My boys have enjoyed the "Real Wheels" series with Dave and Becky. It's all about various types of vehicles and modes of transportation. They are pretty funny and educational too, and each DVD has three episodes on it. For example, "Land, Sea, and Air Adventures" contains these episodes: There Goes a Boat, There Goes the Mail, and There Goes a Helicopter.
On our recent vacation, I packed a duffel bag full of books on cd, as well as cd players and headphones for each child, and extra batteries. My kids kept themselves entertained solely with that for 16 hours. Your girls may be a little young for that yet, but maybe a favorite story book played on the car cd player would entertain them for an hour or two.