Have Cat Questions? Ask a Cat Vet Online.
I suggest that you purchase a condo with closed sides or use a crate with a sheet over it to give your cat a closed in space in which to stay. Many cats are lost at pit stops and rest areas because they are not restrained. Be sure to get a crate with enough room for the cat to move around and be sure there's room for a litter box.
Keep in mind that you will also need a health certificate for the cats if you are traveling over state lines. You can get one from your vet.
You should also talk to the vet about using dramamine or benadryl to control motion sickness.
Let me know if you still have questions.