Page:Highways for the National Defense.pdf/31

30 The widths specified are substantially adequate for all normal civil traffic, including the usual percentages of wide and heavy vehicles, with range of speed from 15 to 70 miles per hour. They are completely adequate for all anticipated military usage.

Three- and four-lane pavement widths.—New three- and four-lane pavements should be constructed to provide a width of 12 feet per lane. Pavements so constructed will be completely adequate for all civil and military traffic. It will be unnecessary to widen any serviceable existing three- and four-lane pavements, the lane width of which is less than this recommended standard.

Shoulders.—On roads carrying a present average daily traffic of more than 1,800 vehicles and on less heavily traveled roads wherever feasible, it is desirable that shoulders be provided continuously on both sides of the pavement of sufficient width to permit parking. For this purpose a minimum width of 8 feet is essential. Such provision is desirable for the accommodation 'of normal civil traffic ; it is highly desirable on roads that are likely to be used with some frequency by military convoy movements, to prevent excessive interference with moving traffic by halted convoys unable to clear the traffic lanes.

If, on roads that are likely to be frequently used by convoys, it is not feasible to provide continuous shoulders of a width sufficient for parking, shoulders of such width should be provided on sections not less than 2,000 feet in length at intervals of not more, than 4 miles. Such shoulder sections should be staggered on the two sides of the road, so as to provide space for parking on one side or the other at intervals averaging not more than 2 miles in length. On roads carrying more than 1,800 vehicles per day this should be regarded as a minimum requirement, and exception should be considered only in mountainous locations involving the heaviest grading.

At no point should shoulders be less than 4 feet wide.

Grades (over 500 feet long).—It will be unnecessary to change the grades of any road otherwise adequate.

On roads which it is necessary for other reasons to construct or reconstruct, it is desirable to hold maximum grades (in excess of 500 feet long) within the limits of 6 percent in mountainous locations and 4 percent in nonmountainous locations.

Grades of the recommended limiting percentages will permit operation by the majority of all passenger automobiles at speeds of 60 and 70 miles per hour, respectively. They will permit operation by the majority of motortrucks, with reasonable carried loads, at speeds in excess of 25 miles per hour, and by the majority of tractor-semitrailer combinations, reasonably loaded, at speeds not less than 15 miles per hour.

Curvature.—It will be unnecessary to revise the curvature of any road otherwise adequate.

On roads which it is necessary for other reasons to construct or reconstruct, it is desirable to hold maximum curvature within the limits of 6 degrees in mountainous locations and 4 degrees in nonmountainous locations.

Curvature corresponding to the recommended limits, combined with maximum practicable superelevations, will permit safe operation at speeds of 60 and 70 miles per hour, respectively. In mountainous areas where heavy grading is required, and on low-priority routes where traffic is light, maximum curvature may be increased to 14 degrees. On low-priority routes in nonmountainous areas where traffic is light, maximum curvature may be increased to 10 degrees.