It spans six lanes of one of Colorado's busiest highways. It is 200 feet wide and 209 feet long — nearly an acre of surface area blanketed in native grasses, shrubs, and soil. And in December 2025, after years of planning, funding, and construction, the Greenland wildlife overpass over Interstate 25 near Larkspur officially became the largest wildlife crossing structure in North America.
For the elk herds, pronghorn, mule deer, black bears, and mountain lions of Douglas County, it is simply a way home.
**The Problem I-25 Created**
Interstate 25 cuts through the heart of Colorado's Front Range, running from Denver to Colorado Springs through some of the state's most ecologically significant land. The Greenland Open Space — a sweeping stretch of grassland and rolling terrain in Douglas County — lies directly in the highway's path.
For decades, that has meant that the wildlife of the Greenland area has faced an impossible barrier. Elk and pronghorn following ancient migration routes, bears moving between seasonal foraging areas, mountain lions ranging across their territories — all of them met I-25 and faced a choice between risking a crossing of six lanes of fast-moving traffic or turning back.
Most years, the results were predictable. Animal-vehicle collisions on this stretch of highway were regular and deadly — dangerous for drivers, fatal for wildlife. Fragmented habitat meant smaller, more isolated populations with less genetic resilience. The migration routes that had shaped these animals over millennia were being severed by asphalt.
**Building the Solution**
The Colorado Department of Transportation (CDOT), working with Douglas County, the Colorado Parks and Wildlife Service, and multiple conservation partners, developed a comprehensive solution: not just a single overpass, but an **18-mile wildlife mitigation system** incorporating the overpass, multiple underpasses, and miles of wildlife-friendly fencing designed to guide animals toward safe crossing points.
At the centre of the system is the Greenland overpass itself — engineered from the ground up to be as naturalistic as possible. The structure carries a thick layer of native soil, planted with grasses and shrubs that blend seamlessly with the surrounding landscape. Animals approaching from either side encounter what appears to be a continuation of their habitat, not a human structure. Motion-triggered cameras will monitor and document which species use it, and how frequently.
The 200-foot width is deliberate — research on wildlife crossing effectiveness consistently shows that wider is better. Narrow bridges are used by some species but avoided by others; wide bridges with natural plantings attract even shy species like mountain lions, which require significant space and cover before they will commit to a crossing.
**What 90% Fewer Collisions Means**
The system is projected to reduce wildlife-vehicle collisions on this stretch by approximately **90%**. That's not just good news for elk and pronghorn — it's significant for driver safety. Animal-vehicle collisions kill thousands of large mammals each year in Colorado and cause serious accidents. On a stretch of highway with documented wildlife activity, a 90% reduction represents a meaningful safety improvement for every commuter who drives between Denver and Colorado Springs.
The fencing component is as important as the crossing structures themselves. Poorly designed wildlife crossings that aren't paired with effective fencing simply change where collisions happen rather than eliminating them. The 18-mile system treats the problem comprehensively: fence the most dangerous stretches, provide crossing opportunities at carefully selected points, and let the animals learn the safe routes.
**Animals Are Already Adapting**
Elk tracks and camera trap footage have already confirmed that animals are exploring the new overpass and the underpasses, tentatively at first, as they begin to understand that these structures are safe passage. Wildlife biologists expect crossing rates to increase significantly over the coming months as individual animals complete trips and the knowledge spreads through their social groups — particularly among elk, which are social animals that follow experienced individuals on migration routes.
Pronghorn, whose migration corridors were particularly fragmented by I-25, are considered one of the primary beneficiaries. Unlike deer, pronghorn don't jump fences well — they evolved on open plains and are built for speed, not vertical agility. The underpasses and overpass in the system have been designed with pronghorn dimensions and behaviour specifically in mind.
**A Model for the American West**
The Greenland overpass joins a growing network of wildlife crossing infrastructure across the American West — from Wyoming's renowned Trapper's Point underpasses to California's Liberty Canyon overpass over Highway 101 near Los Angeles (still under construction). Each crossing adds another link in what conservationists hope will eventually form a continental web of safe passage for wildlife.
The science on wildlife crossings is robust: they work. Animals find them, use them, and use them with increasing frequency over time. Collision rates drop dramatically. Genetic connectivity between previously isolated populations improves. The crossings pay for themselves many times over in avoided vehicle damage, emergency response costs, and wildlife management expenses.
Colorado's Greenland overpass — the biggest of all of them, at least for now — is a statement that infrastructure can be designed to move people and wildlife safely through the same landscape. Both deserve to travel without fear. 🦌
*Sources: Colorado Department of Transportation (CDOT) · Colorado Governor's Office · Denver Gazette · CPR News · Douglas County Open Space · Rocky Mountain Elk Foundation · Axios Denver (December 2025)*
