The Hidden Engineering of Wildlife Crossings
22nd December 2024
This is the Wallis Annenberg Wildlife Crossing under construction over the 101 just outside Los Angeles, California. When it’s finished in a few years, it will be the largest wildlife crossing (*of its kind) on the planet. The bridge is 210 feet (64 meters) long and 174 feet (53 meters) wide, roughly the same breadth as the ten-lane superhighway it crosses. Needless to say, a crossing like this isn’t cheap. The project is estimated to cost about $92 million dollars; it’s a major infrastructure project on par with similar investments in highway work. And it’s not the only example. The Federal Highway Administration recently set aside $350 million federal dollar to fund projects like this. The reasons we’re willing to invest so much into wildlife crossings aren’t as obvious as you might think, and there are some really interesting technical challenges when you’re designing infrastructure for animals. I’m Grady, and this is Practical Engineering.
Roads fundamentally change the environments they cross through. And while on its face, it might seem that it’s always a disaster for wildlife, there are actually some winners amongst the losers. For vultures, crows, coyotes, raccoons, insects, and other decomposers, roads provide a buffet for nature’s scavengers. And they sometimes make for pretty good housing too, at least if you’re a swallow or a bat. In fact, cliff swallows are now so famous for nesting on the underside of highway overpasses that they’re often referred to as bridge swallows. The sides of highways have clear zones kept free from trees and similar obstacles for vehicle safety, but the lack of shade allows tender greens to thrive, creating a salad bar for species from monarch butterfly caterpillars to white-tailed deer.
Of course, especially in the case of deer, this can attract animals into spending time eating dinner in danger. And the truth is that roads mostly range from a mild inconvenience to totally catastrophic for wildlife. In the battle between the two, wildlife usually loses, and in more ways than just getting squished. The ecological impacts of roads extend beyond the guardrails. Habitat loss and fragmentation, noise pollution, runoff, and of course, injecting humans into otherwise wild places are all elements of the environmental challenges caused by roads. It’s actually a pretty complicated subject, and there are even road ecologists whose entire careers are dedicated to the problem. And it’s not just wildlife that’s affected.