Wildlife bridges are often established where highways weave through densely forested areas with almost no human habitations. The manmade roads, however, tend to artificially divide the habitats of wildlife.

Because of this, the highways can make it difficult for animals to migrate or forage for food. In addition, attempts to cross the highways lead to dangerous situations for both animals and human drivers.

As a solution, some governments have invested in ways to help animals to cross highways safely. European countries with large swathes of forests, such as France, the Netherlands, and Switzerland, started building wildlife bridges in the 1950s. The trend has also caught on in the United States, which recently allocated 350 million dollars to build new animal crossings.

Some of these bridges are simple and specific, such as a rope that can be climbed by agile squirrels. On Christmas Island in Australia, there are special bridges used by millions of red crabs during their annual migration.

Other bridges are massive structures built to serve the entire wildlife population. Stretching over multilane highways, the overpasses are covered in vegetation so that they seem to blend in with the surrounding trees and fields. They enable bears, deer, and other animals to make their way across the highways and avoid colliding with automobiles.

Biologists say it often takes many years for local wildlife to get used to the new, animal-friendly infrastructure. So, the long-term effectiveness of wildlife bridges and their impact on animal behavior can be unclear.

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However, local government officials say the immediate benefit can be seen in the reduced number of highway accidents involving wild animals.