A word to the wise: the Yellowstone bison herd couldn't care less whether you had planned on actually driving on the road. It's their park and they'll turn it into their own personal highway whenever they feel like it, dammit. Even if it means backing up traffic for an hour or so and terrorizing the foolish tourists who make the mistake of running out of their cars to see what's causing the holdup.
A closeup bison head shot isn't worth it. Don't get gored. Stay in your car.