On the Road Again by Willie Nelson

Duh. It's Willie and it's got the word "road" right there in the song, dumbass. Also; high.

Kerosene by Big Black

I figure if I'm on a road trip I'm probably on my way to kicking someone's ass, probably family. Big Black helps maintain the mood.

Pretenders by Tattooed Love Boys

If you even think about making a road trip tape without the Pretenders on it you're just being an asshole.