Yellowstone Park regulations say you should stay 75 feet from a bison when you are on foot. Given they can run 30 mph, they can cover 75 feet in something less than two seconds–if they feel inspired to do so. Bison are generally pretty lazy; so presumably the 75 feet must be a distance thought far enough for them not to get irritated enough at a human to motivate them to run after you. Observing and photographing bison from a car seems like a safer bet, especially if you’re trying to get a close-up head shot without a really long telephoto lens. Gotta love the frosty top-knot on this one.