Most cats don't like car rides much, but even if you don't take Felix on trips, he will probably have to ride in a car at least once in a while, even if only to go to the vet. Whether you're driving from sea to shining sea or just a mile to the vet, the safest way for your cat to travel is in a secure carrier, preferably a hard plastic one with washable bedding for comfort.
If you're involved in an accident, a cat carrier will protect your cat from injuries on impact and keep him secure afterward. You don't want your frightened cat slipping out an open door to disappear or be hit on the road. If you're injured, your cat will be safe until someone can take charge of him.
A nervous cat will feel more secure in an enclosed space and could be a serious hazard loose in the car. Some cats also vomit when transported by car, either from motion sickness or, more likely, sheer nerves. Cleanup is much less arduous if the mess is confined to a carrier, and a hard plastic one with removable bedding is the easiest to clean.
You can also find soft carriers, which look like duffel bags with ventilation. They're really designed for air travel in the cabin and don't afford any protection other than control in an accident.
A kitty seatbelt—a harness arrangement that fastens to the car's seatbelt—is a reasonably good alternative to a carrier for some cats; although, again, it won't provide much protection. Most cats prefer the security of a carrier.