Luckily for the animal and human populations alike, there’s been increasing interest in reducing the negative impacts roads have on wildlife over the past few decades. Road ecologists sometimes struggle to drum up support for wildlife crossings at high-traffic freeways (like the Annenberg crossing in LA) because of this effect. As for the design of the bridges themselves, they’re built a lot like highway bridges, usually beam bridges or arches, just with dirt instead of concrete for the deck.