Success of wildlife underpass may encourage state to build more

    The wildlife underpass on Highway 21 between Boise and Idaho City has reduced animal/vehicle collisions. IDFG photo.

    by Nate Green and Joe Evancho

    BOISE — Every spring and fall since 2010, an Idaho Fish and Game truck has driven along Highway 21 in the Boise foothills northeast of town, searching for roadkill. It is during these times of year that deer and elk migrate to and from their wintering grounds on the Boise River Wildlife Management Area.

    Fish and Game staff keep an inventory of the animals that die after getting hit by traffic on the thirteen-mile stretch of the highway between Warm Springs Avenue and Robie Creek Road. A large smear of blood or a crumpled carcass lying at the side of the road are the tell-tale signs that a wildlife-vehicle collision has occurred.

    This section of highway is considered one of the state’s worst for vehicle collisions with between 150-200 mule deer and 5-10 elk killed each year before the completion of the underpass..

    Once the wildlife find their way they continue to use the underpass and they migrate up from the Boise River drainage. IDFG photo.
    Once they find their way to the underpass, wildlife will continue to migrate up from the Boise River drainage in the spring and make the return trip in the fall. IDFG photo.

    But thanks to a wildlife underpass constructed at milepost 18 in 2010, Wildlife Habitat Biologist Krista Muller said that “more mule deer and elk have been going under the highway, than over it.” Muller points to photos that show mule deer and elk, and the occasional fox, coyote or mountain lion, utilizing the underpass.

    During the spring, digital images from movement-sensitive cameras show that an increasing number of animals are crossing under Highway 21 from the lower reaches of the Boise River drainage moving to higher elevations for the summer.

    “They know where the underpass is and they’re definitely using it,” Muller said.

    Funding to finish the fencing

    Another component of the underpass project called for wildlife exclusion fencing on both sides of the highway. The fencing is necessary to funnel wildlife from both sides of the highway to the underpass. Fencing was installed on the west side of the highway on both sides of the underpass in 2010, but additional fencing is needed on the east side of the highway.

    Presently, three-quarters of the fence installation on the east side of the highway has been completed. The task at hand, as it has been since the underpass was completed, is to get the funding to complete the fencing on the east side. Muller said that “approximately $30,000 is still needed to complete this aspect of the project.”

    The project also calls for the installation of a wildlife deterrent system on Spring Shores Road, where there is a gap in the fencing. Fish and Game is considering the installation of an electric mat that will act much like an invisible fence that people use to keep their dogs in their yards.

    This four-foot wide mat produces a very light shock to any animal stepping on it, keeping them from entering the roadway. Additionally, a pedestrian gate or shut-off switch will be installed that will allow people and their pets to cross safely through the system.

    Muller said that “discussions with the appropriate agencies and departments regarding what type of system should be used will be conducted soon. The installation of the system may cost up to $60,000.

    Deer crossing through the underpass at night. Once the final stage of fencing is installed travel safety along the highway will be greatly improved. IDFG photo.
    Deer crossing through the underpass at night. Once the final stage of fencing is installed more studies will be conducted to measure the effectiveness of the program with the belief that travel safety along the highway will be improved. IDFG photo.

    Model for other crossings

    Officials with the Idaho Transportation Department (ITD) say that the Highway 21 underpass could act as a model for similar projects around the state. ITD environmental planners Greg Vitley and Scott Rudel said that the underpass, which was primarily funded by a $1.1 million federal stimulus package, will improve migration routes for deer and elk.

    Though some critics scoff at the price tag, arguing that a few dead deer and elk don’t necessitate a million-dollar project, Vitley and Rudel say that calculations show that the underpass will eventually pay for itself. They estimate that Highway 21 drivers incur about nearly half a million dollars in damages each year through a combination of insurance claims, deductibles, medical costs and disability leave along with the resources spent in recovering road kill. For the particular three-mile section serviced by the underpass, they estimate savings of $20,000-$40,000 annually.

    “We’ve seen safety improvement in this area already,” Vitley said. “Once we put up more fencing, we think we will see a lot more success and we hope that the public will see the benefits of these kinds of structures.”

    “We’re talking 600,000 to a million people living in the area,” Rudel said. “Considering the growing population center, and the fact that the Boise River Wildlife Management Area is critical winter habitat, this problem is not going to go away.”


    The idea for the underpass originated in August 1997 when local, state and federal agencies met to consider ways of mitigating vehicle-wildlife collisions in Idaho. They created a statewide database of wildlife-vehicle collisions and determined that the Warm Springs Avenue-Highway 21 corridor was the highest priority in the state.

    After the formation of the Boise River Wildlife Linkage Partnership – a coalition of government organizations and citizens – ITD began investigating potential areas for constructing wildlife passages along Highway 21. Using more than 30 years of data collected by Fish and Game, they mapped migration routes and pinpointed areas of high deer fatalities.

    In 2009, ITD applied for a grant through the American Recovery and Reinvestment Act and received about $1.1 million and they chose milepost 18 for the underpass based on an analysis that combined environmental and engineering concerns.

    Fencing crosses the valley protecting vehicles and wildlife alike. IDFG photo.
    Fencing crossing the hillside near Lucky Peak helps protect vehicles and wildlife alike by limiting access to the highway. IDFG photo.

    On the east side of the highway, several draws converge into the underpass from the ridges above, creating a natural funnel for wildlife migrating down from the Sawtooth Mountains. The hillsides in this area show terracing from the historical migrations of deer and elk through the corridor.

    The west side of the underpass leads to Lucky Peak Reservoir, Mores Creek and summer pastures. The topography allowed engineers to construct the underpass without cutting off traffic on Highway 21. Essentially, crews were able to tunnel under the highway, retrofitting the underpass rather than constructing an entirely new structure. This helped keep costs relatively low.

    As of January 2015, the project has resulted in a 30-foot wide, 15-foot high underpass, along with 6,600 feet of 8-foot high fence that parallels the highway to the west in order to funnel wildlife toward the underpass. The eastside fencing remains incomplete however, and the final phase of construction includes extending the fences along the uphill side of the highway.

    The Boise River Wildlife Linkage Partnership is still trying to secure funding to continue with the fence construction. The project is scheduled for completion in December of 2017.

    Looking to the future

    Muller says the Boise River Wildlife Linkage Partnership is committed to developing other vehicle underpasses along Highway 21.

    “Now that we’ve got it and can see that it works, I think people are going to say let’s build similar underpass projects in our plans for the future,” she said.