Quote Originally Posted by denkimi View Post
The way gravel behaves. The more cars pass, the deeper the ruts get. If 20 cars make small ruts, 120 cars make big ruts.

But the deepness of course also depends on the condition of the road and the weather.
Have you ever been on Rally Mexico?
Also you need to remember that biggest ruts are made by modern WRC cars, other cars just made them little bit deeper, but WRC cars are making most of the damage.