Page:Historic highways of America (Volume 15).djvu/184

 important, however, as the absorptive power of the road itself, for if a road holds much water the destruction wrought by frost is very great. This trouble is generally due to faulty construction rather than to the material. The density or weight of a rock is also considered of importance, as the heavier the rock the better it stays in place and the better it resists the action of wind and rain.

Only a few of the properties of rock important to road builders have been considered, but if these are borne in mind when a material is to be selected better results are sure to be obtained. In selecting a road material the conditions to which it is to be subjected should first be considered. These are principally the annual rainfall, the average winter temperature, the character of prevailing winds, the grades, and the volume and character of the traffic that is to pass over the road. The climatic conditions are readily obtained from the Weather Bureau, and a satisfactory record of the volume and character of the traffic can be made by any competent person living in view of the road.