I've driven from Maine to Oregon all over the west, and back again, so I have a little experience in this regard. It is a matter of opinion, of course, but for what its worth:
For getting going in the morning, I always did like some crunching southern rock like Top and "Radar Love" is a classic choice. My pick would have to be some Soundgarden, though.
For your typical stretch of highway, I look to a little Lu. Williams, Steve Earle, George Thorogood. I'm normally not an Eagles fan, but not bad for the road.
But, for really logging those miles over those long stretches through the grassy plains of Montana, Wyoming or for taking the desert road from LA to LV, something grooving and mellow to really put you in that nice "hypnotized" state of mind is what I prefer. "Lucious Jackson" comes to mind. Maybe some Steely Dan (like Aja for instance). All of this is preferred in the altered state of mind. The right mix of Stevie Wonder, Sly and the Family Stone, and other 70's groove can be just the thing, too.
I'm starting to miss driving.
TheShrimp