Width of shoulders
Of the 31,831 miles of the system in rural areas, 421 miles are built without shoulders, curbs replacing this normal feature of rural road design. Of the 31,410 miles built with shoulders, 6,273 miles have shoulders less than 4 feet wide; 15,990 miles have shoulders between 4 and 8 feet in width, and on 9,147 miles the shoulders are 8 feet wide or more.
CURVATURE
Of the total length of the system in rural areas, 1,949 miles are located in areas classed as mountainous; 12,572 miles are located in areas of rolling topography; and 17,310 miles lie in generally flat or level areas.
Frequency and sharpness of curves
Both the number and the sharpness of curves, as would naturally be expected, are greatest on the sections of the system in mountainous areas. On the 1,949 miles in such areas there are 5,988 curves, or more than three for every mile, that are sharper than 3 degrees. On the sections in rolling topography, 12,572 miles in total length, there are 9,435 curves sharper than 3 degrees, a frequency of 1 to every 1⅓ miles; and on the 17,310 miles in flat topography the number of curves exceeding 3 degrees is only 6,473 or not much more than 1 in every 3 miles.
On the entire 31,831-mile extent of the system in rura] areas curvature in excess of 3 degrees occurs at 21,896 points, an average of a little
