Page:Highway Needs of the National Defense.pdf/125

Rh Where sight distances within the range of 1,500 to 2,000 feet are not continuously available throughout the length of a two-lane highway, practical capacities are reduced as shown in table 14, where operating speeds of 45 to 50 miles per hour are desired.

These observed facts of the relation of the degree of continuity of passing sight distance to highway capacity are the basis of the sight-distance standard adopted for the interstate highway system. The standard requires that 1,500 feet of sight distance shall be available on percentages of the length of highway sections rising from 16⅔ to 100 percent as the volume of traffic to be served increases from 500 to 800 vehicles in the thirtieth highest hour of the year.

VERTICAL CLEARANCES OF BRIDGES

The design standard providing that all bridges on the interstate system shall have a clear height above the surface of not less than 14 feet, and of not less than 12½ feet above the outer edges of the shoulders, is based on an allowable height of 12½ feet for vehicles plus a reasonable clearance between the vehicle and the bridge.

Data obtained by Nation-wide planning surveys indicate that the occurrence of loaded trucks and combination units of 12½ feet and over and of 14 feet and over is as follows:

The above tabulation shows there is little demand for bridges to accommodate vehicles higher than 12½ feet and practically no demand for a height that will accommodate vehicles over 14 feet high. The demand for vehicles over 12½ feet actually involves only a few commodities. The principal demand for vehicles over 12; feet stems from auto carriers which, when loaded in the most efficient manner as now practiced, have a total height of 13½ feet.

The tolerance of 1½ feet between the allowable height of 12½ feet for vehicles and a 14-foot bridge height is necessary to provide for:


 * 1) The resurfacing of the pavement, which over a period of years may result in several inches of increased thickness.