Why Don’t We Make Roads Perfectly Level?
Roads are made with a curving upper surface, i. e., higher in the middle, in order that the rain will drain away from the road into the gutters or ditches which you find at the sides. You see water has the faculty of running only in one direction, and that is downward. If it cannot go down on one side or the other, it will collect in puddles and make the road impassable. For this reason we build our roads so they are higher in the middle than at the sides—not much higher; only about six inches or so—giving them just the gentle slope toward each side that is necessary to allow the water to run off gradually, but sufficiently sloping to keep the water from collecting in puddles in the road. Thus after the dust has been settled by the first rain that falls, most of the surplus rain that falls on the roads finally runs into the ditches at the side of the road.