Not sure if you've mentioned bison in this post, but if people go to Custer State Park or Yellowstone where big herds roam freely, they shouldn't discount their potential to become a threat if you get too close.
NP rangers at Yellowstone post info saying bison may look sedate but can run three times faster than humans can - visitors are gored every year and some have been killed. They're huge and probably would be tough to stop while charging, but people hop out of their cars or RVs and walk right up to a calf to take pics.
Last year we watched one big boy wander into Fishing Bridge campground's A loop where we were staying, and meander around checking out the RVs.