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 in motor transportation. Crossing deer and other large mammals can create life-threatening hazards on roadways. In the United States alone, there are more than a million automobile accidents per year involving wildlife, racking up more than $8 billion in medical costs and vehicle repairs annually.

More than a Million Animals Killed Every Day

If your point of view considers the well-being of wildlife, the problem is greatly magnified. According to some estimates, automobile collisions kill more than a million animals every day, making them the leading cause of death for many vertebrate species. Still worse is the way major roads and other forms of development can subdivide animal populations and fragment their habitats. Losing access to large areas of their living space makes it much harder for many woodland creatures to forage for food, find mates and carry on their genetic legacies.

As people have become more aware of these dangers, one strategy to mitigate them has gradually gained acceptance: human-made highway crossings designed just for animals. These can come in many forms, depending on the species involved and the geographic features of the land. The most common forms of wildlife crossing are bridges and overpasses, tunnels, viaducts and culverts. Features of this kind are increasingly included in highway planning and road improvement schemes. When successfully implemented, they can greatly reduce the likelihood of collisions, provide a safe corridor for animal transit 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 of various kinds. This is to make them appear like a natural part of the landscape and help invite animal passage. The crossings often work most effectively in conjunction with highway fencing, placed strategically on one or both sides of the entrance to funnel wildlife toward the corridor.

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, elk and other mammals. The Dutch built the world's longest animal crossing, the Natuurbrug Zanderij Crailoo, an overpass that spans more than 0.8 kilometers (0.5 miles). Wildlife crossings can also be found in Australia, Canada and other parts of the world. The idea took a little longer to catch on in the United States, but wildlife bridges and tunnels began appearing there in the 21st century.


One of the earliest U.S. installations was in Davis, California, a college town near the capital of Sacramento. 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 dressed up the area near one entrance as an amphibian-sized village, naming it Toad Hollow. Despite the Davis community's efforts, the local toad population did not adopt the byway.

Far more successful is the system of corridors in Banff National Park in Alberta, Canada. Between 1996 and 2016, 44 structures — six bridges and 38 underpasses — were built for wildlife to traverse the Trans-Canada Highway, the nation's longest road, which bisects the park. During that time, park officials documented 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 stabilize their genetic flow. In addition, the mitigation contributed to an 80 percent reduction in motor accidents involving wildlife. These statistics reveal the full scope of the beneficial effects that can arise from a conscientious effort to integrate conservation principles in highway construction.

Integrating 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 state government introduced about a half dozen wildlife crossings. Several of these were modifications of existing drainage tunnels under roadways. Within these culverts were added ledges 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, several installations of animal crossings have occurred in the western United States. Since 2000, Arizona has constructed at least 20 corridors, including 17 underpasses, each augmented by funnel fencing. Their presence has brought about a 90 percent drop in wildlife-related highway accidents in one stretch of central Arizona known for migrating elk populations. 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, which had recently been widened to six lanes. The project, which would ultimately consist of multiple underpasses and at least two bridges, was intended to provide a means of wildlife access between the northern and southern Cascade Mountain regions. With only the earthen surface completed and no landscaping done, deer were already actively crossing the bridge structure at Snoqualmie Pass.

Some types of wildlife crossings, especially overpasses, can be costly to build, exceeding the typical budget allocations of local and regional transportation authorities. But the structures are proving cost-effective over the long term. Ecologists and conservation experts advocate for building in interventions such as wildlife crossings during legislative planning on 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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