Once a plan is in place for wildlife habitat linkages, the next step is to take advantage of available highway crossing technology. The long-term objective should be to develop a systematic program of wildlife habitat linkages and appropriate wildlife crossings. Starting conservatively, with wildlife crossings that are modestly priced and monitoring their effectiveness, is likely to help DOTs develop experience and confidence that wildlife crossings can be an effective and cost-efficient way to improve highway safety. Each state has a variety of wildlife and highway issues, a variety of habitats, and different social values related to highways and wildlife. One size does not fit all! Be creative and flexible!

TYPES OF WILDLIFE CROSSINGS

Choosing the appropriate wildlife mitigation measures on a segment of highway is a complex decision. To be successful, you may need a variety of treatments. The critical elements of wildlife crossings include target species, structure effectiveness, engineering constraints due to terrain, cost for construction, maintenance, improvements to highway safety, and aesthetics. The following is a summary of the more common types of wildlife crossings:

Signs: This category includes signs with deer or other animals commonly observed on highway rights-of-way. Signs alert motorists to be watchful of wildlife on the roadway, but have little or no impact on mortality rates, habitat connectivity, or human safety. Signs alone are not recommended as a mitigation measure for habitat fragmentation or wildlife mortality. The exception may be in National Parks or places where signs may be used in conjunction with slow traffic speeds. Cost of signing is minimal, which is why it is often utilized.

Interactive Signs: Interactive signs include new technology that incorporates signs that activate only when wildlife is present in highway rights-of-way. Interactive signs have the best potential on lower traffic volume roads, and only detect large animals. Maintenance is a consideration and the efficacy of interactive signs is still being tested. Applications are limited to low volume roads with large animals like deer, elk, mountain sheep, moose, mountain lion, or bears. Night-time speed reduction signs and variable message signs are being utilized in some states as an alternative means of impacting driver behavior rather than the traditional static signs. The cost of interactive signs is moderate.

Culverts: Culverts are used extensively on highways for a variety of applications, mainly for moving water beneath roads. Round and elliptical culverts are used for small streams, intermittent streams, and for cross ditching. Material used for culverts may be metal, cement, or molded plastic. Of these, cement may be the best for crossing structures because it has some attributes of natural ground surfaces and maintains moisture. The application of culverts to wildlife passage primarily benefits small and mid-sized carnivores, up to and including coyotes and bobcats, during times of the year when there is little or no water present. Forty-eight inch culverts should be the minimum size if coyote or bobcat are target species. Fencing associated with 36” and 48” culverts can be an effective ways to move small and mid-sized carnivores across highways. The cost for culverts is relatively low compared to other crossing structure types.

Box Culverts: Box culverts may be superior to round culverts due to the larger interior space compared to similar sized round culverts or arches. Box culverts, depending on size, can be suitable for small, mid-sized, and large carnivores. Box culverts are used commonly on highways in the Southwestern United States to deal with flash floods, offering an opportunity to build or retrofit existing structures to provide wildlife passage for a variety of carnivores. Box culverts have been used successfully in a number of situations. In Florida, large 8’h X 25’w box culverts have been used successfully as highway crossings for Florida panther and black bear. The cost of box culverts is usually modest compared to open-span bridges, bridge extensions, or wildlife overpasses.

Multi-Plate Arches: Multi-plate arches are made using steel curved plates or, occasionally, cement arches. The arches are usually transported to the site and assembled. Most multi-plate arches have been used for large carnivores such as black bear, mountain lion, and grizzly bear, as well as deer and elk. For black bear and mountain lions, as well as many other species, they are often as effective as more expensive crossings. Natural light transmission through multi-plate arches is less than open-span bridges or bridge extensions. Depending on which species are being targeted, this may or may not be of concern. Multi-plate arches are relatively modest in cost compared to other large wildlife crossing structures.

Open-Span Bridges: Open-span bridges have some benefits and drawbacks. The effectiveness of open-span bridges has been shown to be high for most large wildlife including deer, elk, black bear, grizzly bear, mountain lion, wolves, and a variety of smaller species. Openspan bridges should be considered where wildlife habitat connectivity is a high priority and where species like grizzly bear and wolves are present. Open-span bridges are often used in major elk and deer migration routes. Due to the design of open-span bridges, they are often constructed over natural drainages as opposed to fill slopes or flat terrain. Successful examples of open-span bridges for wildlife crossings have been built in Banff National Park, Colorado, Montana, Idaho, and Arizona. In Banff National Park, engineers and biologists consider open-span wildlife crossings to be a good compromise between high wildlife use and cost. Cost, however, can be relatively high, running $1 million and more per structure.

Bridge Extensions: Bridge extensions are probably one of the most beneficial and easily incorporated wildlife crossing structures because they are required for hydrological purposes and wildlife often follow drainages for daily or seasonal movements. Bridge extensions span aquatic ecosystems benefitting stream processes, wetlands, flood protection as well as aquatic organisms. Bridge extensions provide one of the best types of wildlife crossings because they are usually “open” and provide high natural light transmission and minimal confinement. Many wildlife species, including carnivores of all sizes, readily utilize the habitat under bridges, as do species like elk, deer, moose, antelope, mountain sheep, and mountain goats. DOTs often have bridge replacement programs, providing opportunities to leverage wildlife crossings with bridge repair or replacement. Bridge extensions may be costly.

Wildlife Overpasses: Wildlife overpasses, ecoducts, or bridges, are not appropriate everywhere, but they make sense especially when connectivity for multiple species is being considered and the terrain does not allow for an open-span bridge. Wildlife overpasses are often the most effective wildlife crossing and benefit the largest number of species. Species that may shun under the road crossings, like grizzly bear, antelope, and moose may accept wellplaced wildlife overpasses. In Europe, wildlife overpasses are more common because of their efficacy for plant connectivity and invertebrates, as well as for their superior performance for a variety of vertebrate species. Examples of wildlife overpasses can be seen throughout Europe, in Banff National Park (Canada) and Utah (for mule deer). Wildlife overpasses are currently being considered in Montana, Colorado, Idaho, Utah, and Washington. Overpasses are a comprehensive solution with a relatively high cost.