Neither measure wins in every location. A wildlife underpass can provide a permanent route beneath a road, but it works best as a complete system with well-placed fencing, suitable habitat design and maintenance. A scheduled closure or timed access restriction can create a predictable, low-disturbance period when animals move at known times, yet it protects movement only during that window. Choose between them by species, movement timing, collision risk, connectivity goals, traffic needs and how success will be measured. Existing studies do not provide a controlled, head-to-head trial of an underpass against a scheduled closure.
What each measure is designed to do
Underpasses and scheduled closures solve different parts of the road-crossing problem. An underpass changes the road environment by giving animals a route below traffic. A closure changes the traffic environment by removing or reducing vehicles and human activity for a defined period.
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| Decision axis | Underpass with guiding measures | Scheduled closure or timed access restriction |
|---|---|---|
| Timing | Potentially available continuously, subject to design, flooding, debris and maintenance | Effective only during the specified closure or hold window |
| Main ecological role | Provides a route beneath the road; fencing can channel animals and keep them out of traffic lanes | Removes or reduces traffic and human activity during a targeted period |
| Evidence examples | Site-specific evaluations cover deer, other large mammals and amphibians | Studies and documented examples focus on defined seasons, nights or weather-triggered migration periods |
| Key limitation | Poor placement, unsuitable dimensions, fence ends and species differences can reduce use or shift risk | Animals moving outside the window still face the road; enforcement, access and predictable timing affect performance |
| Evaluation | Track structure use, successful crossings, road mortality and movement across the road | Compare animal activity and mortality during closure periods with open periods and suitable reference areas |
The practical question is therefore not “Which percentage is higher?” but “Which intervention matches the animal’s movement pattern and the project’s outcome?”
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How underpasses reduce collisions—and why a tunnel alone is not enough
Fencing turns a structure into a crossing system
The Federal Highway Administration describes underpasses and overpasses as safe crossing opportunities that can mitigate habitat fragmentation and help maintain viable populations over the long term. Its guidance also links collision-reduction performance to associated fencing: the fence keeps animals off the carriageway and funnels them toward a structure. Read the agency’s design discussion in FHWA’s Wildlife-Vehicle Collision Reduction Study, Chapter 8.
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Without a continuous guide, an animal may encounter the road before it reaches the tunnel, follow the fence to an opening, climb around an end, or remain trapped on the wrong side. Design must therefore cover the entire risk segment, not just the concrete or steel passage. Key checks include:
- Placement: locate the passage on a known movement route, near wetlands, cover, denning areas or other attractants used by the target species.
- Species fit: dimensions, light levels, substrate, moisture and openness need to suit the animals expected to use it.
- Fence continuity: connect both sides of the road to the structure and treat fence ends, gates, drainage features and maintenance access as potential escape points.
- Approach habitat: keep the entrance usable and avoid creating a barren, flooded or debris-filled approach that animals will avoid.
- Maintenance: repair breaches, clear obstructions and check that seasonal water, snow or vegetation has not changed the passage conditions.
What the deer study on U.S. Highway 64 found
A North Carolina study evaluated three underpasses with associated fencing using cameras and track surveys. The researchers recorded crossings by at least 20 species. On the treated highway section, wildlife mortalities—primarily white-tailed deer—were approximately 58% lower than on adjacent sections. Deer use of underpass areas was reported as 6.7 times greater after the highway and underpasses were in place.
Those results describe one site and one before/after plus adjacent-section comparison; they are not a guaranteed reduction for every road. The same study found that mortality was lowest near the underpasses and increased with distance within the fenced segment. It also recorded more mortalities in fenced areas than in unfenced areas overall and noted that smaller-ranging animals farther from a passage could try to get through the fence instead of travelling to an underpass. The implication for a construction or highway project is important: a fence can concentrate risk at weak points if passages, spacing and fence ends are not designed together. See the full study at USGS’s summary of the Journal of Wildlife Management evaluation.
What the Utah mitigation package found
A southwestern Utah evaluation compared six years of pre-construction mortality with two years after installation. The treatment combined fencing, jump-outs and underpasses. Deer mortalities fell 98.5% in the treatment section, compared with a 2.9% decline in an unfenced control section. Because fencing and jump-outs were part of the intervention, the result cannot be attributed to the underpasses alone. The details are reported by the USGS-hosted Wildlife Biology study.
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Small tunnels can help amphibians, with different design risks
A Swedish study examined tunnels and guiding fences at three amphibian roadkill hotspots. Estimated amphibians killed or at risk on treated sections fell by 85–100%, while estimated successful crossings increased by 25–340%. The authors also identified possible fence-end effects and called for careful before-and-after monitoring. These figures apply to particular sites, species and estimation methods, not to every small-animal tunnel. The study is available through Ecological Engineering.
What scheduled closures can accomplish
Closures create a predictable low-traffic period
A scheduled closure is a temporary road closure or time-based access restriction, not automatically a brief, on-demand traffic hold at the exact instant an animal approaches. FHWA documents seasonal and weather-triggered closures used for amphibian and reptile migrations. Examples in its 2008 report include River Road in Delaware Water Gap, closed at night in March and April under specified warm, rainy conditions; seasonal day-and-night closures on LaRue Road in Shawnee National Forest; and several spring-night closures on Beekman Road in New Jersey. These are examples recorded in that report, not confirmation of current operating schedules. See FHWA’s chapter on temporary road closures.
For a closure to work, the timing must overlap the movement that creates the risk. Agencies need a clear operating rule, signs and barricades or other access controls, a safe turn-around plan, emergency access procedures and a way to verify that the restriction is being followed. A closure that is too short, starts too late or is rarely enforced may shift traffic to an unprotected route without delivering the intended benefit.
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What the Banff road-closure study found
In Banff National Park, researchers evaluated a 17-kilometre section of the Bow Valley Parkway closed from 20:00 to 08:00 between March 1 and June 25. Remote cameras, road surveys and grizzly-bear GPS data all contributed to the analysis. Wildlife detection rates on the road doubled during closure periods, while detection rates in reference areas did not change. The result supports an increase in wildlife activity or presence on the closed corridor during the closure window; it does not establish a year-round connectivity benefit or a collision-reduction percentage. The study is published in Scientific Reports.
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Closures can therefore be particularly useful where movements are concentrated by season, temperature, rainfall, daylight or another observable trigger. They are less suited to species that cross unpredictably throughout the year, or to roads whose access function cannot tolerate regular restrictions.
Why published percentages cannot be ranked as a league table
The 58% mortality reduction in North Carolina, 98.5% Utah decline, 85–100% amphibian reduction and doubled Banff detection rate measure different outcomes under different designs and time periods. One is a comparison of mortalities on a treated section and adjacent sections; one is a combined mitigation package against a control; one uses estimated amphibian deaths or risk and estimated successful crossings; and one measures wildlife detections during closure periods. They are not a controlled comparison of two alternatives.
A 2024 global assessment reviewed 313 studies and found that only 14% evaluated whether crossing structures changed animal movement across roads. The finding matters because counting animals at a structure is easier than proving that the road’s barrier effect was reduced relative to a baseline or no-action site. The assessment is available from the Journal of Applied Ecology.
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If the priority is continuous connectivity
Prefer a correctly sited passage-and-fence system when animals need to cross outside a narrow, predictable period. Specify the target species first, then design the opening, substrate, lighting, drainage, guide fencing and approach habitat around that species. Include fence ends and maintenance access in the same drawings. A passage that is open all year can address repeated movement, but only if animals can find and use it and the structure remains passable.
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If the priority is a short, predictable migration pulse
Consider a scheduled closure when reliable observations show that most risk occurs during particular nights, seasons or weather conditions. A closure can avoid construction at a sensitive location and immediately remove traffic during the critical window, but it does not protect movements outside that schedule. Plan for public communication, enforcement, emergency response and the possibility of traffic displacement.
If the priority is collision reduction on a busy road
Start with a mortality map and movement data. If collisions are spread across months or occur at several locations, a permanent crossing system may be more appropriate than a narrow closure. If collisions cluster tightly around a weather-triggered event, a timed restriction may be operationally effective. In either case, define the comparison section and period before work begins so that a post-project result is interpretable.
If the priority is maintaining access
Underpasses keep vehicles moving while animals use a separate route, but require capital construction, land disturbance and long-term maintenance. Closures preserve the crossing area without a permanent structure, yet impose recurring restrictions on residents, freight, emergency services or visitors. The final choice is an access-and-ecology decision, not a wildlife percentage alone.
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1. Define the target outcome
Write down whether the project is intended to reduce road mortality, restore movement between habitat patches, increase use of a safe crossing, reduce disturbance, or achieve several of these goals. A camera detection can answer “Did an animal appear here?” It cannot, by itself, answer “Did collisions fall?” or “Was connectivity restored?”
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2. Establish a baseline and comparison
Collect pre-intervention road-mortality records and animal-activity data at the treatment site. Where possible, retain a suitable reference section that remains open and untreated. For an underpass, compare mortality near the structure, along the fenced segment and at fence ends. For a closure, compare open and closed periods across the same season and include reference areas that did not receive the restriction.
3. Monitor several outcomes
- Structure or corridor use: camera records, tracks and other detections identify which species approach and pass.
- Successful crossings: pair entrance and exit observations where possible so an approach is not mistaken for a completed crossing.
- Road mortality: use consistent search routes and timing, recording species, location and carcass condition.
- Movement across the road: telemetry or other movement methods can test whether animals actually move between habitat blocks.
- Human and traffic effects: record closure compliance, traffic volumes, access complaints, emergency events and any displacement to nearby roads.
4. Check for unintended risk
Inspect fence ends, gates, culverts, drainage channels and damaged sections after storms and maintenance work. For closures, check whether vehicles bypass the restriction or create pressure on parallel roads. Review mortality and movement data for locations just outside the treatment area; a local improvement can conceal a shifted problem.
5. Use a long enough and appropriately timed record
Sampling must cover the movement season that motivated the project and account for weather variation. A single high-use week can show that animals were present, but cannot establish a durable change in mortality or connectivity. Report the observation period, method and comparison area with every percentage.
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Using cameras without overclaiming what they show
Remote trail cameras are practical for documenting animals approaching and using underpasses and for comparing activity during open and closed periods. Place cameras so the view distinguishes an entrance approach from a completed passage, protect equipment from flooding and vandalism, synchronize time stamps and record the camera’s active period. Pair the images with road-kill surveys and, where the question is connectivity, a method that follows movement beyond the road.
Camera detections are an evidence layer, not a verdict. More detections may mean animals are using a safer route, or it may mean they are lingering at an entrance without crossing. Fewer detections may reflect a successful diversion, a camera failure or a change in animal abundance. Interpret the images alongside mortality, crossing completion and movement data.
Decision checklist for an agency or project owner
- Which species and life stages are the target, and when do they move?
- Is the objective collision reduction, connectivity, disturbance reduction, or a defined combination?
- Are collision locations concentrated enough for one structure or one closure corridor to address them?
- Can a fence, passage and all fence ends be maintained for the design life?
- Can the road be closed safely, enforced consistently and reopened for emergencies?
- What traffic, resident, freight and emergency-access consequences follow each option?
- What baseline, reference site and monitoring methods will distinguish use from successful movement and reduced mortality?
- What result would trigger repairs, a longer closure, a changed schedule or a second crossing?
Bottom line for crossing decisions
Build an underpass-and-fence system when animals need a reliable route across a road and the project can support species-appropriate design and maintenance. Use a scheduled closure when movement is concentrated in a predictable period and temporary access restrictions are workable. In some landscapes the strongest plan is staged or combined: a closure protects a migration pulse while a permanent crossing is designed, or a passage handles year-round movement while short closures address exceptional peaks. Judge the choice by reduced mortality and demonstrated movement—not by structure use or a single percentage in isolation.
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