On the grade there are 16 curves, 12 between 3 and 4 degrees and 4 between 4 and 5 degrees. Six miles of the grade are surfaced with an undivided pavement 30 feet wide; the remaining 14 miles have a divided four-lane pavement, varying between 20 and 25 feet wide on each side. The average daily traffic in 1948 was 8,650 vehicles, of which 18 percent were trucks and busses.
STOPPING SIGHT DISTANCE
In the 31,831 miles of the interstate system as it is presently improved, there are 21,028 sections of various length, totaling 2,087 miles, on which the road ahead is not visible for a distance sufficient to permit the stopping of vehicles moving at desirable speeds before striking low-lying objects and holes or other dangerous surface conditions. The mileage deficient in this respect is nearly 7 percent of the total rural mileage of the system. Safe operation on these sections can in some cases be provided for by the removal of sight-distance obstructions such as trees, cut slopes, and buildings. In the great majority of cases, however, correction can be made only by changes of the alinement or profile of the road, or both, to provide flatter horizontal and vertical curves.
PASSING SIGHT DISTANCE
Stopping sight distance is necessary for safety of operation on all roads. On two-way roadways of two and three lanes it is equally desirable that the road ahead be visible for a sufficient distance to permit drivers to move into the left or central lane long enough to pass other vehicles without danger of collision with oncoming vehicles. The lack of such safe passing sight distance is the principal reason for the formation of queues behind slow-moving vehicles and a major cause of the obsolescence of highways built in the past. Where passing sight distance is inadequate, safe drivers are restricted in their freedom of movement to almost the same degree as if the lane used for passing were filled with oncoming vehicles; and those who attempt to pass under these conditions are reckless drivers who all too often cause grief to others as well as themselves as a result of their action.
The sight distance required for safe passing varies with the speeds of the passed and passing vehicles and the speed of a possibly oncoming vehicle. For safe passing at the speeds that should be accommodated on the interstate system, a clear sight distance of 1,500 feet ahead is essential. It is not necessary that the road ahead be visible for this distance from every point on the highway. This is a condition that could be fulfilled only at a prohibitive cost. It is essential, however, for reasonable safety, that 1,500 feet of sight distance shall be available continuously over sections of the road sufficiently long to permit the completion of passing maneuvers, and that such sections shall recur at intervals sufficiently short to prevent the building up of queues of vehicles waiting to pass.
One of the more serious deficiencies of the interstate system as it exists today is found in the fact that there are 7,324 miles of the 29,276 miles in rural areas surfaced for two and three lanes—one-fourth of the entire mileage—on which this condition of safe travel is not present.
