Page:America's Highways 1776–1976.djvu/397

 Center markings on curves aided motorists on this highway in 1921, but short sight distances made passing dangerous.

A Policy on Sight Distance for Highways, 1940—provided a scientific approach to answering the controversial question as to the length of sight distance that should be provided to assure safety at curves and crests and also for overtaking and passing slower vehicles on two- and three-lane highways.

In the absence of research results, which were to become available a few years later, certain assumptions were necessary in deriving the values. It is a credit to the Committee that the assumptions were remarkably accurate. In one of the first applications of the design speed concept, the derived minimum sight distance values for stopping ranged from 200 feet for an assumed design speed of 30 m.p.h. to 600 feet for a design speed of 70 m.p.h. It was further postulated that this distance should be measured on a line from the driver’s eye 4.5 feet above the surface to a hypothetical object 4 inches above the pavement. There was considerable conjecture as to the height of the object. Logic would dictate that visibility of the road surface itself would provide the ultimate in safety, but provision of the required sight distance to the road surface would necessitate extremely long vertical curves at hill crests and would, therefore, be very costly. A standing or slow moving vehicle would be the type of obstacle most likely to be encountered on a highway and a height of 2 feet, representative of the height of tail lights for a typical vehicle, was favored by some Committee members. Obstacles a foot or so in height, even though encountered only infrequently on highways, could result in serious accidents if struck by cars and, consequently, such minimum heights were rejected on the grounds of being unsafe. The 4-inch dimension finally agreed upon represented a compromise between economics of construction and severity of hazard. (Both dimensions, height of eye and height of object, were reevaluated in 1961 and, because of reduced height of vehicle, were changed to 3.75 feet for height of eye and 6 inches as a compromise value for height of object.)

Passing sight distances were derived by dividing the passing maneuvers into several component parts and developing time-space relationships for each. Desirable distances, as well as absolute minimums, were developed. For two-lane roads, the “desirable” values ranged from 600 feet for a design speed of 30 m.p.h. to 3,200 feet for a speed of 70 m.p.h. The sight line was from the driver’s eye, assumed to be 4.5 feet above the road surface, to the top of an on-coming car, also assumed to be 4.5 feet in height. Both dimensions were changed to 3.75 feet in 1961 in recognition of changes in vehicle design.

The three-lane highway, like the single-lane highway of 20 years earlier, enjoyed a comparatively short period of popularity, and sight distances for such highways, as well as for two-lane highways, were discussed in this policy. Very few three-lane projects were constructed before 1930 or after 1940. It was soon learned that, while they did provide greater capacity than two-lane roads, the added increment was not great. With the rapid increase in traffic, they were soon taxed to capacity. Moreover, they did not lend themselves to conversion to multilane divided highways. Of greatest concern, however, was their poor accident experience. One reason for this 391