Not sure if you do or not, but it's imperative that a cat be transported in a proper carrier when driving; for their protection and yours (in the event of an accident, a pet become a projectile and can be seriously injured/get out of the vehicle, etc). If possible, I'll put my cat/carrier in the passenger seat (with humans, they're more inclined to get motion sickness if in the back seat). I'll then always cover up the carrier with a large towel or sheet/light blanket....so that kitty cannot 'see' anything whizzing by them. I try to situate the carrier in such a way that it's secure in one spot and not inclined to slide around or move when I'm turning corners.
My friend told me that HER friend has a dog who is very motion sick in cars and she'd heard and tried anti-static strips on the car and dog is no longer motion sick. I just did a quick search on Google and it seems many people use them, for motion sickness in humans/kids (while traveling in a car), and their pets. Seems like a very simple thing to attach to your car. Here's one that seems to be well reputed but again, I've never tried it.
http://www.mizter.com/