ALWAYS ALWAYS in a box (crate). I have seen and known people who say their dog always liked to be free then had them jump out at highway speed and be so busted up they had to be put down. If you have a dog you don't care about let them loose in the back and nothing will probably ever happen. That being said just remember what was already said about Murphys' Law! In a box they won't be injured as readily when you have to hit the binders in an emergency stop.