Do you all use harnesses on your cats? I always believe in having something on the animal so you can grab it if something happens. Much easier than trying to grab a skinny appendage with painful spikes sticking out of it. :)
I would suggest that no matter how your cat rides, etc., that you always have a carrier stored with you. Consider it an emergency item. A freaked out cat on a leash is much harder to handle than a freaked out dog on a leash.
For my parrot, I have his travel cage set up permanently in the RV. He is so much happier in it than in the carrier because he can see everything going on around him. It's secured in place so he and it won't get tossed around. But I ALWAYS have the carrier with us just in case. I normally only use it to move him between the house and the RV, but it's been used when the RV broke down, etc.