During my visit to Yellowstone, I observed elk right in the town Of Mammoth and walking down the main street of Gardiner Montana. Elk were even feeding in the yards next to our motel and on the football field at the school in Gardiner.
In the fall, during the rut season, the bull elk become very aggressive trying to attract females into their harems and protecting their harems against other males. Bull elk will attack people, and even cars, as we observed in Mammoth. When there were a large group of elk, park rangers and volunteers with there to keep people more than 25 yards away from the elk to help protect them. On the Yellowstone National Park site there are videos that demonstrate why you need to keep away from from the large animals. http://www.nps.gov/yell/photosmultimedia/safetyvideos.htm. Visitors need to be reminded that all he animals in the park are wild and can be dangerous.
During the visit, we photographed mainly one herd of elk, that hung around Mammoth and along the road leading toward Gardiner. At times, the bull elk, which we named Ig Boy,, of this herd was very aggressive, especially if a second bull elk was near the herd.