Wildlife crossings give animals a route over or beneath a road instead of across moving traffic. Overpasses, underpasses, adapted culverts, dry ledges and canopy bridges are paired with fencing that keeps animals off the pavement and guides them to the safer passage. They can reduce collisions and reconnect habitat, but results depend on the species, landscape, structure design, fencing and monitoring.
Why roads are dangerous for wildlife
A busy road can cut through feeding grounds, breeding areas, migration routes and daily travel paths. An animal that reaches the pavement may face fast-moving vehicles, while a road itself can separate habitat on opposite sides. The Federal Highway Administration (FHWA) says congressional findings used to create its Wildlife Crossing Program identified more than 1,000,000 wildlife–vehicle collisions each year, more than $8 billion in associated costs, tens of thousands of serious injuries and hundreds of human fatalities. Those figures are the program’s cited congressional findings, not a new 2026 estimate. FHWA Wildlife Crossings Program
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A crossing addresses both problems: it provides a predictable route across the transport corridor and helps preserve movement between habitat patches. That second benefit matters even when an animal is not in immediate danger. A road can function as a long-term barrier, reducing access to food, mates, shelter or seasonal ranges.
How a wildlife-crossing system works
1. A structure carries animals over or under traffic
An overpass forms a land bridge above the road. An underpass carries animals below it. Existing culverts or bridges can sometimes be adapted with a dry ledge so animals have a passable strip beside water. In forested settings, a canopy bridge can connect treetops for species that rarely travel on the ground. The Costa Rica study describes all of these approaches, while FHWA stresses that structure type and dimensions must match the intended species and landscape.
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2. Fencing blocks unsafe entry points
Animals do not automatically find a crossing. Continuous fencing along the road edge can prevent them from stepping onto the carriageway and guide them toward an opening at a crossing. FHWA describes this funneling function as closely linked to crossing performance. Fences need suitable ends, gates and connections; a gap can become a new collision point or allow animals to bypass the structure.
3. Escape features help animals trapped inside the road corridor
If an animal enters the fenced right-of-way from a driveway, damaged fence or other opening, an escape ramp can let it leave without crossing traffic. New Mexico’s U.S. 550 Wildlife Corridors project includes escape ramps in its planned design, along with eight-foot woven-wire game fencing.
4. Habitat cues make the route usable
Animals respond to cover, vegetation, moisture, light and the openness of a passage. The New Mexico project says its crossing areas will use vegetation intended to match adjacent habitat. Designers also consider where animals already move, rather than placing a structure only where construction is easiest.
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| Crossing type | How it works | Design context described by the sources |
|---|---|---|
| Overpass | A planted or otherwise passable structure carries animals above the road. | Used where species will approach an elevated route; the Nevada mule-deer study recorded more successful crossings at overpasses than underpasses in its study setting. |
| Underpass | A passage beneath the road provides a grade-separated route. | Can be built specifically for wildlife or combined with fencing; the North Carolina U.S. 64 project used three underpasses. |
| Adapted culvert or bridge with a dry ledge | A water-management or bridge structure retains a dry strip for animal movement. | Described in the Costa Rica research as one option for roads associated with biological corridors. |
| Canopy bridge | An elevated connection links vegetation above the road. | Useful for arboreal or canopy-moving species that may avoid ground-level passages. |
There is no universally best crossing. A large ungulate, a small mammal, a reptile and a tree-dwelling species may perceive the same structure very differently. FHWA’s technical guidance states: “The location, type, and dimensions of wildlife crossing structures must be carefully planned with regard to the species and surrounding landscape.” Read Chapter 8 of FHWA’s Wildlife-Vehicle Collision Reduction Study
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What the evidence shows
North Carolina: underpasses and fencing reduced reported mortality
A 2010 U.S. Geological Survey study examined a new section of U.S. Highway 64 in North Carolina where three underpasses were installed with associated fencing. Based on collision reports from adjacent highway sections, the fenced highway section experienced approximately 58% fewer wildlife mortalities, primarily involving white-tailed deer. This is a site-specific result, not a guaranteed percentage for every crossing.
The researchers documented use as well as mortality. They recorded 2,433 photographs of nine species after construction and 3,614 track-count crossings representing at least 20 species. Those counts show animals used the system; they are separate from the estimated reduction in deaths. Within fenced sections, mortalities were lowest near underpasses and increased with distance from them, suggesting that some animals still attempted to cross the fence farther from a passage.
Study author Matthew F. McCollister cautioned: “Continuous fencing between underpasses may further reduce the number of vehicle collisions for deer but additional design features (e.g., buried fencing) should be considered for other wildlife species.” USGS study details
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A Nevada Department of Transportation and University of Nevada, Reno study evaluated mule deer along a 6.6-kilometer section of U.S. 93 with fencing and multiple crossing structures. All of the studied structures had some successful use, but researchers observed substantially more successful crossings at overpasses than at underpasses. Their benefit–cost analysis supported the overpass in that setting.
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This finding applies to migratory mule deer on that highway section. It does not establish that overpasses outperform underpasses for every species, road or climate. Read the Nevada crossing study
Costa Rica: regional evidence requires careful interpretation
An Inter-American Development Bank-hosted study of two roads associated with biological corridors in Costa Rica describes underpasses, culverts with dry ledges, overpasses and canopy bridges. It notes that much wildlife-crossing research has been conducted in North America and Europe and has often focused on ungulates. Evidence from one region or animal group should therefore not be treated as a universal performance forecast for tropical forests, small mammals or arboreal species.
The study’s methods include camera traps and roadkill surveys. Its authors describe fencing that guides animals to crossings as recommended practice and emphasize monitoring structure use, road mortality and movement. Read the Costa Rica study
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A photograph of an animal entering a tunnel is useful, but it cannot answer every question. A crossing may attract many animals while failing to reduce deaths if fencing is incomplete or animals still cross elsewhere. Effective evaluations combine several measurements:
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- Structure use: camera traps, track counts or other observations identify species and frequency of visits.
- Completed crossings: researchers distinguish an animal that travels through from one that approaches and retreats.
- Road mortality: carcass and collision records show whether deaths decline on the treated road section and where remaining trouble spots occur.
- Movement and connectivity: telemetry or other movement data can show whether animals continue between habitat areas rather than merely visiting the structure.
- Driver safety: fewer animals on the pavement should reduce collision risk for motorists, a separate outcome from wildlife use.
Before-and-after comparisons, nearby control sections and repeated monitoring help separate a crossing’s effect from changes in traffic, weather, animal abundance or reporting. The studies above illustrate why “animals used it” and “the project reduced mortality” must be reported as different findings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why fencing is often as important as the bridge or tunnel
A crossing without guidance can be an isolated opening in a long road barrier. An animal may approach the structure, reject it, and then continue along the fence until it finds a weak point. That behavior can move the collision problem rather than solve it.
Fencing is designed to make the safe route the easiest available route. Its height, mesh, burial depth, maintenance and connection to the structure depend on the animals present. The North Carolina study’s warning about buried fencing reflects the fact that a fence effective for deer may not stop smaller, climbing or digging species. Gates, drainage points and road intersections also need inspection because each can create an escape or entry point.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesFencing must not become a permanent barrier to movement. Properly paired with crossings, it channels movement through a controlled location. Escape ramps address the exceptional animal that ends up inside the right-of-way, allowing it to leave without reaching traffic.
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Planning a crossing for a real road
- Map wildlife movement and collision hotspots. Use local roadkill records, habitat maps, seasonal migration information and field observations to identify where a road intersects regular movement.
- Define the target species and outcome. Decide whether the priority is reducing deer collisions, maintaining a migration route, reconnecting habitat for several species or protecting arboreal animals.
- Select the structure and dimensions. Compare overpasses, underpasses, adapted culverts, dry ledges and canopy bridges against species behavior, terrain, drainage and available space.
- Design the whole approach. Connect fencing to the structure, close likely bypasses and include escape ramps or other safety features where animals could become trapped.
- Match the surroundings. Use substrate, cover and vegetation that fit the adjacent habitat and make the route recognizable to the animals expected to use it.
- Set a monitoring plan before opening. Record structure use, successful crossings, road mortality, movement and driver-safety indicators so the project has measurable goals.
- Repair and adapt. Inspect fences, remove obstructions, review camera and collision data, and modify weak points when animals continue approaching the road.
A current example: U.S. Highway 550 in New Mexico
The New Mexico Department of Transportation’s U.S. 550 Wildlife Corridors project describes planned overpasses and underpasses, eight-foot woven-wire game fencing, escape ramps and vegetation selected to resemble adjacent habitat. The project is organized in two phases, with final design plans described as due by the end of 2026. That is a stated planning milestone; design or construction status can change, so the New Mexico project page is the current reference.
At the federal level, FHWA’s Wildlife Crossing Pilot Program provides grants intended to reduce wildlife–vehicle collisions and improve habitat connectivity. FHWA also lists work on best practices, data methods, mitigation thresholds and design standards. Program details and funding availability can change over time. See current FHWA program information
Common limits and failure points
- Wrong structure for the animals: A design that works for deer may be ignored by a species that needs canopy cover, water access or a smaller, darker passage.
- Incomplete fencing: Gaps, damaged sections and poorly terminated fence lines can leave animals exposed to traffic.
- Structures placed away from movement routes: Animals may not detour to a crossing that is distant from established travel paths.
- Counting use without counting deaths: High use does not by itself prove that collisions declined.
- Short monitoring periods: A project can appear quiet during a season when the target animals are not migrating. Repeated, seasonal observations are more informative.
- Maintenance neglect: Flooding, fallen trees, vegetation changes or fence damage can make a once-effective route unusable.
These limitations do not make crossings ineffective; they explain why engineering, ecology and follow-up management have to be treated as one system.
Further reading
For a general introduction to road ecology, the Associated Press identifies Ben Goldfarb’s book Crossings: How Road Ecology Is Shaping the Future of Our Planet. The report does not establish a current edition or retailer listing. Read the Associated Press feature. FHWA also lists the Wildlife Crossing Structure Handbook (2011) as a technical resource for professional or student readers.
The practical answer
Wildlife crossings help by replacing a dangerous at-grade road crossing with a route above or below traffic, while fencing directs animals to that route and escape features help those trapped in the roadway corridor. Documented projects show meaningful benefits, including approximately 58% fewer reported mortalities on one fenced North Carolina highway section and higher successful overpass use by mule deer in one Nevada study. Those results are strongest when the structure, fencing and habitat fit the animals that actually move through the landscape and when monitoring verifies both animal movement and reduced road mortality.
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