This study dealt with the effect of alinement, grade, and pavement type and condition on convoy operations. It involved an analysis of the individual speeds of all convoy vehicles and the longitudinal spacings between them. It was found that grades were the only highway feature that materially affected the speed of the military convoys. Highway grades of no more than 6 percent, and preferably 5 percent, would allow practically all military vehicles to maintain speeds of 25 miles per hour as long as the road surfaces were dry and solid. These data help produce better controls for military convoy operations.
Traffic Control Devices
Units along a highway that regulate or advise the vehicle operators are broadly known as traffic control devices. Presently they consist of traffic signals, signs, pavement markings and other markers or delineations along the roadway. The earliest forms were milestones and notched trees. With the advent of the automobile, signing and markings became prevalent in the early 1900’s on principal highways in and around the larger cities. Most of these early devices were developed by innovative people based primarily on their imagination and ingenuity.
The U.S. literature until the mid 1920’s is practically void of records of research on devices used to control traffic. Instead, a unit was conceived, installed, observed and conclusions drawn for future guidance. Naming, numbering, and marking of roads began with motorist clubs, chambers of commerce, women’s clubs and, to some extent, the States. Being individually initiated, the overall result was a colorful chaos. To bring order out of this chaos and thus help to speed the traveler upon his way, the American Association of State Highway Officials agreed in 1925 that the main roads of the country should be marked with standardized information and direction signs. A 1927 manual, developed by a joint board of Federal and State highway officials, published sign details and the numbering system of U.S. highways. This incorporated the numbering system adopted in 1925 for the Federal-aid highway system, which is still in effect.
In the late 1920’s, a uniform route numbering system for U.S. highways was adopted, and these routes were identified with a distinctive shield design.
The various control devices now used for traffic management were mostly developed through private industrial research. However, as early as 1934 the Bureau of Public Roads had begun a series of intersection studies to determine the delay caused by different control methods. A report in that year showed that for an intersection having a total volume of 2,000 vehicles per hour, operation without any mechanical control incurred the least delay to traffic. “Of all the control methods [studied], officer control permitted the fastest movement of traffic, closely followed by the shortest fixed-time control, and traffic-actuated control.”[1] This study not only set the pattern for much future research in this area, but also established methods and procedures for the conduct of studies of the effect of traffic control devices on the safety and efficiency of traffic operations.
The first national rural manual on traffic control signs was issued by AASHO in 1927, and the Manual on Street Traffic Signs, Signals and Markings (urban) was issued in 1930 by the National Conference on Street and Highway Safety. These first efforts at national standards gave details on sign shape, color, legend and mountings. While colors were prescribed, data were lacking as to their suitability. To fill this need, the Bureau of Public Roads undertook a study, completed in September 1933, on the visibility and legibility of several alternative color combinations of signs visible by day and by night, with and without reflector buttons. This study showed that the black on yellow combination was more effective than either black on white or white on black. It also determined for the first time the effectiveness of reflecting buttons in various sizes and spacings.[2]
The first combined rural-urban Manual on Uniform Traffic Control Devices for Streets and Highways was issued in 1935. With each new edition of the Manual, revisions were made based on research conducted by Public Roads, State highway departments, universities, cities, industries, and others. For example, in the 1955 Manual the stop sign was changed from black on yellow to white on red, a standard in use today. This came about as a result of research tests which demonstrated that motorists observe the red sign more effectively than the yellow.[3]
The Interstate manual adopted by AASHO and approved by the Bureau of Public Roads in 1958 for signing and pavement marking of the Interstate System was the result of recommendations based on their joint engineering and psychological research. The use of lowercase lettering became widespread after research indicated that these letters are more easily read by the average driver. There was diversity of opinion on the color and reflectorization of directional signs. A research project was undertaken by Public Roads in 1957, which gave substantial support for the final selection of green as the approved color for directional sign background on the Interstate System.[4]
- ↑ E. Holmes, The Effect of Control Methods on Traffic Flow, Public Roads, Vol. 14, No. 12, Feb. 1934, p. 240.
- ↑ F. Mills, The Comparative Visibility of Standard Luminous and Nonluminous Highway Signs, Public Roads, Vol. 14 No. 7, Sept. 1933, p. 111.
- ↑ G. Sessions, Traffic Devices: Historical Aspects Thereof (Institute of Traffic Engineers, Washington DC 1971) p. 121.
- ↑ Id., p. 123.
