NPR has a list from John Sandford's new book (Broken Prey) -- 100 best songs from the rock era for the road. It's a good list. He has most of my essentials, such as "Born to be Wild." I would add that you can't have road music without "Runnin' Down a Dream" from Tom Petty and the Heartbreakers and maybe a little Rush, say their album of covers, "Feedback." It's cool stuff (and what I'm listening to at the moment).
Lynyrd Skynyrd's "Call Me the Breeze" is a good one to add to that list, too, and you have to have "Freebird" for a good road trip. I would include "Evenflow" by Pearl Jam and "Superunknown" by Soundgarden. (Rock is rock, after all. Throw in some Foo Fighters while you're at it.) And Sandford has neglected to include anything by the Doors! So I would add the full version of "Light my Fire."