Yes. Wildlife crossings can substantially reduce animal–vehicle collisions, especially when paired with continuous exclusion fencing that guides animals to the crossing. The size of the benefit depends on the species, fence length and design, road layout, maintenance, and how results are measured. Published percentages are study findings, not guarantees for every project.
What a wildlife crossing actually does
A wildlife crossing is an overpass, underpass, culvert, or similar route that lets animals move across a road without entering traffic lanes. On its own, a structure may be easy for some animals to miss. The strongest evidence concerns a complete system: fencing keeps wildlife off the pavement, and the fence line funnels animals toward a safe crossing point.
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This distinction matters when interpreting claims about effectiveness. A project described as a “crossing” often includes several underpasses or an overpass, miles of fencing, gates, drainage features, and maintenance. If collisions fall afterward, the result generally reflects the combined system rather than the concrete or steel structure alone.
Why fencing is central
Animals approaching a road usually follow familiar travel routes. A continuous fence blocks direct access to the roadway and creates a guided path. When the fence terminates at a crossing, animals can pass under or over the road instead of searching for a gap. Fence ends, damaged sections, poorly designed gates, and culverts that are too small can defeat that intended path.
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Fencing also changes where risk occurs. If an animal cannot reach the crossing or finds a breach, it may climb over, dig under, or walk around the barrier and enter the traffic corridor. For that reason, a project evaluation should examine collision locations near fence ends and gaps, not just the area immediately beside the structure.
How large can the reduction be?
The broadest figure in the available evidence comes from the Federal Highway Administration’s 2008 Wildlife-Vehicle Collision Reduction Study: Report to Congress. Its executive summary states: “Numerous studies in the last 20 years have demonstrated that wildlife fencing, with or without wildlife crossing structures, can reduce collisions with deer and other large animals by 87 percent on average (80–99 percent).”
That 87 percent is an average across numerous studies. It applies to deer and other large animals represented in those studies, not to every species, road, climate, or crossing design. The 80–99 percent range shows that many projects performed very well, while the average should not be treated as a guaranteed outcome at a proposed site.
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|---|---|---|---|
| Federal Highway Administration, 2008 synthesis | Wildlife fencing, with or without crossing structures, across numerous studies | 87% average reduction; range 80–99% | Collisions with deer and other large animals |
| Van Manen and McCollister, 2010, North Carolina | Three underpasses and associated fencing on a new section of US Highway 64 | Approximately 58% fewer wildlife mortalities | Collision reports compared with adjacent highway sections, primarily involving white-tailed deer |
| Van Manen and McCollister, 2010, North Carolina | Underpass areas monitored before and after construction | 6.7 times greater average white-tailed deer use after construction | Observed use of underpass areas, not a collision percentage |
| Comparative study, 2016 | Wildlife fencing and crossing structures of different lengths | Fences of 5 km or less averaged 52.7% effectiveness, with a 0–94% range; fences longer than 5 km typically exceeded 80% | Collision-reduction effectiveness, with substantial variation among projects |
| Virginia Transportation Research Council, 2020 | Fence additions at existing isolated underpasses | Benefits exceeded fencing costs in 1.8 years; average savings exceeded $2.3 million per site | Economic result for the study sites, based on crash-reduction benefits |
A field example: three underpasses in North Carolina
Frank Van Manen and Matthew F. McCollister studied a section of US Highway 64 in North Carolina with three underpasses and associated fencing. Comparing collision reports with adjacent highway sections, they reported approximately 58 percent fewer wildlife mortalities, primarily white-tailed deer.
The same study documented evidence that animals used the structures. Before construction, researchers recorded 242 deer-use occasions. After completion, cameras captured 2,433 photographs involving nine species; deer accounted for 93 percent of the photographed crossings. Track counts recorded 3,614 wildlife crossings by at least 20 species.
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These figures answer different questions. The mortality comparison indicates fewer reported deaths, while photographs and tracks show animals using the route. A high crossing count does not automatically prove that every animal reached the road safely, and a lower collision count does not by itself show whether the crossing serves the target species or maintains habitat connections.
Risk did not disappear everywhere along the fence
Deaths were lowest near underpasses inside the fenced sections and increased with distance from the structures. Animals with smaller home ranges were less likely to travel far enough to find an underpass. Some attempted to get over or under the fence, creating residual collision risk away from the structures.
This pattern explains why a project can be successful overall while still having dangerous spots. Monitoring should map mortality along the entire fenced corridor, including approaches, fence ends, road junctions, and places where animals may breach the barrier.
Why fence length and layout change the result
A 2016 comparative study found that short fence sections of 5 kilometers or less had an average collision-reduction effectiveness of 52.7 percent, with results ranging from 0 to 94 percent. Longer fences, exceeding 5 kilometers, typically achieved reductions above 80 percent in that comparison.
The finding does not establish a universal cutoff or guarantee that a longer fence will work. Longer systems can cover more approaches and reduce the number of unprotected entry points, but they also require more inspection, repairs, escape features, and coordination with adjoining land. A short, well-designed installation may outperform a poorly maintained long one.
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Fence placement must match animal movement. A barrier that stops abruptly beside a high-use travel route can concentrate animals at its end and increase attempts to cross the road. A system should connect the crossing to habitat on both sides, maintain a continuous barrier, and include features that let trapped animals leave the road side safely without creating a new opening for entry.
Collision reduction, crossing use, and connectivity are different outcomes
Three measures are often discussed together but should be reported separately:
- Collision reduction: the change in documented vehicle strikes or wildlife mortality, with the comparison road sections, monitoring period, and species identified.
- Crossing use: photographs, tracks, genetic detections, or direct observations showing which animals use the structure and how often.
- Habitat connectivity: whether animals can move between habitat patches on opposite sides of the road, including whether the route is used by the target species over time.
A structure can show frequent use yet leave collisions nearby if animals also try to cross elsewhere. Conversely, collisions may decline before monitoring demonstrates regular use by every target species. Agencies should publish all three measures rather than presenting one as a substitute for the others.
What determines whether a project works?
Target species and behavior
Deer, elk, black bears, amphibians, reptiles, and small mammals do not respond identically to the same fence or structure. Body size, climbing and digging ability, home-range size, seasonal movements, and fear of enclosed spaces affect whether an animal approaches and uses a crossing. The North Carolina results were dominated by white-tailed deer, so they cannot be transferred directly to every species in another region.
Structure location and approach habitat
Crossings work best when placed on known movement routes, near water or established trails where appropriate, and connected to suitable habitat. A technically adequate underpass in an area animals rarely visit may record little use. Conversely, a heavily used route needs enough capacity and visibility for animals to approach without excessive disturbance.
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Fence continuity and maintenance
Fence performance declines when vegetation pushes it down, flooding undermines it, gates remain open, or road work creates temporary gaps. Inspection schedules should cover the full length, not only the crossing entrances. Repairs need to be fast enough to prevent animals from learning that a breach provides access to the roadway.
Traffic and roadside conditions
Vehicle speed, traffic volume, lighting, median barriers, drainage channels, nearby development, and snow or flooding can all affect collision risk and crossing behavior. Results from a rural highway may not apply to a divided suburban road or a low-speed urban corridor.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to evaluate a proposed or completed crossing
Readers reviewing an agency plan, environmental assessment, or post-construction report can look for a consistent set of questions.
- Define the target species. Check whether the design and monitoring focus on the animals responsible for most collisions or on species whose habitat is being reconnected.
- Describe the whole intervention. Record the number and type of structures, fence length, fence height and configuration, escape features, gates, and connections to adjacent habitat.
- Establish a comparison. Look for before-and-after collision data, nearby untreated road sections, or both. A single year of counts cannot show whether a change came from the crossing rather than weather, traffic, hunting, reporting practices, or animal abundance.
- Separate mortality from use. Confirm that crash totals, camera detections, track counts, and other observations are reported as distinct measures.
- Check the monitoring period. Seasonal migrations and population cycles can produce large year-to-year changes. Longer monitoring provides a more reliable test than a brief post-construction snapshot.
- Map residual risk. Ask whether collisions cluster at fence ends, road intersections, drainage points, or damaged sections, and whether corrective work is planned.
- Assess connectivity. Determine whether animals can move across the corridor and reach habitat on both sides, not merely whether they avoid the pavement at one location.
Do overpasses work better than underpasses?
The evidence summarized here does not establish one universally best structure type for all species, traffic settings, and geographies. Overpasses can provide an open route that some large mammals readily approach, while underpasses can fit beneath existing roads and may suit species that prefer covered travel. Cost, drainage, soil, right-of-way, habitat, and the behavior of local wildlife all influence the choice.
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Can the investment pay for itself?
The Virginia Transportation Research Council reported that, at its study sites, adding fencing to existing isolated underpasses produced crash-reduction benefits that exceeded fencing costs in 1.8 years. Average savings were above $2.3 million per site. Those are site-specific economic results, not a universal payback period. Local traffic volume, crash severity, animal abundance, construction costs, land acquisition, and maintenance obligations can produce very different totals.
Economic evaluations should include the value of avoided human injuries, vehicle damage, emergency response, and wildlife mortality, while clearly stating which costs and benefits were counted. A short payback at one location should not be used as a promise for every crossing proposal.
What a responsible conclusion looks like
Wildlife crossings reduce collisions most reliably when they are designed as complete systems. The Federal Highway Administration’s 2008 synthesis found an average 87 percent reduction, with an 80–99 percent range, for fencing with or without crossing structures in studies of deer and other large animals. A North Carolina project with three underpasses and fencing reported approximately 58 percent fewer wildlife mortalities, while also documenting substantial animal use. A separate comparison found much more variable results for short fence sections than for longer ones.
Those findings support a qualified “yes,” not a guarantee. A credible project report should identify the species, fence configuration, comparison roads, monitoring period, collision definition, crossing-use method, and remaining risks at fence ends or gaps. When those details are available, readers can judge whether a reported percentage describes a durable safety and connectivity benefit or only a short-term change at one site.
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