What a car picks up between salt trucks
A New England car wears three kinds of dirt. There is dried grime baked onto the lower panels, there are wheels packed with brake dust and road film, and there is road salt, which hides underneath where you cannot see it and keeps working long after the roads dry out.
Most tunnels fail at one of those. Chain conveyors grab a wheel and drag the car along by it, which is hard on rims and the underbody. A quick pass of soap at speed skips the stuck-on grime entirely. And a wash that never gets low enough leaves the salt exactly where it does the damage.
The tunnel at the barn is built around those three problems, in that order.
