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Wildlife crossings are engineered passages that let animals move across roads while reducing the chance of vehicle collisions. The safest projects do not treat a bridge, tunnel, or culvert as a stand-alone object. Transportation engineers, wildlife biologists, hydrologists, and land managers first identify the movement problem, then match a crossing and its approaches to the species, habitat, terrain, traffic conditions, and long-term maintenance plan. Fencing commonly works with the structure by keeping animals off the pavement and guiding them toward the protected route.
The Federal Highway Administration (FHWA) describes this work in Wildlife Crossing Structure Handbook: Design and Evaluation in North America, Report FHWA-CFL/TD-11-003, published in March 2011 by Anthony P. Clevenger and Marcel P. Huijser. The handbook is technical guidance, not a regulation, specification, or universal design standard. Local engineering and ecological expertise remain essential because a measure that works for one species or landscape may perform differently elsewhere.
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What a wildlife crossing is designed to accomplish
A road can harm wildlife in two related ways. Vehicles may kill or injure animals, and the road itself can divide habitat that animals need for feeding, breeding, shelter, or seasonal movement. A crossing project addresses both problems when it creates a safe route and reconnects habitat on opposite sides of the road.
That produces two distinct objectives:
- Reduce wildlife–vehicle collisions. Animals are prevented or discouraged from entering traffic lanes and are directed to a safer route.
- Maintain or restore movement between wildlife populations. Animals retain access to habitat and mates, helping prevent a road from becoming a long-term barrier.
These objectives overlap but are not identical. A structure may be used frequently without proving that populations are connected over generations. Conversely, a crossing used by only a few animals may still be important if those animals represent a rare movement route. Evaluation must therefore test the outcome the project was actually intended to deliver.
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FHWA summarizes the design principle this way: “The response of particular wildlife species to these measures may vary across North America.” In practice, there is no universally best “wildlife bridge.” The right solution depends on the animals, landscape, road, and management capacity at a particular site.
How planners decide where a crossing belongs
1. Define the road problem
Planning starts with evidence of the impact: collision records, carcass surveys, reports from road crews and residents, wildlife tracking, habitat fragmentation, or a known migration route. Teams examine traffic volume and speed, road width, median barriers, drainage, sight distance, nearby development, and the locations where animals already attempt to cross.
A crossing should be part of a broader road-mitigation plan rather than an isolated response to one dramatic collision. The plan identifies which risks are being addressed, over what road length, and for which species or species groups.
2. Map habitat and movement corridors
Engineers and biologists study the land on both sides of the road. Useful information includes protected habitat, wetlands and streams, denning or nesting areas, seasonal ranges, topography, existing trails, and barriers such as buildings, secondary roads, or steep slopes. The approaches to a structure must lead into habitat that animals can actually reach and use.
Land ownership and future development matter as well. A crossing can lose value if one approach is later isolated by construction or if a connected corridor is narrowed. Conservation easements, public land management, and compatible private-land agreements may be needed to protect the route beyond the road right-of-way.
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3. Establish baseline conditions
Before construction, the project team records existing road-kill rates, animal movement, habitat condition, and any current fencing or drainage features. Baseline information gives evaluators something to compare with post-construction conditions and helps distinguish a genuine improvement from a change in reporting or animal abundance.
4. Match the design to the site
Crossing location and form are selected together. A deep valley may favor a viaduct, while a road embankment may allow an underpass. A forested route used by arboreal animals may require a canopy connection rather than a ground-level tunnel. A watercourse crossing must preserve the function of the stream while providing a usable path for terrestrial animals.
FHWA cautions against automatically extrapolating results across political boundaries or landscapes. Design dimensions and prescriptions in its handbook should be treated as technical guidance for the contexts described there, not as universal construction specifications.
Crossing types and when each can fit
The FHWA handbook catalogs 11 structure types spanning above-road and below-road solutions. Some are wildlife-only; others can accommodate people as well. The following comparison focuses on the decisions that distinguish them, not on ranking one type above another.
| Structure type | Typical design logic | Key planning questions |
|---|---|---|
| Landscape bridge | A broad, planted connection that makes the road crossing resemble the surrounding landscape. | Can the project create continuous habitat, soil depth, drainage, and plant cover across the span? |
| Wildlife overpass | An above-road passage dedicated primarily to animals. | Which species will approach an exposed structure, and how should noise, light, and human activity be controlled? |
| Multi-use overpass | An overpass shared by wildlife and people under a managed layout. | Can recreation or maintenance traffic be separated from sensitive wildlife movement? |
| Canopy crossing | An elevated connection for arboreal or climbing animals moving through trees. | Are anchor points, tree cover, span, and inspection access adequate for the target species? |
| Viaduct or flyover | A roadway structure that leaves a wider, more natural passage beneath it. | Will the open area preserve movement, stream function, and habitat continuity? |
| Large-mammal underpass | A substantial below-road passage sized and configured for large animals. | Is the opening large enough and natural-looking enough for the target animals to enter? |
| Multi-use underpass | A below-road route that may serve wildlife and people. | How will human use, lighting, surface treatment, and timing affect wildlife use? |
| Underpass with waterflow | A passage that integrates a stream or drainage channel with an animal route. | Can hydraulic capacity, bank conditions, flood resilience, and dry passage be maintained? |
| Small-to-medium mammal underpass | A smaller tunnel or box passage for species that do not need a large opening. | Are entrances, substrate, moisture, and cover suited to the target group? |
| Modified culvert | An existing or new drainage culvert adapted to improve animal passage. | Can ledges, shelves, natural substrate, or entrance improvements keep the route usable during normal flows? |
| Herpetile tunnel | A passage intended for reptiles and amphibians, often with closely associated guidance features. | Will moisture, temperature, substrate, seasonal timing, and barriers meet the needs of the target species? |
The table describes design logic, not a substitute for drawings or calculations. Structure dimensions, loads, drainage, fire and life-safety requirements, and accessibility obligations must be developed by qualified professionals under the rules that apply to the project location.
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How the structure is made usable
Approaches must look and function like a route
Animals do not perceive a crossing as a map line. They respond to cover, scent, noise, light, grade, surface texture, moisture, and the presence of people or predators. Designers shape the approaches so the entrance is visible and reachable from the movement corridor. Planting, retained vegetation, boulders, logs, natural substrate, and controlled drainage may help the route feel continuous with adjacent habitat, provided they suit the target species and do not create maintenance or safety hazards.
Road geometry and site constraints set limits
Existing embankments, bridge foundations, utilities, streams, groundwater, rock, snow, and flood levels can determine whether a passage is practical. Construction staging must also keep workers and drivers safe while limiting disturbance to the animals the project is intended to help. A design that is ecologically attractive but impossible to build, inspect, or maintain will not meet its long-term objective.
Human access is a deliberate choice
Some structures are wildlife-only; others are multi-use. Where people are permitted, the layout, sight lines, lighting, surface, and operating rules should prevent human activity from blocking wildlife movement. A multi-use designation should never be assumed to be harmless for every species.
Why fencing is usually part of the crossing
Fencing keeps animals from entering the roadway and funnels them toward the protected structure. FHWA’s Collision Reduction Study states that performance in reducing wildlife–vehicle collisions and creating crossing opportunities is linked to associated fencing that keeps animals off the road and directs them toward overpasses and underpasses.
Fence design is species-specific
Mesh, height, burial or ground contact, spacing, strength, and the treatment of corners all depend on the animals being addressed. A fence that stops a large mammal may not stop a small mammal, reptile, or amphibian. Some species climb, dig, squeeze through openings, or use water channels, so the barrier and the crossing must be designed as one system.
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Fence ends and gaps can undermine the project
Fence termination points need careful placement. If an end simply stops beside the pavement, animals may walk around it and re-enter traffic. Connections to natural barriers, additional fencing, gates, or other control features can close these routes. Culverts, driveways, median openings, maintenance access points, and drainage channels also require deliberate treatment.
Maintenance is part of the design
Storms, fallen trees, snow, vandalism, burrowing, and vehicle impacts can create openings. Inspection schedules should identify who repairs damage, how quickly it must be addressed, and how access will be provided without creating a new hazard. A fence that is effective on its opening day but neglected later cannot deliver reliable collision reduction.
Other measures that may support the crossing
Depending on the project’s goals and setting, agencies may combine a crossing and fence with gates, escape ramps for animals that enter the road side of a fence, warning signs, speed management, animal-detection systems, or adaptations to existing infrastructure. These are not automatic requirements. Each measure should have a defined purpose, a responsible operator, and a way to determine whether it works.
For example, an escape ramp may help an animal that has bypassed a fence return to the safe side, while a gate may be needed for authorized maintenance access. Detection or warning systems may be useful where animal movements are concentrated in time or where a full structural solution is not yet possible. Such additions should complement, not conceal, a poorly located crossing or a defective barrier.
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How projects are evaluated after construction
Set objectives before choosing cameras or surveys
FHWA recommends establishing goals and objectives, measuring baseline conditions, setting performance targets, selecting suitable monitoring methods, and designing a study capable of evaluating performance. The method follows the question. Counting animals that enter a tunnel is different from determining whether a population has regained connectivity or whether collisions have declined.
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Monitor use and outcomes
Monitoring can include track and substrate checks, remote cameras, road-kill surveys, telemetry, genetic or demographic studies, maintenance inspections, and traffic or speed data where relevant. A project may need several methods because no single observation captures every species or ecological outcome. Sampling should cover seasons and conditions that matter to the target animals, including migration or breeding periods.
Use results to correct the system
Monitoring may reveal a damaged fence, an entrance that animals avoid, an unexpected species using the passage, or a habitat link that planners did not document initially. Managers can then repair barriers, modify approaches, adjust human access, improve drainage, or refine future crossings. Adaptive management makes evaluation an operating function rather than a publicity exercise performed only once.
What the collision-reduction numbers do—and do not—mean
An FHWA workshop summary reports wildlife–vehicle-collision reductions of 80–100 percent on longer fenced road segments. The summary excerpt does not provide all underlying study details, so the range must not be presented as a guaranteed result for every crossing or as the effect of a structure alone. It describes a context in which fencing and crossing measures were associated with longer treated road sections. Agencies should verify the original study conditions before using the figure for forecasting or public commitments.
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Common design mistakes to avoid
- Choosing a bridge because it is visually familiar. A tunnel, culvert modification, canopy route, or viaduct may better fit the species and terrain.
- Building the structure without a barrier system. Unfenced approaches can leave animals on the pavement and reduce the safety benefit.
- Ignoring the corridor beyond the right-of-way. A crossing cannot restore movement if surrounding land uses block both entrances.
- Copying dimensions from another landscape. FHWA guidance is context-specific, and species responses vary across North America.
- Measuring use but not the intended outcome. Frequent passage does not by itself prove population connectivity or long-term conservation success.
- Underfunding inspection and repairs. Fence gaps, drainage failures, and damaged entrances can develop after construction.
- Adding every possible device. Gates, ramps, signs, detection systems, and speed measures should be selected only when they address a documented need.
A practical planning checklist
- Describe the problem. Identify collision locations, movement barriers, target species, and the road segment affected.
- Collect baseline data. Record road-kill patterns, animal movements, habitat, drainage, traffic, and existing barriers before construction.
- Protect the corridor. Confirm that both approaches connect to viable habitat and account for ownership and future land-use changes.
- Select the crossing family. Compare above-road, below-road, canopy, water-integrated, culvert, and multi-use options against species behavior and site constraints.
- Design the whole system. Coordinate the structure, approaches, fencing, fence ends, gates, escape features, drainage, and maintenance access.
- Set measurable targets. Decide whether success means fewer collisions, more safe passages, restored connectivity, or a defined combination.
- Fund monitoring and maintenance. Assign responsibilities, schedule inspections, and provide a process for adaptive changes.
- Report results with context. State the species, geography, treated length, dates, methods, and limitations whenever performance is published.
The central design lesson
A wildlife crossing succeeds when it functions as a connected, maintained mitigation system: a suitable route in the right landscape, paired with barriers and approaches that guide animals to it, and followed by monitoring that tests the project’s actual goals. FHWA’s 2011 handbook offers a North American technical framework, but local ecology and engineering determine the final answer. Treating the structure, fencing, habitat corridor, construction constraints, and evaluation plan as one project gives animals a safer way across roads without assuming that any single design works everywhere.
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