=Surface Drainage.=--The drainage method which is by far the most nearly general in application is that which utilizes open ditches, and the system which employs these ditches is usually referred to as surface drainage. The full possibilities of this method of minimizing the effects of storm water are rarely fully utilized in road construction. Very frequently, deterioration of a road surface is directly attributable to failure to provide adequately for the removal of the storm water or water from the melting of snow that has fallen on the road, or water that flows to the road from land adjacent thereto. Surface water can usually most cheaply and expeditiously be carried away in open ditches, although special conditions are occasionally encountered which require supplementary tile drains. The cross section commonly adopted for roads lends itself naturally to the construction of drainage ditches at the sides of the traveled way, and these are usually the principal dependence for the disposal of storm water. =Run-off.=--The capacity required of side ditches to insure satisfactory surface drainage will be affected by the amount and nature of the precipitation in the region where the road is built. The annual rainfall in a region may amount to several feet, but may be well distributed throughout the year with an absence of excessive rainfall for short periods, that is, flood conditions may rarely occur. In other areas, the annual rainfall may be comparatively small but the precipitation occurs at a very high rate, that is, flood conditions may be common, or it may be at a low rate extending over a considerable period. These peculiarities must be known before an adequate drainage system can be planned. It is almost universally true in the United States that precipitation at a very high rate will be for a relatively short duration, and during these short periods, which usually do not exceed thirty minutes, a portion of the water that falls on the areas adjacent to the road and that drains to the road ditches will soak into the soil and therefore not reach the ditches along the road. The extent to which the water is taken up by the soil will vary with the porosity and slope of the land and the character of the growth thereon. Cultivated land will absorb nearly all of the water from showers up to fifteen or twenty minutes duration; grass land a somewhat smaller percentage; and hard baked or other impervious soil will absorb a comparatively small amount. Rocky ground and steep slopes will absorb very little storm water. The surface of the road is designed to turn water rapidly to the ditches, but when the material is the natural soil, there is always considerable absorption of storm water. Surfaces such as sandclay, gravel and macadam do not absorb to exceed 10 per cent of the precipitation during short showers. Bituminous surfaces, brick and concrete pavements, do not absorb an appreciable amount of storm water.