Put invideo cameras (3-4 should do the trick), all facing the rear of vehicles as they go through. By doing so, you will be able to find out why the accident happened. It is easy to assume that any time two cars hit, the first one skipped a roller due to putting on the brakes and caused the collision. Often that is the case. But what about the one that knocks their vehicle out of neutral and gets a running head start, runs 20-30' down the track at 5-10 MPH and rear ends the vehicle in front of them. Been there done that. Cameras catch that and with a good DVR not only can you catch it but you can also make them a copy of it for them to take to the local police station and for their insurance company when they make a claim. Either way, whether they put their foot on the brake or took off in drive, you're responsibility is absolved.
We had one last winter, a young guy who had been through several times not only take off down the track, but jump both wheels to the right off the inside guide rail and drove virtually all the way down the tunnel, sideswiped a rinse arch breaking a nozzle, jump back on the conveyor just before running over the tire shine and continue out the exit.
He came back and told me the conveyor did and was I going to fix the scratch now on his vehicle. We went in the office and reveiwed the video. Upon viewing it I told him that it was his fault and that if he pursued the claim that I would be sending a copy of this to his insurance company along with a bill for the damage he caused to my equipment. He left and brought his mother back. She reviewed the video, looked at him and called him a 'dumbass' right in my office!
If you run a car wash today without many video cameras you can never know what is going on when you are not there nor avoid the headaches of customers doing 'stupid customer tricks' and then outright lying about what happened. It is worth its weight in gold.