US route endpoint in
West Yellowstone

That was the original north end of US 191.  Then, for a brief period from 1979-1981, it was the south end of US 191.  Today it is the south end of the northern segments of both US 191 and US 287 (which both resume at the South entrance), as well as the east end of US 20's western segment (which resumes at the East entrance).  

Here we are looking the opposite direction, at the point where all three highways are signed for the first time west of the Park (the three routes overlap for only a short distance before they go their separate ways):