A few years ago, we decided before a long cross country trip to get seat belt harnesses for our two dogs. One figured out what the red button on the seat belt does and continually released herself whenever we belted her in. And the other got so stressed out, he turned around so may times on the seat, he choked himself. So the harnesses, for us, were a waste of money.
We now have two Collies - 54 and 62 lbs. - so they're good sized dogs, along with a mini Aussie puppy that will get up to about 30 lbs. We crate the puppy in a car carrier while we are on the road but give the big dogs the run of the coach. Without fail, they will lay down in the floor behind our seats, or one will take the couch and the other will take the doghouse. They don't get up and wander at all until they need to go outside. Then, they'll come up and let us know it's time. If one or both were to begin to get rowdy, we would leash them to the seatbelts on the couch. The biggest problem this has caused us so far is my wife trying to step between them when going for a soda in the fridge.
Because I travel with precious cargo, I make sure to pay attention to the road and what is in front of me at all times. The primary cause of accidents is not paying attention. Because Collies have a tendency for motion sickness, and because I tow, I have gotten into the habit of always making slow starts, turns, lane changes and stops or slowdowns. This combined with watching the road as far ahead as visually possible allows me more stopping distance in the event an accident is about to occur. Knock on wood, but I have never had to slam on the brakes yet in my coach, and I have never had a dog come forward due to a sudden slowdown or stop. Accordingly, I feel giving my dogs freedom to move around and lay comfortably on the floor is worth the chance of an accident and not having them belted in. Of course, no one can ever drive defensively enough to avoid every accident and you can never trust the other driver, but I feel good about putting in over 2,000,000 miles without having an accident on the road. Of course, I'm the guy who runs into his own mailbox trying to back the coach in the driveway. ;D