In Idaho, the freeway limit is 80 and that applies to all lanes. If you're doing 80 in the left lane and someone wants to pass, you usually have to slow down to 70 or so to move right. Since you're already going the speed limit and passing cars & trucks doing it, I see no reason to slow down so someone can break the law. If I can do it without slowing down, then I'll move over.
The question is why are you driving in the left lane then? I believe the law in MT is to travel in the right hand lane unless passing or turning. Though I've never heard of anyone actually getting ticketed for it. I wish they would enforce it though. You also are permitted to go 10 over in order to pass.
