HIGHWAY SURVEYS, PLANS, AND COMPUTATIONS

HIGHWAY SURVEYS, PLANS, AND COMPUTATIONS

INTRODUCTION:

Early highway locations were not based on engineering principles. For example, of the 2 ½ million mi of rural highways that had accumulated by 1890, many were positioned by the successive development of the trails. In the vast Mississippi valley area, roads commonly followed the north-south and east-west section and township lines. With state aid and state highway construction in the 1890-1920 decades came better road location. Then, with the development and increased use of the motor vehicle, alignments were improved and grades flattened. Even so, before World War II, rural highway locations in settled areas involved mainly higher standards for the width, line, and grade of existing roads. In urban areas the primary concern was with street realignment and widening and with subdivision layout. Only in the sparsely settled portions of the far west and pacific states and certain mountains areas were major new locations over long distances undertaken. These were carried out primarily on the ground using traditional techniques based on the transit, level, and tape.

            Since World War II, highway location practices have been revolutionized. First, the principle was established that access to major highway facilities must be controlled to protect them from encroachments by land-use activities; this forced the adoption of new locations for many major arteries in both rural and urban areas. For example, it has been estimated that 75-85% of the interstate system lies on new locations. Second, new techniques for surveying, mapping, and computation that have developed during and since the war have largely supplanted traditional methods, at least for the larger highway agencies.

            For modern highway location, engineers must do far more than determine a route that with reasonable economy meets certain minimum requirements regarding curvature and grade. Locations must blend curvature, grade, and other roadway elements to produce an easy-riding, free-flowing traffic artery that has high capacity and meets exacting safety standards. Furthermore, as indicated by Table 3-1 and 3-2, they must recognize and evaluate its economic and environmental impact on existent and future community, industrial, business, residential, science, and recreational values.

            Before surveying and mapping for any highway location are begun, tentative decisions regarding design speed, roadway cross sections, and maximum grade must be made. These, to be sound, must rest on estimates of the amount, character, and hourly distribution of traffic, coupled with knowledge of the area to be the location survey progresses, choices between possible routes and decisions regarding design alternatives must be selected. These often are made with the active participation of local officials and community groups.

            The remainder of this chapter deals with photogrammetry as a tool for highway engineers; some of the problems of highway location in rural and urban areas and the surveys required solving them; and also the realms of highway plans, specifications, and computations. The effect of such environmental factors as noise and air pollution on location is discussed in chapter 12.