Page:America's Highways 1776–1976.djvu/387

 Highway design is the art of anticipating the roadway requirements of motor vehicle traffic and of satisfying these requirements in the preparation of construction plans. It involves the selection of dimensional values for geometric features such as widths, radii of curvature, rates of cross section slopes and longitudinal gradients; the determination of the requirements for the roadway structure (pavement and base courses) and for the bridges, drainage and other structures ; the preservation and restoration of ground cover and plant growths; the safety and efficiency of traffic operations, including all forms of traffic control devices; and the melding of the highway’s alinement and gradient with the landscape in a manner least disturbing to the natural environment. Engineering analyses during the highway design must attain an acceptable balance between the broad controls of economic limitations, land space usage and environmental and social considerations.

The roads existing during the early years of the automobile were not designed for motor vehicle usage; they were simply wagon roads. For many years after the automobile came into wide use, these same roads were adapted to automobile travel with little or no change in location, alinement or geometries. Even after the automobile became commonplace, a number of years, or even decades, were to elapse before highways were constructed to meet the unique requirements of motor vehicle traffic.

The transitional period from horsedrawn to motor vehicle traffic occurred at a time when railroad transportation was approaching maturity. Trains shared with the automobile one distinguishing characteristic when compared with horsedrawn vehicles—both traveled at a faster speed. It was natural, then, that when travel speed became recognizable as a factor in highway design, the leaders in this art turned to the practicing railroad designers for guidance in determining alinement design details to satisfy the requirements of the higher speeds of motor vehicles.

While building upon these bases of knowledge, highway designers were often in a dilemma as to which features of wagon roads should be retained, modified, or rejected, which characteristics of railroad practice should be incorporated, and what new concepts were needed to satisfy the peculiar qualities of automobile traffic. Resolution of these matters was often the subject of heated debate with the answers frequently being dictated by economics.

From the standpoint of highway design, the automobile era logically falls into four fairly distinct periods: (1) 1900–1920: the preemptory period during which the motor vehicle usurped the wagon road in its contemporary state of improvement; (2) 1920–1930: the uplifting period during which existing roads were adapted to motor vehicular use, largely by shaping the cross section, draining and surfacing; (3) 1930–1940: the period of stabilization in the design of conventional highway during which the dynamics of motor vehicle traffic were recognized as a major force to be dealt with; and (4) 1940–1976: the era of the freeway.

It is significant that throughout the brief period of development of the modern highway system there has been a lag of a decade or more between the time of recognition of a new design principle, or a standardization of a dimensional value, or the advocacy of a concept, and the time that results of these advances in design begin to show in completed highways. For example, a discovery or determination in 1910 that highway curves should be superelevated to offset centrifugal force found little application until after 1920 when the extensive program of surfacing highways came into full swing.

The categorization of the four design periods reflects the period of application of design principles, rather than the period of their discovery. Each of these periods is discussed in terms of the design characteristics in vogue at the time and of the factors that influenced these characteristics.

The automobile came on the scene at a time when roadbuilding was undergoing a renaissance after 50 years of neglect. The better rural roads of the mid-19th century were waterbound macadam, 12 to 15 feet wide—a width that was adequate for two loaded wagons to pass each other with the horses at a walk. To. shed water quickly, these roads were crowned on both the straight sections and the curves, and the crown, 6 to 8 inches higher in the center, was steep enough to make driving at the sides uncomfortable. Consequently all traffic ran in the middle, taking to the righthand side of the road only to pass other vehicles.

In 1913, even mountain roads such as this earth road in Wise County, Va., had relatively flat grades to accommodate horsedrawn vehicles. 381