Roads are one of the most widespread human impacts on wildlife. They kill animals directly, injure and sometimes kill drivers, and split habitats into isolated patches. Over the past few decades, engineers and biologists have developed a solution that works remarkably well when it is designed carefully: wildlife crossings combined with fencing. This article explains how crossings work, what the evidence shows and why they are becoming a standard part of highway planning.
The Scale of the Problem
Collisions between vehicles and animals are common and costly. According to The Pew Charitable Trusts, a motorist in the United States hits a large animal roughly every 26 seconds, such collisions kill about 200 people a year, and their societal costs exceed 10 billion dollars annually. State Farm, one large insurer, estimated about 1.7 million animal-collision claims between July 2024 and June 2025. Deer account for most collisions, but elk, moose, bears, pronghorn and many smaller animals are also involved.
For wildlife, the harm goes beyond individual deaths. Busy highways act as barriers that prevent animals from reaching food, water, mates and seasonal ranges. Over time, isolated populations can become inbred and more vulnerable to local extinction. Migratory species such as mule deer and pronghorn may be cut off from routes they have used for generations.
What a Wildlife Crossing Is
A wildlife crossing is any structure that lets animals pass over or under a road without entering traffic. Overpasses are wide bridges covered with soil and vegetation so they look and feel like a continuation of the landscape. Underpasses range from large open-span bridges that elk and moose can walk beneath to culverts and small tunnels used by amphibians, reptiles and small mammals. Different species prefer different designs: elk, deer and pronghorn often favor open overpasses with long sight lines, while black bears and cougars readily use underpasses.
Why Fencing Is Essential
A crossing without fencing is often ignored, because animals simply continue crossing the road where they always have. Fencing along the highway guides animals toward the structures. Effective systems also include features that prevent animals from getting trapped on the road side of the fence: escape ramps or jump-outs that let an animal that wanders onto the roadway climb out, and wildlife guards, similar to cattle guards, at places where the fence must be interrupted for side roads. The combination of crossings, fencing and escape features is what produces the large reductions in collisions seen at successful sites.
Evidence From Banff National Park
The longest-running example is along the Trans-Canada Highway through Banff National Park in Alberta. According to Parks Canada, the park has 6 overpasses and 38 underpasses, along with 82 kilometers of highway fencing. Wildlife-vehicle collisions fell by more than 80 percent after fencing and crossings were installed, and by more than 96 percent for elk and deer. By 2012, eleven species of large mammals, including grizzly and black bears, wolves, cougars, moose, elk, deer, bighorn sheep, wolverines and lynx, had been recorded using the crossings more than 150,000 times since monitoring began in 1996. The long monitoring record also showed that some species, especially grizzly bears and wolves, needed several years to learn to use the new structures, a reminder that evaluations should not be rushed.
Evidence From Colorado’s Highway 9
In the United States, a frequently cited project is on Colorado State Highway 9 between Green Mountain Reservoir and Kremmling, a stretch that crosses mule deer winter range. Completed in 2016 by the Colorado Department of Transportation with Colorado Parks and Wildlife and partners, the 15.7 million dollar project included two overpasses, five underpasses, 10.3 miles of fencing, 29 wildlife guards and 61 escape ramps. Monitoring from late 2015 to 2020, presented by Colorado Parks and Wildlife biologist Michelle Cowardin, found that wildlife-vehicle collisions fell by more than 90 percent. Mule deer crossed using the structures 112,678 times over five years, elk used the underpasses 489 times, and seventeen species in total were recorded using them.
Do They Pay for Themselves?
Crossings are expensive to build, but studies frequently find that, on high-collision road segments, the costs avoided, including vehicle repairs, medical care, emergency response and the value of the wildlife killed, can exceed construction costs over the life of the structure. The economics are strongest where collision rates are high and where crossings are placed precisely on known wildlife movement corridors, which is why agencies rely on roadkill data, GPS-collar tracking and local knowledge to choose locations. Pew notes that crossings with directional fencing can reduce collisions by up to 97 percent for certain species.
A Landmark Project in California
The Wallis Annenberg Wildlife Crossing over U.S. Highway 101 at Liberty Canyon in Agoura Hills, near Los Angeles, is described as the largest wildlife crossing in the world, spanning ten lanes of freeway and an adjacent road. The project, which costs about 114 million dollars, aims to reconnect the Santa Monica Mountains with habitat to the north. Mountain lions in the Santa Monica Mountains have been hemmed in by freeways and development, leading to inbreeding that threatens their long-term survival; the famous cougar P-22 became a symbol of that isolation. According to Spectrum News, a ribbon-cutting was scheduled for December 2, 2026. The crossing is also designed for smaller animals, including reptiles, small mammals and insects, with native plantings across the top.
Federal Funding and Policy
The U.S. Infrastructure Investment and Jobs Act created the Wildlife Crossings Pilot Program with an initial five-year allocation of 350 million dollars. Pew reports that applications have far exceeded available funding, and in 2026 Congress was considering bills to extend and expand the program as part of the next federal surface transportation reauthorization. Many states, including Colorado, Nevada, Utah, Wyoming and Washington, have also built their own crossing programs.
Limits and Design Lessons
Crossings are not a cure-all. Poorly placed structures, gaps in fencing or noisy, brightly lit designs can lead animals to avoid them. Fencing ends are a known weak spot where collisions can concentrate if the fence stops too soon. Smaller animals need structures suited to their size and behavior. And crossings work best as part of larger efforts to protect the habitat on both sides of the road; a bridge that leads to a housing development does little for wildlife.
The Bottom Line
Well-designed wildlife crossings paired with fencing have reduced wildlife-vehicle collisions by more than 80 to 90 percent at some of the best-studied sites in North America. They protect drivers, prevent costly crashes and reconnect landscapes that roads have divided, making them one of the clearest examples of infrastructure that benefits both people and wildlife.
Sources
- Parks Canada: Wildlife crossing structures and research, Banff National Park
- Summit Daily: Study: Crossings on Highway 9 reduced wildlife collisions by 90%
- The Pew Charitable Trusts: Experts highlight urgent need for federal investment in wildlife crossings
- Spectrum News: The Wallis Annenberg Wildlife Crossing on track for December 2026
Featured image: Coolcaesar / Wikimedia Commons, CC BY-SA 4.0
