"America" by Simon and Garfunkel. Especially good if you are planning on taking the New Jersey Turnpike or hitchhiking from Saginaw. Not that your route takes you through Saginaw or New Jersey.
some good old classic rock. Boston and Styx,etc
The Fratelli's newest album (but only if it's accompanied by their last one)
Fingersmoran (I'm completely biased because I'm sleeping with him, but it really is awesome roadtrip music)
The new Death Cab, Jason Mraz and Coldplay albums (but only because I haven't had a chance to listen to them all the way through yet and roadtrips are the best way to manage that)
Old school Michael Jackson and Bon Jovi because you just can't go wrong.
I like Tom Petty for road tripping. And Zeppelin.
Road trips = fun! I'm taking one myself next month. Since I'm the old geiser over here and don't really know what's hot & trendy these days, I'm going to recommend some real grown up music: Art Tatum. Great piano jazz guy. I just bought his CD as part of my Father's Day gift to my dad - he loves it. That way, you and your sis can keep up the chitchat without breaking into song. Because there are no words to be had.
I like the song "Washington is Closer PRETTY PLEASE," by Michael Jackson Timberlake.
Oh! Pardon my mature taste in music.
Journey's Greatest Hits
Tom Petty- Wildflowers and Greatest Hits
Bob Seger- Greatest Hits
The Cars- Greatest Hits
and
311- Greatest Hits
311- Don't Tread On Me
311- 311
I know I can't really call it a road trip unless I've blared some vintage Madonna, but that may be, uh, too much for you. 
I would have to say that The Allman Brothers - Jessica is the greatest open road summer driving song ever.
I usually go with the old school greatest hits stuff that a lot of people mentioned. I always include Bob Marley in that list though.
I also always bring along a couple of albums I've been waiting to listen to and listen to them from beginning to end.
Good call verybadcat! Journey Greatest Hits is a super road trip album.
I'm into the new MGMT & Hot Chip--it would keep the mood up.
new coldplay album, viva la vida
I LOVE audio books! And, if you have a library card, and you should because the Chicago public library is my favorite library of all time and if you don't go there regularly I will be sad, you can go to www.chipublib.com and down load books for FREE! Also you can go to the Netlibrary.com and sign up for an account. Audio books can be your new driving-vacation-tradition.
I think you have to go with a good load of Zepplin. That'll last you at least 10 hours!
I love road trips!! Go for a good alternative list of a little bit of everything 
Id say start it off slow - maybe some Jack Johnson or Allison Krause & Robert Plant. Then move your way onto something more rock like - The Grapes or Rodrigo y Gabriela. At some point you'd need to sprinkle in a variety of Allmans, James Brown, Widespread and The Roots.
Of course, you should roll down the windows singing along with New York, New York by Frank Sinatra as you pass across that border.
Los Lonely Boys, Simon & Garfunkel: Concert in Central Park (1984), Billy Joel, OAR, and definitely Amy Winehouse. She may be a nutcase, but bitch can SING.
Explosions in the Sky - The Earth is Not a Cold Dead Place
It lasts a long time and is pretty cathartic for being motion.
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