Fifty miles east of Seattle, a bridge crosses a steep stretch of Interstate 90 known as Snoqualmie Pass. This is no ordinary bridge, meant for automobiles or pedestrians. Covered in topsoil, boulders, and seedlings, it is intended to convey wild animals from one side of the highway to the other — and it’s working.

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Roads have few equals as a destroyer of animal life. Vehicles claim more wild terrestrial animals — perhaps more than a million per day in the US alone — than any other form of direct human-caused mortality, like hunting, oil spills, or wildfires. And it’s not just common critters like squirrels that get flattened (though we should worry about their welfare, too). At least 21 species are imperiled by cars in the US, and one recent study found that collisions may soon wipe out globally threatened creatures like maned wolves, brown hyenas, and leopards. We are, quite literally, driving some of the world’s rarest animals to extinction.

For more than half a century, countries have attempted to solve this problem using wildlife crossings. France constructed the world’s first crossings, known as passages à faune, in the 1950s, followed by Germany and the Netherlands. During the 1970s and ’80s, a handful of American states, including Wyoming, Florida, and New Jersey, built their own crossings. Many showed promise: After a 100-foot passage was installed beneath I-70 in Colorado, for instance, hundreds of mule deer trotted through each summer.

Yet crossings were slow to catch on in the US, for several reasons. Few states rigorously collected data on animal collisions, masking the problem’s severity. Some early structures were poorly designed or monitored, casting doubt on their efficacy. And even when agencies did document successful crossings, tight budgets rarely had room for more. “The unfortunate thing to date is that the most effective solution is also the most expensive,” wrote one California official in 1980.

Over time, though, collisions became impossible to ignore. As human populations grew, traffic spiked in rural areas. Meanwhile, elk, bear, moose, and especially deer were bouncing back after centuries of exploitation. When speeding cars struck these hefty mammals, the crashes could be catastrophic for both parties. In 1995, researchers estimated that deer collisions caused 29,000 injuries and around 200 human deaths every year in the US. Animal crashes had become a public safety crisis.