I also recommend keeping a window cracked in the car as fresh air seems to help too.
Some kitties do better if they cannot see out of the car, the moving landscape seems to make them dizzier and thus more nauseous. Using a carrier placed on the floor in the car seems to help.
If this doesn't work discuss a medication called Cerenia (maropitant) with your veterinarian. It works very well to combat nausea and lasts at least 8 to 12 hours.
If you truly believe this is all anxiety related, although it seldom is, you can try products such Feliway which is a synthetic version of a calming pheromone cats produce to mark things as safe. It comes in a spray as well as impregnated collars. Spraying the carrier that they travel in may help.
Another option is a calming homeopathic remedy called Bach's Rescue Remedy. It can be used along with Feliway. See this link for further information: http://www.bachflower.com/Pets.htm
If that isn't enough your veterinarian can prescribe anti-anxiety medications for them. Possibilities include fluoxetine, or buspirone.
Please let me know if you have any further questions.