AASHTO Journal - Montana Considers Wildlife Undercrossing for I-90

As wildlife-vehicle collisions on I-90 in western Montana pile up, the Montana Department of Transportation is launching a feasibility study for construction of a wildlife undercrossing – one that could also restore the natural flow of a creek to help with fish crossings.

[Above photo by HDR Engineering]

The study area is a 7.3-mile segment of I-90, centered on Sixmile Creek in Missoula County. Home to grizzly bears, black bears, elk, and deer, it is a “key linkage area” for wildlife between the Ninemile Valley and Bitterroot Range, according to Ben Schendel, project manager for Montana DOT.

Less than a mile south of I-90, Sixmile Creek connects to the larger Clark Fork River, which attracts wildlife from the forest and helps form a “movement corridor for multiple species of wildlife between large tracts of protected public lands to the north and south,” Schendel said. “Opportunities for wildlife to safely cross I-90 in this area are currently limited.”

More than a third of reported crashes in the corridor involve wildlife, nearly triple the statewide ratio, according to Montana DOT data. Multiple news sources reported in July that 11 elk trying to cross I-90 in the corridor were killed in one collision with a tractor-trailer. Additionally, a recent State Farm study showed that drivers in Montana were the second-most likely in the country to be involved in a wildlife-vehicle collision, with the odds placed at 1-in-53. (West Virginia ranked first at 1-in-40, the study found.)

Wildlife-vehicle collisions can be deadly for animals and motor vehicle occupants. According to a 2021 report by the Pew Research Center, as many as 2 million such collisions occur every year in the U.S., killing about 200 people, injuring about 26,000, and causing at least $8 billion in damages and costs.

The proposed project would replace a 60-year-old culvert under I-90 with two vehicular bridges, eastbound and westbound, that would leave enough room below for a rehabilitated Sixmile Creek and a wildlife undercrossing. The pipe culvert under I-90 chokes down Sixmile Creek, altering its natural characteristics, leaving no room for wildlife to cross, and hindered the ability of trout to spawn.

Nearly 14 miles of eight-foot wildlife fencing along the interstate would steer wildlife to the undercrossing. Double cattle guards, electric mats and earthen jump outs near the fencing would discourage the wildlife from getting into the interstate right-of-way.

The Montana Fish, Wildlife, and Parks agency has “long recognized the Sixmile Creek culvert as an impediment to upstream passage for all fish species,” Schendel said. “The project would improve fish passage by removing the culvert and reconstruction the stream channel under the new bridges.”

Montana DOT held an in-person open house in October about the project and has a virtual open house on its project website. The site will take public comments through November 12.

While the agency currently does not have funds to build this crossing, it has scheduled activities in anticipation of a future funding decision, including stream design, preliminary bridge layout, and developing construction cost estimates.

A final report on the study is scheduled for August 2026, Montana DOT noted.