Your cat may get scared and hide under the seat or escape when the car stops. Make sure your cat is wearing her collar and ID tag even though she's inside of her carrier. Some cat owners also put a leash on their cats just in case they have to take them out of their carriers. Place a towel on the bottom of the carrier so your cat will be comfortable. Towels are easier to clean than blankets if your cat gets carsick.
Some cats get carsick even if they've been in the car a few times.
To prevent carsickness, feed your cat dry food before a trip or withhold its food entirely. Canned food is rich and moist and it's more likely to make your cat sick. Make sure you let your cat uses her litter box before the trip. If you're taking your cat on a long car trip, pack some frozen water bottles so your cat will have cold water, and some dry food. You can put your cat in a large carrier with a small litter box inside.