Wildlife Crossings

Wildlife Crossings

Bridges and tunnels specifically designed for animals can reduce the environmental impact of highways.


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Biology, Ecology, Conservation

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Roadkill is a serious problem on U.S. roadways. Deer and other large animals create life-threatening hazards when they cross roads and highways. In the United States alone, there are more than a million automobile accidents per year involving wildlife. That comes to more than $8 billion in medical costs and vehicle repairs annually.

More than a Million Animals Killed Every Day

If you are concerned with the well-being of wildlife, the problem is greatly magnified. According to some estimates, automobile collisions kill more than a million animals every day. For many species, it is the leading cause of death. Still worse is the way major roads and other forms of development can split animal populations and destroy their habitats. Losing access to large areas of their habitats makes it much harder for many animals to find food and a mate.

As people have become more aware of these dangers, highway crossings for animals have become more widespread. These can come in many forms, depending on the species involved and the geographic features of the land. The most common forms are bridges and overpasses, tunnels, viaducts and culverts. Animal crossings can greatly reduce the likelihood of collisions, provide a safe corridor for animals and help reduce the ecological impact of highways by reconnecting animal habitats.

Green Bridges

Wildlife bridges, often called "green bridges" in the United Kingdom, are usually covered in native vegetation. The purpose is to make them appear like a natural part of the landscape and encourage animals to use them. The crossings often work most effectively when fencing is placed on one or both sides of the road to funnel wildlife toward the crossing.

The concept was first developed in France in the 1950s. It took off in the Netherlands, where more than 600 crossings have been constructed to protect badgers (Meles meles), elk and other mammals. The Dutch built the world's longest animal crossing, the Natuurbrug Zanderij Crailoo, an overpass which spans more than 0.8 kilometers (0.5 miles). Wildlife crossings can also be found in Australia, Canada and other parts of the world. In the United States, the idea took a little longer to catch on, but wildlife bridges and tunnels began appearing here less than 20 years ago.


One of the earliest U.S. crossings was in Davis, California. In 1995, the city built a 15-centimeter (six-inch) wide tunnel, or "ecoduct," to allow frogs to pass under Pole Line Road toward a wetland on the other side. The town's postmaster decorated an area near one entrance as an amphibian-sized village, and named it Toad Hollow. Despite the Davis community's efforts, the local toad population did not use it.

Far more successful is the system of corridors in Banff National Park in Alberta, Canada. Between 1996 and 2016, six bridges and 38 underpasses were built for wildlife to cross the Trans-Canada Highway, which bisects the park. During that time, park officials counted more than 150,000 crossings by mammals, such as elk (Cervus canadensis), moose (Alces alces), black bears (Ursus americanus), cougar (Puma concolor), and grizzly bears (Ursus arctos). Studies demonstrated that the corridors have specifically helped the grizzly bears maintain a wide enough selection of mates to create genetic diversity.

Wildlife populations need to maintain genetic diversity in order to stay healthy. Having a wide variety of genes improves a species' ability to respond to stresses, such as changes in the environment.

Integrating Conservation and Highway Construction

After adding the corridors, Banff saw an 80 percent reduction in motor accidents involving wildlife. These statistics reveal the benefits that can come from a conscientious effort to integrate conservation and highway construction.

In parts of Queensland, Australia, the koala (Phascolarctos cinereus) population has declined at an alarming speed over the past few decades. Auto accidents were among the leading causes of koala deaths in southeast Queensland between 1997 and 2011, according to a government report. Between 2010 and 2013, the Queensland government built about a half dozen wildlife crossings. Several of these were based on existing drainage tunnels under roadways. The government built ledges in the tunnels, which were wide enough for small animals to scurry across without getting wet. One ecologist studying the sites expressed surprise at how quickly the koalas adapted and began using the tunnels.

20 Crossings Bring Huge Drop in Animal-Related Accidents

In recent years, animal crossings have been built in the western United States. Since 2000, Arizona has constructed at least 20 corridors, each with funnel fencing. These crossings have brought about a 90 percent drop in wildlife-related highway accidents in a part of central Arizona known for migrating elk. The state spent $9.5 million on a bridge and overpass along Oracle Road, just north of Tucson. In 2018, Washington state authorities were working on a wildlife bridge spanning Interstate 90, a project that would ultimately consist of multiple underpasses and at least two bridges. It was intended to provide a way for wildlife to pass between the northern and southern Cascade Mountain regions. Even with only the earthen surface completed and no landscaping, deer were already crossing the bridge at one of the passes.

Some types of wildlife crossings, especially overpasses, can be costly to build, but the structures are proving cost-effective over the long term. Ecologists and conservation experts advocate for building wildlife crossings when state legislatures plan highway construction and renovation.

Media Credits

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Tyson Brown, National Geographic Society
National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
André Gabrielli, National Geographic Society
Last Updated

October 19, 2023

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