lateral placement of vehicles; distances for passing; speeds of vehicles; spacing of vehicles in the traffic stream; and hill-climbing abilities of larger vehicles. Improved measuring instruments and methods for analyzing the large volume of data were also developed.
The Bureau immediately applied these early results in design and construction programs. One direct application was the analysis of data on transverse positions of vehicles on bridges to assist in the establishment of standards for bridge widths. The basic relations found between vehicle speeds and spacings with varying volumes of commercial traffic and under various alinement conditions have been used and have aided in estimating the likelihood of congestion in tunnels and on bridges.
These early studies in principles of highway capacity were developed and later expanded by O. K. Normann, the “Mr. Capacity” of highway research. He established that the practical or working capacity of a highway is a relative value, being the number of vehicles that a highway can carry without restricting the speed or movement of vehicles to an extent that drivers find intolerable, He also determined that the minimum spacing the average driver allows between his vehicle and a vehicle ahead varies for different highway conditions, as well as the different speeds, and that the theoretical or possible capacities vary accordingly.
A major milestone was the determination of possible capacities, which are still used by design, traffic, and operating engineers today. On analyses of the data assembled by 1941, Normann concluded that “The possible capacities are about 2,000 vehicles per hour for both lanes of a 2-lane highway, 4,000 vehicles per hour for two lanes of a 4-lane highway, and up to 3,600 vehicles per hour for the best 3-lane highways.”[1] The first edition of the Highway Capacity Manual, published in 1950, and the latest edition, published in 1965, provide the identical capacity values for two- and four-lane highways. The value for three-lane, two-way highways has been increased only slightly to 4,000 vehicles per hour in the latest edition.
Both manuals have been widely used, the first having been translated into nine languages. These successively refined evaluations proved to be major tools for engineers developing the details of current highway programs. With them, designers were able to determine the number of traffic lanes and other geometric features that should be provided to accommodate the predicted volumes and types of vehicles in operation at practical speeds on a given highway.
Continuing research in the 1940’s regarding characteristics of individual drivers showed that “persons traveling long distances drive faster and generally have newer cars than local travelers; young persons drive somewhat faster than older persons; men drive somewhat faster than women; and the newer vehicles are driven faster than older vehicles.”[2] Two important conclusions were developed from this series of studies: (1) Highways built to accommodate a high percentage of drivers traveling on long trips should, therefore, be designed for higher speeds than highways on which trips are predominantly short, and (2) there is no justification for a design of high-ways to accommodate speeds in excess of 70 miles per hour under any condition. For about two decades 70 miles per hour has been the advocated upper design speed for the Interstate System and other main highways.
Truck Performance
Data obtained and procedures developed in the analysis of passing practices in the late 1930’s have resulted in guidelines for proper design of two-lane, two-way highways, including the principles of stopping and passing sight distances. Also, the more fundamental data then developed from the hill- climbing or gradeability studies are still in use today in solving problems of highway design and traffic regulation. A basic conclusion of the gradeability study was that for motor trucks even to approach reasonable speeds on grades, grades must be reduced to 3 percent or less. Where such grades are not practical, these data led to the concept of an additional uphill or climbing lane on highways carrying substantial numbers of heavy trucks.
The desirability of design with grades as flat as practical was recognized in the 1954 AASHO design policy guides. The 1956 Interstate standards named maximum design grades of 3, 4, and 5 percent (except in rugged terrain) for design speeds of 70, 60, and 50 m.p.h., respectively.
In 1948, a major study was conducted on the fuel consumption of trucks in relation to their weight and power. Cooperating in the study were vehicle manufacturing and operating groups, the Department of the Army, the Pennsylvania Department of Highways, the Pennsylvania Turnpike Commission, and the Bureau of Public Roads. Important findings developed then, and still in use today, for commercial vehicles were: (1) On any highway section, gasoline consumption and travel time vary in a definite manner with the rate of rise and fall on the highway; (2) gasoline consumption was definitely related to the gross weight and the travel time to the weight-power ratio of the vehicle; and (3) the results were applicable to paved highways and gasoline-powered vehicles in any part of the country.[3]
This automatic traffic recorder was installed on US 240 in Maryland in 1938. It used photoelectric cells to project two beams across the road, and when a vehicle broke the two beams simultaneously, a cumulative count was recorded on a tape.
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- ↑ O. K. Normann, Highway Capacity, Proceedings, 21st Annual Meeting, Vol. 21 (Highway Research Board, Washington, D.C., 1941) p. 379.
- ↑ Public Roads Administration, Highway Practice In the United States of America (Federal Works Agency, Washington, D.C., 1949) p. 65.
- ↑ C. Saal, Time and Gasoline Consumption In Motor Truck Operation, Research Report 9-A (Highway Research Board, Washington, D.C., 1950) p. 16.
