According to law  rules must be confined cats in car crate or by latch system that prevents pet disturbing driver. Moreover, in case of sudden braking will prevent cat from hurting. When we drive with animal in the car, we do it as smooth as possible, avoiding hard acceleration or braking, because they are not aware and we can scared them. If they are not accustom, to longer trips, start with a simple walk around the block and reward it something positive.