IMHO it's being in the carrier, since they start complaining before they're even in the car. As a young college idiot I drove 3 days with a cat and a friend, and we didn't have a carrier. The cat found herself a "safe spot under a seat, and since it wasn't the driver's seat we let her stay there. She was quiet, seemed pretty calm, even slept. If I knew my cat would stay in a safe spot like that, and would get back in the carrier to go into the vet's office, I might let her out for the drive. But that's not gonna happen.