Page:Historic highways of America (Volume 15).djvu/105

 rolled or compacted by traffic. This varies from about one in twenty to one in thirty on a macadam road, to one in forty or one in sixty on the various classes of pavements, and for asphalt sometimes as low as one in eighty.

Where the road is constructed on a grade or hill the slope from the center to the sides should be slightly steeper than that on the level road. The best cross section for roads on grades is the one made up from two plane surfaces sloping uniformly from the center to the sides. This is done so as to avoid the danger of overturning near the side ditches, which would necessarily be increased if the elliptical form were used. The slope from the center to the sides must be steep enough to lead the water into the side ditches instead of allowing it to run down the middle of the road. Every wheel track on an inclined roadway becomes a channel for carrying down the water, and unless the curvature is sufficient these tracks are quickly deepened into water courses which cut into and sometimes destroy the best improved road.

In order to prevent the washing out of