Page:America's Highways 1776–1976.djvu/396



The period 1930—1940 was one of discovery, as well as of stabilization. On the debit side, administrators were dismayed to discover that many highways paved during the previous decade, in the expectation they would last a lifetime, were already obsolete. There were several reasons for this—the alinement was not suited to current speeds, sight distances were too short, pavements and shoulders were too narrow, and traffic volume had simply outgrown the capacities of many two-lane roads. Moreover, the seemingly unlimited capacity of multilane roads near cities was being rapidly depleted as a result of uncontrolled access. Pavements were deteriorating because of the frequency of heavy axle loads. Trucks were seriously interfering with the free movement of passenger cars where there were long steep grades.

On the plus side, the most outstanding discovery, or more properly, achievement, was the recognition that most problems could be resolved through a co-ordinated research effort coupled with an organized program to exchange knowledge and information gained in the various States.

A coordinated research program was given a strong boost by the U.S. Congress in 1934 through passage of the Hayden-Cartwright Act. This Act authorized the use of up to 1½ percent of Federal-aid highway funds for planning and research.

Secondly, distilling the results of experience and research and promoting the best in design practices commensurate with economic benefits found expression through the AASHO Committee on Planning and Design Policies.

In February 1937 a proposal was approved by AASHO to establish a special committee consisting of three key officials from the Bureau of Public Roads (BPR) and 12 outstanding design engineers from the States. This committee was named the Committee on Administrative Design Policies. It was subsequently renamed the Committee on Planning and Design Policies, and membership was increased to include 20 members from the States with BPR furnishing a single member, a nonvoting secretary. A unique feature of the committee working process was the provision of a small force of experts assigned by BPR to devote full time, if necessary, to the work of the Committee.

The Committee’s mode of operation was to outline a general program of work, after which the BPR task force gathered and evaluated all the known information on each subject. If there were gaps in the existing knowledge, the BPR engineers identified them for further study. Eventually the staff prepared a tentative discussion, with indicated design controls, guide values and other conclusions for that subject. This was then criticized, evaluated, and supplemented by the Committee members and reworked until a policy acceptable to them was produced. The resulting policy was submitted through the Committee on Standards to the AASHO Executive Committee for ballot by the several States; with a two-thirds favorable vote, it became an approved policy, and also, in effect, the national design policy of the United States on that particular subject.

The Committee soon developed design policy brochures on seven projects: (1) Highway classifications; (2) sight distance; (3) marking and signing no-passing zones; (4) highway types; (5) intersections at grade; (6) rotary intersections; and (7) grade separations. These booklets have considerable historical significance since, together, they are the fundamental structure upon which all subsequent geometric design policy for highways has been based.

A Policy on Highway Classification, 1938—offered a method of classifying highways to indicate the service expected of them. Three factors were considered in the classification: (1) Traffic volume, (2) character of traffic, and (3) design speed.

“Traffic volume” represented the number of vehicles per hour and was defined as the average of the probable maximum hourly traffic of several peak days. Prior to this time, it had been common practice to consider traffic on a daily basis if, in fact, it was considered at all in geometric design. In later years, research was to permit a degree of refinement, and the 30th highest hourly volume of the design year became the criterion of traffic volume classification for design purposes.

“Character of traffic” was used to denote the relative number, or percentage, of trucks and buses in the traffic stream in order that allowance could be made for them in design and operation of highways. Three categories were utilized: “P” for traffic composed entirely of passenger cars or types of trucks which did not impede smooth traffic operation; “T” for traffic in which the percentage of trucks likely to use the highway was such that movements of passenger cars would be interfered with and, consequently, should be given detailed considerations; and “M” for mixed traffic where the percentage of trucks was between that for “T” and “P.” While the classification was necessarily vague, it did serve a useful purpose in recognizing that truck traffic was an essential factor that must be dealt with. This classification stood for many years and was discontinued only after a means was devised in 1950 for converting truck volumes into equivalent passenger car volumes.

The “assumed design speed” was used for correlating the design features of a length of highway that affect, or are affected by, the speed of operation. For this purpose, assumed design speed was chosen as being representative of the maximum approximately uniform speed that probably would be adopted by the faster group of drivers, exclusive of the reckless few, that would use the highway. Speed classifications of 30, 40, 50, 60 and 70 miles per hour were agreed upon, and guidance was offered as to conditions under which each might be appropriate.

Adoption of speed as a design criterion was in recognition of the fallacy of the previously held belief that drivers could be relied upon to detect sharp curvature and short sight distances sufficiently well to adjust their speed to conditions. The false assumption that drivers would reduce their speed to as low as 15 or 20 miles per hour at curves was a major cause for high accident rates and the early obsolescenseobsolescence [sic] of many highways paved prior to 1930. The design speed concept was truly a landmark innovation in highway engineering. 390