BTW…To get from Missoula to St. Regis requires riding on two segments of I-90. One is about 3 miles. Forget how long the second one is. Maybe 4 miles. There is another option for a third segment, but there is a way to avoid it. It’s just a bit convoluted. I’ve never bothered with it.
You also have to get on I-90 west of St. Regis for a much longer stretch if you don’t follow the route I suggested above or take the Olympian Trail west of town.