Animals are smarter than we give them credit for. They know roads are dangerous to cross, and know how to avoid doing so.
According to a study by the University of Maryland, almost 60 different species of animals have been spotted using underground drainage culverts to safely cross roads. Not only do they use the tunnels, they also show preferences regarding tunnel design, and teach their young how to use the tunnels safely. Deer avoid tunnels with rocky floors, herons like tunnels big enough to flap their wings in, and just about all the observed species like tunnels with unobstructed views.
The study used motion-capture cameras to photograph thousands of animals using the tunnels. A few examples are reproduced below. They clearly show why this behavior is as beneficial to humans as it is to animals: no car driver wants to hit a giant horned buck or a stinky skunk.