Page:America's Highways 1776–1976.djvu/399

 A Policy on Highway Types (Geometric), 1940—outlined the distinctions between the two-, three-, and four-lane highway types and divided highways. It dealt primarily with pavement widths and factors of driver behavior and highway design that affect width. The policy related the traffic volume, the character of traffic (“P,” “M” or “T”), and design speed to the minimum width of pavement that should be provided. For two-lane roads, the minimum width of surfacing varied from 16 feet for the lowest classification upward to 24 feet. (Single-lane roads were acceptable for volumes below five vehicles per hour.) Turning paths were developed for three design vehicles—a passenger car, a truck, and a tractor-semitrailer.

The Committee lamented the absence of better highway capacity information that would permit delineation between the traffic warrants for two-, three-, and four-lane pavements. Because of the intervention of World War II and for other reasons, such information would not become available for another 10 years, although the basic research was already well advanced.

The policy discussed median design for divided highways at considerable length, as well as curbs, sidewalks, guardrails and shoulders. Shoulders 8 to 10 feet wide, clear of all obstructions, were recommended.

A Policy on Intersections at Grade, 1940—treated the subject in great detail, utilizing turning paths for a design passenger car and a design truck. Design requirements were developed for various types of intersections from a simple crossing to the more elaborate types of channelized intersections with relatively high-speed turning roadways and speed change lanes. Innovations included three-centered compound curves for the pavement edge at turns, minimum radii for separate turning roadways as related to design speed, pavement widths for such roadways, and sight distance requirements at intersections not controlled by signals. Vehicle dimensions have changed since this policy was prepared, but in all other respects the concepts have stood the test of time with little need for modification.

A Policy on Rotary Intersections was not completed until 1941. During the period 1930–1940, rotary intersections were thought to be a considerable improvement over conventional intersections, so much so that they were often constructed as substitutes for grade-separated interchanges. Events were to prove that, like the three-lane road, their effective life was usually rather short because of their limited capacity and the rapid rate of traffic growth.

The policy provided guidance in selecting various design dimensions such as radius of the central island, roadway widths, and lengths of weaving sections as related to design speed.

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