I always put mine in cages before I put them in the car. There is no reason to have a cat running loose while driving.
I remember when only old women treated their dogs like babies and took them everywhere, now you can't even go to the grocery store without morons of every age and sex taking their little mutts with them.
Current cat is caged for car rides, but I used to have a leash trained cat that would curl up by my left foot when I drove.