missible for the minimum design speed suggested for such class of terrain.
DEFICIENCY OF GRADIENT
The standards prescribe an absolute maximum of 6 percent for grades on the system. »There are 668 grades that exceed this limit, 560 on rural, and 108 on urban sections of the system. The aggregate length of these excessive grades is 243 miles, of which about 28 miles are in urban areas.
Grades of 3 to 6 percent
While grades up to 6 percent are permitted by the standards, any grade over 3 percent, if it is long enough, will slow heavy trucks to a speed at which they become annoyingly obstructive of the free flow of lighter traffic. If, under these conditions, the pavement is of two lanes, and especially if passing sight distance is unduly restricted, the obstruction of the heavier vehicles may become dangerous as well as annoying.
Under these conditions a remedy is desirable, and it may be applied by reduction of the grade to a maximum of 3 percent, or by the construction of additional lanes on both sides of the highway, or by the construction of an added lane for slow-moving trucks on the uphill side for at least the distance in which they will be undesirably reduced in speed.
On rural sections of the system there are 5,430 grades of 6 percent or less but more than 3 percent. They aggregate about 2,350 miles in length. Of these, 5,018 grades, totaling about 2,180 miles, are on two-lane or three-lane highways; the remainder are on multilane highways of adequate and inadequate width.
Widening needed on grades
Most of these grades between 3 and 6 percent in steepness are short enough, or exist on roadways wide enough, or for other reasons are acceptable, so that no improvement is required for the correction of a grade deficiency. But 916 of them, totaling more than 400 miles in length, should be widened in some way to abate the nuisance of slow-moving trucks. Of these instances, 592 are on two-lane roads that should be widened on the uphill side for an aggregate distance of about 300 miles by the addition of a truck lane; 284 are on two- and three-lane highways for a total length of 114 miles that require widening to four lanes; and 40 are on multilane highways that require widening. There are only 21 miles in the last category.
Steeper grades
In addition to the above widening necessitated on grades of less than 6 percent, a similar treatment is required on 65 of the grades of 6 percent or more, involving the desirable widening of 26 miles, most of which is now two or three lanes wide.
DEFICIENCY OF SIGHT DISTANCE
Safe stopping distance
Within the 31,831 miles of the rural system there are 21,028 sections, totaling 2,087 miles in length—nearly 7 percent of the total mileage—on which sight distance sufficient to permit safe stopping is not avail-
