Tunnels For Bears
Actually, the tunnel located just a bit north of Yellowstone isn't only for bears. It'll handle deer, elk, whatever kind of critters want to walk through it to avoid the traffic overhead on I-90.
The tunnels are modeled after some used in Canada, along with bridges, that allow wildlife to avoid traffic when it comes time to cross the highway corridor. I first saw some of those bridges/tunnels back in the 1990s when my wife and I went to Canada for a cycling trip. Those overpasses are landscaped like the surrounding forest, so the animals feel pretty much at home crossing them.
The idea is that by providing these types of relatively safe corridors, migrating wildlife and wildlife that simply likes to roam -- such as bears, wolves, wolverines, etc -- will be able to co-mingle their genes with populations north of Yellowstone and thus prevent isolated gene pools.
Anyway, on each side of the interstate the tunnels in Montana will involve about two miles of fencing -- one mile on each side of the underpass -- to funnel wildlife into the tunnel. You can read about the project here.