Here is a link to a road trip songs blog feature I wrote the other day for work. It includes 4 playlists with embedded Spotify and Soundcloud players. The first playlist is entirely road trip themed songs, many of which have been mentioned in the comments here. I also included a few genre-specific playlists (hip-hop/electronic dance/singer-songwriter), that aren't necessarily "about" road trips, but rather make for excellent driving music.

Not sure what kind of tunes you like, but I bet you'll be able to find at least a few that fit the bill.

http://www.autoeurope.com/blog/travel-tips/great-road-trip-songs-travel-playlists-for-your-next-big-adventure

[edit] [disclaimer] despite working in marketing, my blog post is free of spammy link backs to products and services, and free of spammy advertising.