The following are design elements that should be considered when building wildlife crossings for carnivores and other wildlife species:

Keep It Natural: The more naturally a wildlife crossing fits into the surrounding area, the more likely animals will use it. A natural appearance is created when the vegetation approaching a crossing structure is similar to that of adjacent habitat, or when a structure is designed with a natural bottom. A natural-appearing structure also includes a minimal amount of features that would either intimidate or obstruct wildlife such as livestock fencing, cement walkways, rip-rap, construction debris, unnecessary fill, signing, or poles. A natural appearance is particularly important for wary species like grizzly bears and wolves.

Video footage from the United States and Europe indicates a wide array of wildlife behavioral responses to highway crossings. When the appearance of wildlife crossings is unnatural, approaching animals will watch the crossing, sometimes for several hours. After watching the crossing, some animals will cross, some will not, some will run through, and some will run or walk partway through and return without crossing successfully. It may cost slightly more to make a crossing appear natural, but it is money well-spent.

Location: The general location of wildlife crossings can be assessed from aerial photos. The precise location of each structure should be made after considerable field work has been done to determine the best location. In most situations, wildlife crossings should be located precisely where animals naturally approach a highway, or where they have historically done so. Often, animals choose areas to cross where there is a specific terrain feature, vegetation, or a reduction in the number of lanes. Ridges, valley bottoms, stream and river courses, and wooded corridors often are choice locations. Location of wildlife crossings is one of the critical factors in optimizing their use.

Approaches: How an animal approaches wildlife crossings may be the deciding factor in whether or not wildlife use a structure. Approaches should include habitat factors like vegetation near or at the crossing entrance. Several animals have shown preferences for using a structure where the distance between cover is the shortest. For example, studies in Canada indicate wolverine moved long distances parallel to highways to find areas where distances across roads was minimal and vegetation was close to the right-of-way.

Vegetation: Vegetation provides many benefits for a wildlife crossing. It minimizes the distance animals must travel between habitats on both sides of the highway. It shields animals from light and noise. It also provides cover which is often important to animals that are feeling vulnerable. When rights-of-way are cleared for highways, vegetation should be left at locations where wildlife crossings are planned. Also, trees and shrubs should be planted in approaches and between lanes for divided highways.

Discord Elements: Discord elements in the approach area may reduce a wildlife crossing’s effectiveness. Excess road-fill material should not be placed at wildlife crossings. Bright pieces of metal, boxes, or other construction material should be removed from the crossing structure and approaches. Farmers or ranchers should not store equipment, hay, or other unnatural material in or near wildlife crossings or approaches. Sediment fences make use by many species difficult or impossible. Rip-rap is difficult for many species to traverse, especially ungulates and amphibians.

Line of Sight: Animals approaching wildlife crossings should be able to see through the structure to suitable habitat on the opposite side of the highway.

Crossing Bottom Material and Design

To the extent possible, the bottom of structures should have similar soil as would occur if the structure were not there. Often, wildlife crossing bottoms are made up of coarse material from road cuts, cement, or metal. For some species that are more adaptable like coyote, black bear, raccoon, and opossum, bottom material may not be a significant factor. But, for other species including deer, elk, moose and other ungulates, crossing bottom material is important.

Bridges can provide both stream crossings and wildlife crossings. It is preferable to maintain a natural stream bank and let wildlife choose where to make trails or cross within a structure. Hardened vertical walls on structures, such as those made of building blocks and cement, seem to be less desirable than those of natural fill material (soil or loose gravel). Avoidance of such designs has come mostly from ungulates and may not apply to carnivores. Likewise, avoidance or fear of vertical walls may fade after animals adapt over time. Elaborate pathways are likely unnecessary and add cost.

Fencing: Fencing is as critical as the wildlife crossing structures and approaches. Most wildlife are extremely wary and will avoid confinement or unnatural situations. Given the choice between going through unfamiliar wildlife crossing structures and crossing highway pavement, many will choose the latter. Fencing forces wildlife to use the crossings. Over time, research indicates wildlife species will be more comfortable using wildlife crossings. Adaptations to using wildlife crossings may take up to five years. Young animals brought through wildlife crossings by their parents may readily accept crossings. Without fencing, most of these animals would not use the structures (Clevenger et al 2001).

Fencing Length: There may be many fencing options. Properly placed and well-designed wildlife crossings require less fencing than crossings that are minimally accepted by wildlife. If animals have a high resistance to using the structure, they may travel along fences for long distances, trying to find less intimidating places to cross the highway. Continuous fencing linking multiple crossing structures, such as in Banff National Park and in some parts of Florida, is not feasible in most highway situations. In these cases, wing-fencing is employed. There are no simple answers to the length of wing-fences from a crossing structure. Sometimes there are natural features that funnel animals into wildlife crossings and perhaps wing-fencing can be limited to a few hundred feet on each end. Most of the time, wing-fencing should be built for ½ mile or more if large carnivores, deer, and elk are target species.

Fencing Placement: Wildlife fencing should be designed to minimize the corral or shoot effect. This is done by constructing fencing to the top of wildlife crossings, rather than the bottom, making the approach to a wildlife crossing as wide as possible (see examples on pg. 14). When fencing between lanes of a divided highway, build the fencing parallel to the highway for a short distance so it does not look like a narrow, confining chute.

Large Carnivores: For large carnivores and deer and elk, 8’h page wire fencing is standard (Reed 1995). Bears, wolves, coyotes, and other carnivores may try to dig under fences or climb over. Burying fencing underground reduces the possibility of wildlife digging under the fence and also increases the lifetime of fencing reducing maintenance costs. Regardless of the degree of maintenance required, maintenance of fencing should be included in long-term project budgets.

Mid-Sized and Small Carnivores: Fencing is also important for small and mid-sized carnivores, although there is less information on what fence type works best. For many species, standard height highway fencing (4’h wire mesh) should be adequate. Skunks and other small carnivores will fit through 4” mesh size. In Europe, a variety of fencing material is used, including variable mesh fencing that has small-sized mesh openings at the bottom and 4”h x 4”w page wire on top. One half inch mesh screening is used in Europe for badger, amphibians and other small animals. Three or four foot high, 2”h x 4”w page wire should be adequate to funnel small carnivores into 36” culverts.

Often, 5-wire barb wire fencing is used to exclude livestock from using the crossing structures. Unfortunately, such fencing also may prevent or discourage wildlife from using the crossing. Recommended wildlife-friendly livestock fencing consists of a 3-wire design with minimal use of barbed wire, and the fencing should not be located immediately in front of structure entrances. The bottom wire should be high enough (normally 16 to 18 inches) to allow young animals to travel under the fence.

Escape Ramps: Even under the best situations, wildlife find ways to enter the right-of-way. Escape ramps help larger species to avoid being trapped.

Lane Configuration: Often highway configuration can be used to benefit wildlife crossings. Constructing two wildlife crossings in a divided highway often results in a more effective situation than a single crossing spanning four to six traffic lanes. These options should be considered when highway widening projects are undertaken.

Highway Bridge Heights: Wildlife commonly follow riparian habitat or drainages, and bridges may already exist in places where wildlife naturally cross highways. Highway bridges represent opportunities to improve wildlife habitat connectivity, reduce wildlife mortality and reduce animal-vehicle collisions. Bridges are constantly being replaced as they become old and unsafe or highways are improved. For example, Oregon Department of Transportation qualitatively evaluated wildlife passage at a selection of bridges that were being replaced. Bridge design criteria that facilitate wildlife passage are the same as other wildlife crossings. For deer and most large carnivores, at least 10’h clearances are recommended. If elk, grizzly bear, or moose are present, bridge heights should be at least 12’ to 13’ high or more. For mid-sized and small carnivores, at least 3’ or 4’h clearance above the highwater zone is recommended. There must be an adequate stream bank to allow use by target species.

Bridges are often high and open enough to allow enough sunlight to penetrate and allow growth of shrubs and grasses. Some bridges have been built with steel girders that make loud noises when traffic crosses. In Arizona, noise has been identified as extremely disturbing to elk trying to use crossings. Bridge material should minimize traffic noise.

Structure Size and Type: Design criteria that include structure size and type are “essential elements” for effective wildlife crossing structures. Size and type of structures affect both wildlife use and cost.

For example, a 13’h x 23’w multi-plate arch may cost $250,000. Open-span wildlife crossings often cost $1 million, or more. And, a wildlife overpass can easily cost $5 million or more. So, size and type of structure will matter to a highway engineer and highway departments. Small increases in structure size, or what may seem like subtle changes in design, may have a large effect on costs. All other things being equal, biologists should recommend the most cost-efficient design that will work for the target species.

Please refer to the Crossing Structure Type and Size - Alternatives By Species table for a matrix of structure size and type alternatives.