I do remember reading an article back in the day about the safety of car harnesses for dogs. I read that most of the ones out there would not hold up to an accident at 50mph, something about the plastc buckles breaking I think. There was only one that the article found (which I can't remember) that would hold up to an accident going 50mph. This particular harness looked a nightmare to put on the dog however - very complicated.
That said, we use a regular car harness for Misha and buckle in a Sherpa bag with Merlin in it. Both are excellent travelers.