Somebody mentioned the narrow roads and heavy traffic in Yellowstone. There could be bumper to bumper motor homes, trailers and buses. Chances are very good there will also be herds of bison on the road, and those guys are big, quick and unpredictable. There might also be "bear jams" with vehicles stopped to look at bears which might be black or grizzly, off the road or on it. I think being eaten by a griz while camping is only one of the concerns I would have.