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"It's looking promising so far," says Mike McVaugh, the CDOT traffic and safety engineer overseeing the project. "The crews are seeing fewer carcasses."
Colorado's wildlife experts have benefited from studying what Canada and other states have done, as well as from detailed field work, examining critical crossing points and how the animals respond to different types of structures. It's not enough, they say, to stick a pipe under a highway and expect critters to come flocking to it.
"There's so much variability, even within a given species," explains Chris Haas, a senior biologist at SWCA Environmental Associates. "But we do have a lot of information about what works and what doesn't. You have to make sure that you're spending the money in the places that are most likely to be used."
Haas spent years studying various forms of wildlife crossings for California's highway department. More recently, as a research associate at Colorado State University, he was involved in an extensive CDOT study of Colorado crossings, including key linkage points on Wolf Creek Pass and U.S. 285. One new underpass on 285 near Conifer was a hit from day one, he says: "The mule deer were using it even before it officially opened."
The structure worked, he suggests, because it was built in the right place, with the right kind of vegetation and the needs of its customers in mind. Large ungulates, such as deer and elk, use culverts warily; they prefer the larger spaces under span bridges. "The longer the culvert is, the higher it needs to be to reduce the tunneling effect," Haas notes.
Some prey species want unencumbered sightlines and avoid going under bridges that have ledges where predators might lurk; others seek lots of cover. Bobcats and coyotes will use drainage-pipe culverts, but not if they get too small; yet Haas has also seen a moose use a culvert that was hardly optimal for its size. Many animals balk at a concrete surface or a submerged one. Then there are juveniles or transients, non-alpha members of a pack or herd, which might use structures that others don't.
A host of other considerations go into siting and design. Should fencing be used to help channel wildlife to the right spots? How long can the barrier be before animals attempt to breach it? And why are they trying to cross the road in the first place — in other words, where are they trying to go?
"Getting them across the road safely is just one part of the puzzle," says ecologist Kintsch. "If they don't have protected habitat on both sides, you don't have a linkage. You don't have a place for them to go. The land needs to be managed appropriately, too."
The argument for a wildlife bridge on I-70 near Vail Pass arises out of years of study by state agencies and environmental groups, seeking to identify the most advantageous points for breaching the Berlin Wall. The location is an ideal one, backers say, in part because of the presence of the relatively undisturbed Holy Cross and Eagle's Nest wilderness areas on either side of I-70.
"It isn't an area where you're going to see the most roadkill," Kintsch says. "It's not a major migratory pathway; you don't have huge numbers of deer and elk passing through on a seasonal basis. But it is a high-priority crossing for ecological reasons."
Three years ago, when she was a program director at the Southern Rockies Ecosystem Project (now part of the Center for Native Ecosystems), Kintsch helped coordinate a "citizen science" wildlife monitoring effort. Volunteers set up motion-triggered cameras at potential crossing areas along I-70 between Copper Mountain and Vail and returned frequently to swap out memory cards. By the time the project concluded in 2008, the cameras had recorded thousands of images of wildlife — a teeming, largely nocturnal world of deer, elk, bears, coyotes and others on the fringes of the highway.
"We got a good sense of how active they were," says Paige Bonaker, a staff biologist at the Center for Native Ecosystems who worked on the project. By using hair snares, scat and track surveys and other methods, Bonaker was able to determine that there were additional animals in close proximity to the interstate, including pine martens, that weren't always picked up by the cameras.
Among the busiest crossing points were areas down the valley on the west side of the pass, closer to Vail, where high bridge spans had left roomy passage underneath. The bridges hadn't been built with wildlife in mind, but they worked perfectly; the four-legged pedestrians had little problem moving at will through the area.
So why build an expensive overpass further up the pass? Because the bridges end as you head east, leaving miles of deadly at-grade crossing. Because large animals prefer the openness of an overpass to the confined space of a culvert. Because an interagency panel convened to study the issue identified milepost 187.4 as the ideal site, in terms of vegetation, topography, a relative lack of a human recreational presence (compared to, say, the top of the pass) and high usage by wildlife.