present state were characterized, on the basis of curvature alone, as generally safe for speeds of 40 to 50 miles an hour, the same curve-frequency test applied to the sections in rolling topography would rate them as safe on the average for 60- to 70-mile speed. This, it should be added at once, is a rather broad generalization, because there are curves in some places on the system in rolling topography that are about as sharp as any in the mountainous areas. By similar broad generalization and like qualification, the greater part of the system in flat rural areas can be characterized, on the basis of curvature alone, as good for speeds approaching 70 miles an hour.
Since adequacy of design is determined by a number of conditions besides curvature, the general characterizations indicated in respect to curvature must be strictly so limited. All conditions considered, a very small part of the system as it now exists is satisfactory for travel at 70 miles an hour.
GRADES
The steeper grades on a highway become serious impediments to movements over the highway only when they are of substantial length. Vehicles climb short, steep grades on the momentum gathered on approaching down grades or stretches of level or light upgrade.
Classification of grades
For this reason, in the survey of the grades existing on the system, grades of various steepness were recorded as significant only where they exceeded certain lengths, varying with the rate of gradient. The determining lengths were 2,000 feet for grades between 3 and 4 percent, 1,000 feet for grades between 4 and 5 percent, and 500 feet for grades of over 5-percent steepness.
Grades of the several degrees of steepness were recorded when, either as continuous sections of single gradient or as an average of different grades within a continuous section, they extended to at least the determining lengths corresponding to the three grade classes. It follows that no grades of 3 percent or less were recorded except as, in combination with steeper grades, they averaged above 3, 4, or 5 percent for the required determining distances.
Grades steeper than 3 percent
As thus defined, there are 6,646 grades totaling 2,770 miles on the entire designated system that are steeper than 3 percent; 656 of these grades, totaling 226 miles, are on urban sections of the system; 5,990, totaling 2,544 miles, are on the rural portion of the system.
Steeper than 6 percent, there are on the entire system 668 grades, totaling 243 miles, of which 108, totaling 28 miles, are on urban sections, and 560, totaling 215 miles, are on rural sections.
The steepest grades
The two steepest grades on the entire system are both in urban areas. Both average more than 10 percent. One is on US Route 40 in the town of Old Washington, Ohio (population, 297). The other is on US Route 70 in Marion, N. C. (population, 2,889).
The grade on US Route 40 averages just over 10 percent on a length of about one-third mile, but it includes a section of 14-percent grade one-fifth mile in length. An hourly traffic of 430 vehicles traveled this grade, up and down, in 1948. The road has an 18-foot two-lane bituminous pavement of high type.
