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Neither wildlife crossing bridges (overpasses) nor underpasses is universally better. The effective choice depends on the species a road project must serve, the animals’ established movement routes, local topography, structure dimensions, approach habitat, and whether fencing safely funnels wildlife to the crossing. Federal Highway Administration (FHWA) guidance reports that grizzly bears, deer, and elk tend to use overpasses more than underpasses, while black bears and mountain lions use underpasses more frequently. A Nevada study also recorded substantially more successful mule-deer crossings at overpasses than at underpasses, but that result applies to one species and project.
Underpasses can be highly effective when their dimensions, cover, location, and fencing suit the target animals. Studies in Florida and North Carolina documented large reductions in wildlife mortality and thousands of crossings after underpasses were paired with barriers or fencing. Those results cannot be ranked directly against the Nevada overpass result because the projects measured different species, designs, locations, and outcomes.
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What the two crossing types do
An overpass carries wildlife over a road on a landscaped bridge or broad earth-covered structure. An underpass carries animals below the road through a tunnel, box culvert, arched passage, or similar opening. Both are intended to separate animals from traffic while preserving movement between habitat patches.
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Which animals tend to use each option?
Species differ in how they respond to overhead openness, confinement, light, noise, vegetation, and unfamiliar surfaces. FHWA guidance summarizes broad tendencies, not rules that apply to every population or design.
Animals often associated with overpass use
- Grizzly bears
- Deer, including mule deer and white-tailed deer
- Elk
These animals may be more willing to cross an open, vegetated span that resembles ordinary ground. Large migratory herds can also benefit when a broad overpass accommodates several animals at once and provides continuity with the surrounding landscape.
Animals often associated with underpass use
- Black bears
- Mountain lions
- Smaller mammals, reptiles, amphibians, and other species documented at culverts and underpasses
Some animals prefer cover or already use drainage corridors and narrow travel routes. An underpass can provide that concealment, particularly where a tunnel is sized and furnished for the species rather than treated as a generic pipe.
These tendencies do not make a species incapable of using the other structure. Local populations, previous experience, vegetation, traffic noise, lighting, and the presence of fencing can change behavior. The practical question is not “Which structure is best in general?” but “Which design will the target animals accept at this location?”
Overpasses and underpasses compared
| Decision factor | Overpass (wildlife bridge) | Underpass |
|---|---|---|
| Typical behavioral fit | FHWA reports greater use by grizzly bears, deer, and elk; open sightlines can resemble natural terrain. | FHWA reports more frequent use by black bears and mountain lions; cover can suit species that avoid exposed crossings. |
| Landscape requirement | Must connect naturally to terrain on both sides and provide usable approach habitat. | Must be placed on an existing route or drainage corridor and offer adequate light, width, height, and dry or suitable footing. |
| Important design variables | Width, natural substrate, vegetation, visual continuity, edge treatment, and safe approaches. | Opening dimensions, length, substrate, drainage, light levels, vegetation at entrances, and resistance to flooding or debris blockage. |
| Role of fencing | Fencing guides animals to the bridge and prevents them from reaching the road at its edges. | Fencing guides animals to tunnel entrances and prevents road access between passages. |
| Evidence example | Nevada Department of Transportation reported substantially more successful mule-deer crossings at overpasses than underpasses on a 6.6-kilometre section of US 93 north of Wells (2015). | US Geological Survey studies recorded 3,614 crossings of at least 20 species in North Carolina and major mortality reductions in Florida after underpasses were combined with fencing or a barrier wall. |
| Cost information | Use current, local construction and lifecycle estimates; FHWA examples from its 2008 report are historical. | Use current, local construction and lifecycle estimates; costs vary with excavation, utilities, drainage, geology, and road staging. |
The table shows why a simple head-to-head ranking is misleading. The Nevada comparison involved mule deer and fencing on both sides of the road. The North Carolina intervention used three underpasses and associated fencing. The Florida project combined a barrier wall with culverts. Their results answer different questions.
Why location can matter more than the structure label
FHWA guidance places location and species needs at the center of crossing design. Animals repeatedly use particular ridgelines, stream valleys, migration routes, bedding areas, and gaps in developed terrain. A crossing that intersects those routes is more likely to be discovered and adopted than one selected only because a road segment is convenient to build.
Use movement evidence before choosing a design
- Map road-kill clusters and collision reports, while recognizing that carcass records can be incomplete.
- Identify trails, tracks, camera detections, seasonal migration routes, and habitat connections on both sides of the road.
- Check how terrain, streams, wetlands, rock outcrops, and existing drainage channels constrain possible entrances.
- Consider seasonal changes. A route used during migration may not be the route used for daily foraging or dispersal.
Placement should also account for where fencing can begin and end safely. A structure that sits on a movement route but leaves an unfenced road edge nearby may simply shift collisions to the next opening.
Match the approach habitat
FHWA identifies nearby vegetation and structure dimensions as design considerations. Animals need an approach that feels continuous with the habitat they are leaving. Sudden open pavement, bright lighting, steep embankments, or heavy human activity can deter use. Conversely, dense vegetation immediately at an entrance can hide predators or obstruct visibility. The right balance depends on the species and site.
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How fencing changes the result
Fencing is not an accessory added after the crossing is designed. It excludes animals from traffic lanes and funnels them toward a safer route. The fence must connect continuously to the structure, block climb-through and dig-under points, and remain functional after storms, maintenance work, vegetation growth, and animal damage.
Common fencing failure points
- Gaps where fence sections meet bridge abutments, tunnel portals, drainage channels, or property boundaries.
- Access gates or maintenance openings that are left unsecured.
- Fence ends that terminate near the road and create a concentrated collision point.
- Washouts, fallen trees, snow damage, or burrowing that create openings below the mesh.
- Vegetation or debris that pushes animals over, under, or through the barrier.
North Carolina monitoring found higher mortality in fenced areas than in unfenced areas and observed that mortality increased with distance from underpasses. That pattern illustrates why a fence can reduce collisions near a crossing yet create risk where animals encounter an opening, a termination, or a route that does not lead efficiently to a passage.
Build maintenance into the plan
Inspect fences and gates on a schedule tied to weather, traffic maintenance, and animal movement seasons. Repair breaches promptly, clear blocked entrances, and document every repair. A crossing that performs well at opening can lose effectiveness as fencing deteriorates or surrounding land use changes.
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Florida: a barrier wall and culverts
A 2004 US Geological Survey study by Dodd, Barichivich, and Smith examined a heavily traveled Florida highway where a barrier wall and culverts were installed. The study recorded 2,411 road kills before construction and 158 afterward, excluding hylid treefrogs, in the studied area and observation periods.
Across the Paynes Prairie basin survey area, the authors reported a 65% mortality reduction when hylid treefrogs were included and a 93.5% reduction when they were excluded. Many remaining kills occurred around an access point and a fence segment, while most nonhylid kills in the wider surveyed area were beyond the barrier system.
These figures measure a specific barrier-and-culvert system. They are not an isolated test of every underpass design, and they should not be treated as a universal percentage for roads elsewhere.
North Carolina: three underpasses and fencing
A US Geological Survey study record from 2010 documented 3,614 wildlife crossings by at least 20 species after three underpasses and associated fencing were installed on US 64 in North Carolina. Comparisons with adjacent highway sections suggested approximately 58% fewer wildlife mortalities on the new highway section, primarily involving white-tailed deer.
The same study found higher mortality in fenced areas than unfenced areas and an increase in mortality with distance from underpasses. Those findings reinforce that passage counts and collision outcomes must be examined together. A tunnel can be used frequently while poorly maintained fence ends still expose animals to traffic.
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Nevada: mule deer at overpasses and underpasses
A 2015 Nevada Department of Transportation report evaluated wildlife crossing structures on a 6.6-kilometre section of US 93 north of Wells. Its abstract reports substantially more successful mule-deer crossings at overpasses than at underpasses. The result is informative for mule deer in that project, where fencing was present on both sides of the road, but the report does not provide a universal percentage that can be applied to other roads or species.
The Nevada, North Carolina, and Florida studies used different interventions and outcome measures. Their counts and percentages should therefore be read as project-specific evidence rather than a standardized league table.
Choose a crossing with a species-and-site decision process
- Define the target species and outcome. Decide whether the priority is reducing vehicle collisions, maintaining seasonal migration, reconnecting habitat, protecting a threatened population, or serving several goals. Specify whether “success” means an animal entering, completing a crossing, avoiding a retreat, or contributing to a measurable reduction in road mortality.
- Map the movement corridor. Combine road-kill data, field signs, camera records, telemetry where available, habitat maps, and local ecological knowledge. Identify routes used by the animals the project is designed to protect.
- Screen structure types against behavior. Consider open versus covered travel, body size, herd behavior, tolerance of confined spaces, and likely reactions to noise, light, water, and human activity. Use FHWA species tendencies as a starting point, not a substitute for local evidence.
- Check engineering and terrain constraints. Evaluate road width, embankments, geology, groundwater, drainage, utilities, construction staging, and long-term access. An underpass may be impractical where flooding cannot be controlled; an overpass may be impractical where approaches cannot be graded into stable habitat.
- Design the approaches and fencing together. Connect barriers continuously to each entrance, eliminate foreseeable gaps, provide suitable substrate and vegetation, and plan gates and maintenance routes that do not become escape points.
- Set a monitoring design before construction. Record baseline collisions and animal activity, establish camera or track-monitoring locations, define sampling seasons, and decide how long animals may need to adapt before results are judged.
- Review and correct. Use monitoring to find blocked entrances, fence breaches, repeated retreats, or mortality hotspots. Modify vegetation, gates, fencing, drainage, or lighting when evidence shows a problem.
How to measure whether a crossing works
Different metrics answer different questions. A credible evaluation reports them separately rather than presenting one number as proof of overall success.
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Passage counts
Track pads, cameras, and other methods can count approaches or completed passages. Counts show use, but they can overstate benefit if the same animals are recorded repeatedly or if many approaches end in retreat.
Successful crossings and retreats
A completed crossing demonstrates that an animal reached the other side. Recording failed approaches, hesitation, and retreat can reveal problems with dimensions, lighting, noise, predators, water, or fencing even when total visits appear high.
Road mortality and collision changes
Collision data measure risk on the road, not simply interest in the structure. Compare equivalent observation periods and account for traffic volume, weather, detection effort, and road segments beyond the fence. The Florida and North Carolina studies illustrate why the intervention area and comparison areas must be stated.
Connectivity and population outcomes
Maintaining movement between habitat patches may require genetic, demographic, or long-term occupancy evidence. Short-term passage observations alone do not prove population recovery. A project can reduce collisions immediately while its effect on reproduction, gene flow, or population size takes years to evaluate.
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FHWA notes that performance studies are often short-term or sporadic. Such sampling can miss adaptation periods, seasonal movement, and species that are not the original monitoring target. Animals may require time to learn a new route, and early avoidance does not necessarily predict long-term use.
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Monitoring should span the seasons relevant to the target species and continue long enough to identify maintenance failures and changes in surrounding land use. Report the observation window, methods, species detected, and whether counts represent unique animals or passage events.
Costs, feasibility, and what not to infer
There is no reliable single price for an overpass or underpass. Construction and lifecycle costs depend on current local labor and material rates, road geometry, land acquisition, utilities, drainage, geology, traffic-control requirements, fencing length, monitoring, and maintenance. Compare complete project budgets rather than the structure shell alone.
FHWA’s Wildlife-Vehicle Collision Reduction Study: Report to Congress from August 2008 contains historical cost examples. Those figures should not be reused as current prices without updating them for the location, date, and project scope.
Feasibility also includes the cost of failure. A cheaper passage that animals avoid, or that directs them to a fence end, can leave collision risk unresolved and require retrofits. Conversely, an expensive overpass is not automatically justified if the target species prefers covered routes or if the site cannot support safe approaches.
Common mistakes and better alternatives
Declaring a universal winner
Mistake: Treating the Nevada mule-deer result as proof that every overpass outperforms every underpass.
Better approach: State the species, site, design, fencing, study period, and outcome whenever citing a result.
Counting visits as completed crossings
Mistake: Reporting camera detections at an entrance as successful connectivity.
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Installing fencing without an end-to-end plan
Mistake: Assuming any fence automatically makes the road safer.
Better approach: Tie fencing into both sides of the structure, secure gates, inspect after storms, and analyze mortality at fence ends and access points.
Ignoring non-target species
Mistake: Designing for deer while overlooking amphibians, reptiles, carnivores, or smaller mammals.
Better approach: Set a target-species list and monitor for other wildlife that may use or be excluded by the design.
Judging too soon
Mistake: Treating a short or sporadic survey as a long-term verdict.
Better approach: Include adaptation periods, seasonal sampling, maintenance checks, and appropriate population or connectivity measures.
Practical verdict
Choose an overpass when the target animals are likely to prefer open, broad, vegetated travel and the landscape can support natural approaches; FHWA’s reported tendencies and the Nevada mule-deer study make overpasses especially compelling candidates for some deer, elk, and large-bear corridors. Choose an underpass when the target species favors cover, already travels through a drainage or enclosed route, or when a well-dimensioned passage can be integrated into the terrain; FHWA reports frequent underpass use by black bears and mountain lions, and field studies show that underpasses paired with effective fencing can substantially reduce mortality.
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In either case, the decisive package is a correctly sited structure, dimensions and habitat suited to the animals, continuous fencing, durable maintenance, and monitoring that measures completed crossings and road mortality separately. The evidence supports a conditional, species-specific decision—not a universal ranking of bridges over tunnels or tunnels over bridges.
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