Excellent idea, but my cat escaped!
We are planning to move up north so the cats will be in the car for at least 5 hours. The harness is a brilliant idea. I tried it out on one of my cats, tightened it up so it fitted snugly, then went for a drive. It took her 10 minutes to escape from the harness and climb onto my lap! I don't want them escaping while I'm on the motorway, so I have a contingency plan of a collar and lead in addition to the harness with the lead tied to the seat belt. Or else it's back to the cat basket!