Why did the Bison Cross the Road?

The once endangered North American Bison were saved from extinction and reintroduced into the wild in Yellowstone. They have repopulated very successfully, and now have a very strong presence in the park. We ran into a large herd of them about an hour into our drive on the "Grand Loop." They were munching on the grasses by the road, and some of them ventured onto the pavement. These had no fear of humans, and did not respond to the car horns. In the video you can see how close they got to us. Shortly after this video was taken one even scraped up against the back of our car as he crossed the road.