As to the highway, you will be surprised at the difference in traffic between here & NYC. Not even a remote comparison. While the speed limit here is 75 on highways, the traffic is light, even at the busiest times. The only scary thing is the semi truckers that drive like a bat out of hell. Going South from Missoula, like you will be, I doubt there will be too many truckers, or even much traffic for that matter. There's not all sorts of interchanges or confusing turnoffs. Just get on the Highway going South. It's a straight path & easy to find the right turnoff. Just look for the signs. It's pretty simple. Even for someone not used to driving highways, it's an easy drive.