Along a 14-mile stretch of Colorado's Interstate 25, a concrete artery that carries 100,000 cars per day through the exurbs south of Denver, wildlife once faced a deadly choice: turn back or die trying to cross. Over just two years, from 2018 to 2020, the Colorado Department of Transportation tallied collisions with 76 deer, 15 bears, and 10 pumas on that single section of highway. But mule deer, elk, and black bears weren't just being hit—they were being permanently severed from the landscapes they needed to survive, unable to move between alpine forests and Colorado's eastern prairies.
That's where a global movement to heal fragmented ecosystems is making tangible progress. Animal collisions with vehicles cost the U.S. economy more than $10 billion annually, a fact that has motivated both conservation and public safety efforts. Roads pose what researchers call a "wall" effect: some species won't even attempt to cross. Yet decades of evidence shows that wildlife crossings—underpasses and overpasses, paired with roadside fencing—can dramatically reduce these collisions while reconnecting broken corridors that animals desperately need for survival, especially as they seek food, water, and refuge from increasingly extreme wildfires and weather.
Colorado's solution stands as one of the most ambitious examples of this emerging practice. In 2021, the state completed five spacious, dirt-floored underpasses and installed more than 25 miles of roadside fencing to allow wildlife safe passage beneath I-25. Then in December 2025, Colorado finished constructing an overpass near Greenland that measures 200 feet wide by 209 feet long—61 by 64 meters—making it one of the largest human-made wildlife crossings on Earth. That structure arcs over six lanes of traffic, a feat of engineering that signals how seriously this work has become.
The results are compelling. CDOT projects that this network of six passages will reduce roadkill by 90 percent and reunite 40,000 acres of fragmented habitat—an area roughly the size of a small county. "We're connecting some huge wildlife populations," says Andy Hough, environmental resources coordinator for Douglas County, who worked with CDOT to plan the crossings. The project demonstrates what's possible when transportation agencies prioritize ecological corridors alongside human mobility.
This Colorado work sits within a fast-growing international movement. Wildlife crossings originated in France during the 1950s with green bridges built for deer, then spread throughout Europe as countries like Germany, Switzerland, and the Netherlands recognized that fragmented landscapes couldn't sustain viable animal populations. The Dutch invested heavily in both overpasses and "toad tunnels" for migrating amphibians, establishing themselves as leaders in the field. Now, similar ambitions are emerging in North America: New Jersey recently passed legislation requiring its transportation and wildlife agencies to plan statewide wildlife crossings, particularly to create safe passage for bobcats in the mountainous northeast region known as "Bobcat Alley."
Experts believe the challenge is solvable within a generation. Renee Callahan, executive director of ARC Solutions, a group studying and advocating for wildlife crossings globally, frames it simply: "With sufficient investment, we could solve this problem in a generation." The question now is whether governments and communities will commit the resources needed to transform roads from barriers into connectors—allowing species from Florida panthers to Asiatic cheetahs to move freely through landscapes that humans and animals increasingly must share.