Make sure the cat is in a carrier. The first time I took a cat in the car, she tried to jump on the back of the drivers seat and sunk her clasw into the back of my shoulders. Not good for a driver.
I had another cat that I traveled with and always had a harness on her. It is too easy to get out of just a plain collar. I never let her out unless she was fastened to a leash. She never minded the harness and leash and went everywhere with us.