I have a seat belt for my Parsons terrier in the back seat of the landy, and front seat of my Beetle! It is just a straightforward clip in strap and I have used it just on his collar, but if we were involved in a crash I think he would break his neck, so now I clip it to his harness, which will take an impact without choking him. He doesn't seem to mind either way and it stops him trying to chew the windscreen wipers from the inside, and in the beetle, there is a 'shelf' on which he would happily sit if he didn't have a seat belt on! Long journeys I still prefer to crate him.