A wildlife crossing is a bridge or a tunnel, plus the fence that funnels animals to it, built so an animal can get across a road without meeting a car. It is also where the confusion starts, because the crossing is the part you can see from the driver's seat, and the part that decides whether it works is the part you cannot.
This post is for the people who fund, build, and operate roads: what a wildlife crossing is in engineering terms, the four components that make one work, which species will actually use which structure, and why a crossing with no protected land on either side is a bridge to nowhere.
what a wildlife crossing is
A wildlife crossing is a purpose-built structure — an overpass, an underpass, or a retrofitted culvert — combined with fencing that guides animals to it, so they can move across a highway, railway, or canal without entering traffic. Transportation agencies file it under safety and structures. Biologists file it under connectivity. Both are right.
Two clarifications, because they are the ones that get lost between the agency and the advocate.
First, a crossing is gray infrastructure. It is concrete, steel, earth fill, and chain-link, designed to a load rating and inspected on a schedule like any other bridge. Planting the deck does not make it habitat; it makes it a bridge animals are willing to walk over. If you arrived here from the living infrastructure dialect: a crossing is not that. It is gray structure that serves a living route.
Second, a crossing is not the corridor. The wildlife corridor is the land — the valley, the ridgeline, the riparian strip — that animals actually move through. A crossing is the point where that land meets the pavement and needs help. Hold that distinction; the rest of this post depends on it.
A wildlife crossing is gray infrastructure that serves a living route. It is not the route.
the four parts of a working crossing
Engineers talk about a crossing system, not a bridge, because a structure by itself does very little. Four components carry the load.
overpass
A wildlife overpass is a bridge over the road, typically topped with soil and vegetation and bermed or walled at the edges to screen traffic noise and headlights. It is the most expensive option and the one that open-country species need. Elk, mule deer, and especially pronghorn want to see the far side before they commit; pronghorn in particular avoid enclosed spaces and will walk around a tunnel rather than through it. Build for them and you build wide, open, and with clear sightlines.
underpass
A wildlife underpass runs the animal beneath the road: a span bridge over a creek or draw, an arched culvert, a box structure. It is cheaper than an overpass, often rides along with a drainage crossing the road already needed, and suits species that tolerate or prefer cover. Black bears, mountain lions, coyotes, and bobcats use them readily. So do deer where the opening is generous. Florida's panther underpasses on Interstate 75 are the long-running U.S. proof. The Florida panther post is the adjacent reading on that animal, not the engineering history.
culvert retrofit
A culvert retrofit modifies a drainage structure that already exists so animals can use it: a dry ledge or shelf above the waterline, a natural substrate floor, a wider or taller opening, and removal of the perched outlet or debris that blocks fish and salamanders from moving upstream. It is the least glamorous of the four and often the best value per dollar, because the hole in the road is already there.
fencing
Wildlife exclusion fencing runs along both sides of the highway and funnels animals to the structure. It is not optional and it is not an accessory. An overpass with a gap in the fence a mile away is a very expensive photo opportunity, because deer do not read signs; they cross where they always crossed. Escape ramps let an animal that ends up inside the fence get back out. Where a fence ends, crashes cluster, which is why end-of-fence treatment is its own design problem.
Fencing without a structure also fails, in a different way. It keeps the herd off the road and off the other half of its range. That is a wall, not a crossing.
who uses what
Species are not interchangeable and neither are structures. The table is the short version of what Banff's monitoring and a generation of road-ecology fieldwork have taught agencies.
| structure type | who uses it | what it cannot do alone |
|---|---|---|
| overpass (open, vegetated, bermed edges) | elk, pronghorn, mule deer, grizzly bear, wolves — species that want sightlines | keep animals off the road between structures; supply habitat at either end |
| underpass (span bridge, arch, box) | black bear, mountain lion, bobcat, coyote; deer where openings are large; smaller carnivores where cover is present | draw open-country herds; stay usable when it floods or fills with debris |
| culvert retrofit (ledge, substrate, enlarged opening) | fish, amphibians, turtles, small mammals, some mid-sized carnivores | serve large ungulates; steer small animals in without low barrier fencing |
| exclusion fencing + escape ramps | everything above — by directing it | provide passage; a fence without a structure severs the range |
Two consequences follow. A system usually needs more than one kind of structure, because a canyon that funnels both elk and salamanders needs an open span and a small tunnel, not a compromise between them. And the species you design for sets the cost, which is why the largest structures in North America are the ones built for elk and pronghorn.
four crossings, one sentence each
Banff. Parks Canada paired overpasses and underpasses on the Trans-Canada Highway with continuous fencing, and wildlife-vehicle collisions on the mitigated sections fell by more than 80 percent — the longest-running wildlife-crossing research program in the world, and the reason every agency now says "structure plus fence."
Wallis Annenberg Wildlife Crossing. Over the 101 freeway at Liberty Canyon in Agoura Hills, California, the roughly 210-by-165-foot vegetated overpass — about $114 million, state transportation funds plus private philanthropy — is scheduled for dedication on December 2, 2026, built to reconnect a mountain lion population that had been inbreeding on an urban island.
I-25 Greenland overpass. Completed by CDOT in late 2025 between Larkspur and Monument, Colorado, at roughly 200 by 209 feet the largest wildlife overpass in North America, designed open and wide for elk and pronghorn and finishing a system whose underpasses and fencing were already in the ground.
I-70 West Vail Pass. Across Colorado on Interstate 70, CDOT is finishing a set of underpasses sized for large and for small-to-medium animals, with fencing to guide them in, as part of an auxiliary-lanes project due to wrap in late 2026 — the ordinary case, where crossings ride along with a road job.
None of these is a critique target. The gap this series is about is what surrounds them.
do wildlife crossings work?
Yes, when the structure and the fence are built together and sited where animals already move. The Banff result — more than 80 percent fewer collisions on mitigated sections — has held for decades. CDOT expects its I-25 South Gap system of underpasses, the Greenland overpass, and fencing to cut wildlife-vehicle crashes on that stretch by about 90 percent. The Federal Highway Administration puts large-animal collisions in the United States at one to two million a year, so the arithmetic on a busy migration route is not subtle.
What the numbers do not say is that a crossing restores a route. It removes the pavement from the list of things that kill the animal on that route. Whether there is still a route — winter range on one side, summer range on the other, a place to go — depends on the land.
A crossing takes the road off the list of things that kill the animal. It cannot put back the land that was already gone.
the book that named the field
Ben Goldfarb's Crossings (2023) is the reason a general reader can now say "road ecology" without a footnote. Its opening fact is scale — some 40 million miles of roadway on Earth, about 64 million kilometers — and its argument is that the largest human artifact on the planet has been mostly invisible as a conservation problem. He spends time on the overpasses and the toad tunnels, but the thesis that survives the book is simpler: to protect wildlife, you have to protect how they move. This series takes that as its spine, and the migration post picks it up. Here it does one job — it explains why the transportation agency and the conservation biologist have started showing up at the same meetings.
the structure is not the route
Here is the trap the ribbon-cutting hides. A crossing is funded as a capital project: a line in a DOT budget, a federal pilot grant, or a named gift. It gets built, it gets dedicated, and too often the surrounding land is treated as someone else's problem. But an overpass connects whatever is on either end of it. If one end is a subdivision in ten years, the structure still stands and the route is still severed.
The land on both sides is a different asset with a different owner and a different funding problem. It is ranches, county open space, conservation easements, a land trust's balance sheet. It is not in the transportation budget and it does not come with a ribbon. Greenland works because Douglas County and its partners spent decades holding open land on both sides of Interstate 25 before the bridge went in. Annenberg's partners announced a long-term research, restoration, and maintenance commitment alongside the dedication because, as the National Wildlife Federation's Beth Pratt put it at the dedication announcement, it is "a beginning, not an ending."
A crossing designer should not hear that a vegetated bridge is the corridor. A conservationist should not hear that easements alone will get the herd across six lanes. The route needs both.
build the structure and hold the land
On ensurance, two accounts are meant to split the work — the structure and the land — even though the live corridor page still also names crossings. The split is the job. The current pages are not yet a clean wall between them.
wildlife-crossing.syndicate coordinates funding for the structure: feasibility studies, construction of overpasses and underpasses, culvert retrofits, fencing. wildlife-corridor.syndicate coordinates funding for the land — easements, acquisition, habitat on both sides of the pavement. Each is a named account that receives proceeds and routes them to a named route, so a transportation agency, an infrastructure investor, or a county can fund the piece it is responsible for and see the other piece funded next to it.
Both accounts are live. Volumes are small and we say so. The point at this stage is the distinction, not the size: if you are a DOT, the structure is your job and the land is someone else's; if you are a land trust, it is the reverse. A shared ledger for the route is how the two stop finishing their halves in different decades.
Read next: the carcass is the invoice — what the collision data can and cannot tell you about hawks, owls, bears, and foxes. Start at the hub: wildlife corridors: the infrastructure of survival. If you operate roads or rail: what this looks like for infrastructure operators.
frequently asked questions
what is a wildlife crossing?
A wildlife crossing is a purpose-built structure — an overpass, an underpass, or a retrofitted culvert — paired with fencing that guides animals to it, so wildlife can cross a road, railway, or canal without entering traffic. It is gray infrastructure that serves a living route; the surrounding land is the corridor.
what is a wildlife overpass?
A wildlife overpass is a bridge that carries animals over a road, usually topped with soil and vegetation and bermed or walled at the edges to screen traffic. It is the structure open-country species such as elk, pronghorn, and mule deer will use, because they need to see the far side.
what is a wildlife underpass?
A wildlife underpass carries animals beneath a road through a span bridge, arch, or box structure, often built into a drainage crossing. Black bears, mountain lions, coyotes, and deer use underpasses readily; it is the cheaper structure and the more common one.
do wildlife crossings work?
Yes, when structures and exclusion fencing are built together at a location animals already use. Banff's mitigated Trans-Canada sections cut wildlife-vehicle collisions by more than 80 percent, and CDOT expects roughly 90 percent fewer wildlife crashes on the I-25 South Gap system. A crossing alone does not restore a route; the land on both sides has to stay habitat.
the series
- wildlife corridors: the infrastructure of survival — the hub
- what a wildlife crossing actually is — overpass, underpass, culvert, fence (this post)
- the carcass is the invoice — roadkill, and the species numbers we do and do not have
- to protect wildlife, protect how they move — Goldfarb and the migration case
- who pays for a wildlife crossing — the payor map
- a bridge without the land is still an island — hold the route
Adjacent reading: elk.syndicate and winter range, the Caloosahatchee crossing, what living infrastructure actually is.