One of the treats of taking a Yellowstone tour (www.usaparktours.com/yellowstone) is that sooner or later you're likely to be caught in a buffalo jam!
Yellowstone National Park is known for its buffalo jams, where bison walk along the roads, sometimes creating traffic jams which can last for well over an hour. You don't need a whole herd of bison to start a buffalo jam. There are times when people, especially in the summer months, haven't seen a bison on the lower loop yet, and all of a sudden they come across one or two of them, and everyone slows down, or stops in the middle of the road, to grab some pictures or videos, and this is just what happened here!
The Yellowstone National Park Service even stations rangers in certain parts of Yellowstone, to ensure that cars keep on moving, but Yellowstone is such a vast park that rangers can't be everywhere.
I came across this buffalo jam just to the west of Nez Perce Creek, approaching the Lower Geyser Basin.