Mt200 is another route across the whole state - and by the same number into ID.
Since the eastern part of the state is flat, there isn't a lot of difference between routes. They are all straight, flat, fast, and sparsely settled.
Years ago, following an auto accident on US2, the hospital administrator in Glasgow put my toddler son and me on a mail plane to Billings - it was a small 2 engine plane, with half a dozen stops across the eastern half of the state.
Looking on the map, I remembered that I used US12 for part of a drive between Billings and Glacier Nat Park. I stopped to view prong horns. But I remember the part of US89 north of 12, which crosses a modest set of mountains.