Considerations of safety require that the length of road ahead visible to drivers of vehicles be at all points at least as great as the distance required to stop a vehicle moving at the assumed design speed, allowing 2 seconds for perception and brake reaction and a braking distance based upon a uniform coefficient of friction of 0.4. The distance thus required is 614 feet. Three-degree curvature in unwidened cuts allows a minimum sight distance of 690 feet; 4° curvature under similar circumstances provides a minimum sight distance of 590 feet. The higher of these standards, therefore, would conform to the requirements of safety corresponding to the assumed design speed of 70 miles per hour, the lower would be only slightly deficient; but 6° curvature under similar conditions would provide seriously insufficient sight distance for safety of operation. On the 153.8 miles located to the latter standard it might be necessary in places either to widen cuts where they occur on the sharper curves or indicate the necessity for caution by warning signs.
Modern passenger cars in good condition are capable of maintaining relatively high speeds on 3- and 4-percent grades and even on grades of 6 percent. Motortrucks, as at present designed, are incapable of such performance. Tests by the Bureau of Public Roads indicate that the larger vehicles now in general use, when loaded in accordance with the manufacturer’s gross-weight rating, cannot be expected to climb 4-percent grades at a speed greater than 25 miles per hour. On 3-percent grades a speed of 30 miles per hour is the maximum that may be expected under the same conditions. With vehicles loaded 50 percent over the manufacturer’s rating—a practice that is not uncommon—the corresponding maximum speeds for 4- and 3-percent grades are 16 and 22 miles per hour respectively. Under either condition of loading a 6-percent grade slows the larger modern trucks to a crawling speed.
On grades of steepness approaching either the 3- or 4-percent standards and, of course, on the exceptional 6-percent grades, ample provision is required to permit the passing of trucks by passenger cars.
On 2-lane roads the sight distance, wherever possible, should be sufficient to permit passing of the slower vehicles when the visible length of road is clear of opposing traffic, even if opposing traffic appears after the operation of passing is begun. Where such a sight distance cannot feasibly be obtained signs must be placed to prohibit passing. Such limited sections should be permitted only as exceptions, and in no case should their continuous length exceed about 2 miles.
PAVEMENTS AND THEIR FOUNDATIONS
For roads of the character of those proposed only high-type pavements are appropriate. The pavement thickness should be designed in each section to be consistent with the wheel loads to be expected. A smooth-riding surface, as nonskid and glareproof as possible, should result. Subbases should be used wherever required; and, both for pavements and subbases, good local materials should preferably be used, if available.
Material encountered in foundations should be analyzed, and, if found to be unstable, should be replaced or improved by stabilization,
