If you are driving in Yellowstone and the traffic comes to a stop most likely there has been an animal sighting and instead of pulling off the road someone has stopped their car in the middle of the road to take a photograph. Or if you find yourself in a long line of traffic moving very slowly chances are it is a bison jam.
We encountered this twice: once when on the photo tour and as we were heading towards the north entrance on our way out of the park. In both cases the bison were on their way to a pasture to graze and instead of having to go through the heavy brush they wisely used the paved road. And in Yellowstone they have the right of way! So we enjoyed the scenery, watched a mother and calf stop to sniff our car before moving on, and for the first time since learning how to drive enjoyed being stuck in a jam.