move through their environment.”

Nine years after the alarm bells of the 2016 UDOT report, Frey and her research team at Utah State University found that wildlife collisions on this particular stretch of I-80 has plummeted by 77 percent. The catalyst for that drop now stands directly over the highway—the Parley’s Canyon Wildlife Crossing—a 350-foot overpass spanning 50 feet wide, built specifically to give animals safe passage across six lanes of traffic.

Overpasses are like bridges for animals. And like humans, an animal is just as likely to go back the way it came if it doesn’t feel safe. Changes in migration pressures, species learning curves, climate change, and the structure of the bridge itself are part of the careful set of considerations that can make or break the success of these multi-million dollar projects, noted Makeda Hansen, coordinator of the DWR’s Wildlife Migration Initiative.

A large-scale review of decades worth of papers and research on wildlife crossings found that these “bridges” are not always as successful as simple fencing or underpasses to direct animal migration because animals will not intuitively cross them. So why take the chance?

Joshua Sletten, the WSP Project manager who oversaw everything from the engineering to the public forums for the project, was convinced an overpass was the only viable choice. While an underpass or culvert would have been cheaper, forcing large animals into a dark, 300-foot tunnel beneath the highway posed a major hurdle.

“We felt like that confined space for animals, because it needed to be over 300 feet long, would be too uncomfortable for most big animals to use,” he says.

Bridging the gap in Parley’s Canyon

Moose—which pose the highest risk of fatal collisions for both drivers and animals—were a top concern for designers. These giants are notoriously cautious and stubborn, often taking years to trust artificial structures. Yet the Parley’s Canyon overpass proved to be a rare exception. Over two years of monitoring trail camera feeds, Frey and her team observed how quickly local species adapted to the crossing.

They found there to be virtually no learning curve for animals in this part of the Wasatch—mule deer and moose crossed with little to no hesitation from early on with successful crossing rates of 93 and 100 percent. Any researcher can confirm this kind of result is a scientific pipe dream. Frey suspects this quick uptake is, in part, thanks to the natural flow of the topography leading up to the overpass entrances and exits. 

“At the end of the overpass, is what we call the apron,” Frey clarifies while pushing up her dark-framed glasses from her home office. Behind her is a poster about habitat connectivity that she claims covers the mess on the other side.

The goal of the apron is to do the opposite, fanning out naturally from the overpass entrance and exit so that animals have clear visibility into the wilderness beyond. For a mule deer or moose, this visibility can be life or death—especially if a mountain lion or bear is lurking unseen. The next 10 miles is lined with fencing to continue to guide animals in the right direction for good measure.

Their travels don’t stop after a meager 20 or so miles. Long-range movement is critical to the survival of animals like mule deer and moose, both as individuals and entire populations. While a human might be making their daily 30-mile journey from home to the office, that might be a small section in the 100-mile trek ahead of a mule deer in search of water in Utah’s harsh seasonal changes. One laced with barrier after barrier. 

“When we look at migration corridors, we are trying to understand where there are barriers to connectivity, like roadways or places where we're seeing mortality,” says Hansen. ”But we're also looking 510 miles out from that road crossing.”

It’s More Than Just a Couple of Deer

And in the case of major roadways, what goes in does not always come out the same. Researchers in California found this to be the case when examining the populations of two carnivores on either side of one of America’s busiest highways in Los Angeles. The animals that crossed rarely went on to reproduce, one of many other signs that pointed toward clear genetic isolation created by barriers like highways and home developments.

According to Hansen, this isn’t a one-off scenario, it’s a larger trend in which the highways become a man-made form of natural selection, with the young, old, weak, or pregnant animals finding themselves at the hands of vehicle collisions. These population level impacts, particularly when they hone in on the reproducing individuals, are far more alarming than a dead deer on the side of the road. 

“It’s important to make sure that populations aren't getting isolated for genetic reasons,” Hansen explains. When the pregnant females who are responsible for introducing new genes into the population start dying off, gene flow can become severely restricted. Inbreeding increases and harmful traits like weakened immune systems and low fertility rear their ugly heads. This kind of genetic isolation can be a primary driver of a species toward extinction. And once that vehicle is set in motion, it becomes much more difficult to stop.

Building a narrower crossing was a calculated risk. In Banff National Park, Canada’s famous Redearth Overpass spans 197 feet across the Trans-Canada Highway. Utah’s Parley’s Canyon bridge, by comparison, sits at just 45 feet wide—a compromise driven by limited funding and a lack of regional research on mountain terrain overpasses.

Wildlife officials initially pushed back against the smaller design, but project managers pointed to the surrounding environment for proof that it could work.

"We see all these game trails around the mountains, often going through narrow spaces," says Sletten. "Had we not been willing to accept that risk, the project probably would not have happened."

That gamble paid off. The wide array of species adapting to the crossing—from small mammals to full-grown moose—helped win over skeptics. Financially, the project effectively paid for itself within five years by preventing costly collisions.

Experts note that the success in Parley's Canyon demonstrates that wildlife infrastructure does not always require massive capital to make a profound ecological impact. On an increasingly crowded planet where humans and animals inevitably cross paths, solutions come down to designing spaces where both can move safely.