Boulder County has identified six potential wildlife crossing locations along U.S. 36 between Boulder and Lyons and is moving into early engineering work as Colorado prepares to create a new statewide funding source for wildlife collision prevention projects.
The proposed crossings, which could include overpasses, underpasses and fencing between Nelson and Hygiene roads, are intended to reduce collisions along one of Colorado’s deadliest highway corridors for wildlife.
The effort received a boost this spring after state lawmakers passed legislation creating an optional $5 vehicle registration fee beginning 2027 to help fund wildlife collision prevention projects statewide. Gov. Jared Polis is expected to sign the bill.
County officials say the timing could significantly improve the county’s chances of securing long-term funding for the project, which remains in the planning stages but has broad public backing.
Highway 36, also known as North Foothills Highway, is among the worst roads in Colorado for wildlife-vehicle collisions. The corridor cuts through thousands of acres of open space at the ecological transition between the mountains and plains.
“The highway was constructed in the transition space, and that is really hard on migratory animals,” said county Environmental Resources Specialist Susan Spaulding.
The stretch averages 3.2 animal collisions per mile annually, compared with a countywide average of 0.26.
Many animals require large ranges to find food and shelter, while others migrate seasonally across long distances. In Boulder County, animals often cross Highway 36 while moving to lower elevations during winter and higher elevations during summer.
Wildlife collisions are deadly for both animals and motorists and can carry steep financial costs. Coloradans incur an estimated $320 million per year in costs related to wildlife collisions, the highest rate in the West, Spaulding said.
County residents told officials in a survey that many avoid Highway 36 at night. Some said they do not allow their teenagers to drive the corridor after dark or during dawn and dusk, when animal activity is highest.
Environmental journalist Hillary Rosner recalled driving north of Boulder one evening in early summer. “There were just so many animals darting out onto the road, from deer to raccoons and other kinds of smaller mammals.”
Rosner recently published “Roam,” a book about fragmented natural habitats and efforts to reconnect them. Allowing wildlife to move across long distances is critical for healthy ecosystems, she said, because limiting wildlife movement can not only cut animals off from food and shelter but also disrupt broader ecosystems.
The proposed solution is a network of wildlife crossings, including overpasses, underpasses and potentially culverts, at six identified locations between Nelson and Hygiene. Fencing would funnel animals toward the crossings. Rosner compared it to the way CU Boulder guides students to underpasses beneath Broadway.
Wildlife crossings have proved highly effective elsewhere, with studies showing they can reduce collisions by as much as 90%. Colorado has already built dozens of crossings, including projects on Highway 13 north of Craig and Highway 160 west of Pagosa Springs. A crossing on I-25 between Castle Rock and Monument cost around $15 million to construct.
“Wildlife overpasses are such a simple, obvious solution,” Rosner said. “They’re also cool because they illustrate the problem.”
What the county is planning
Boulder County began the corridor project in 2024 with public outreach. The county has since refined the proposal and expects pre-engineering work to finish by the end of June. County staff did not provide a cost estimate, saying expenses will depend on the final design, but a 2024 county study estimated one overpass project would cost around $16 million to $18 million, including construction, fencing and maintenance.
“With all big-picture transportation projects, it takes a long time. They’re completed, they’re long term, so you just gotta keep moving forward step by step,” Spaulding said.
The proposed project area stretches from Lefthand Creek Bridge to near the St. Vrain River. Officials identified crossing locations using elk-collaring data and assessments of where construction would be feasible. Elk and moose are expected to be the largest animals using the crossings, though the structures are also meant to help small mammals, reptiles and amphibians.
Once designs are approved by Colorado Parks and Wildlife and the Colorado Department of Transportation, the county plans to move into structural engineering, including fencing design.
Spaulding said funding remains one of the largest challenges but noted that wildlife crossings have bipartisan political support because they benefit both public safety and wildlife conservation. A 2021 bipartisan infrastructure law allocated $350 million for wildlife crossing projects nationwide.
“We’re encouraged that grants will continue to be supported at the federal level long term,” Spaulding said.
In Colorado, Polis is expected to sign the Wildlife Collision Prevention bill, which would create the opt-in $5 vehicle registration fee beginning in January 2027.
“It’s going to provide reliable and consistent funding for projects, either directly or as matching funds,” Spaulding said.
The county has also seen strong local support for wildlife corridors. Surveys found that 94% of respondents supported the proposal. Officials say Boulder County’s extensive protected open space network gives them confidence to know that land for the crossings will remain preserved for decades.
The stretch of highway between Boulder and Lyons is also especially dangerous for cyclists. The North Boulder Bikeway, which would create an 11-mile separated bike path, is also under development. Some cyclist safety structures, including retaining walls and barriers, can inhibit wildlife movement. Staff on the two projects are coordinating to avoid conflict.
“Our road ecology team is also working closely with bikeway designers to make sure the projects don’t get in each other’s way,” Spaulding said.
The county’s survey on wildlife crossings is still open and has received over 800 comments.
