Traffic and Safety
Capacity and Design
In response to Chief MacDonald’s concern, research was directed in the 1920’s into three major areas: (1) Road construction and maintenance, (2) the economics of road operation, and (3) the economic value of highways to a community. Although elements of highway capacity and design were mentioned, it was W. K. Hatt, Director of the Highway Research Committee of the National Research Council, who in 1921 raised the following questions: “What is the capacity of a road of a given width for any particular type of vehicle as expressed in vehicles per hour, ton-miles per year, etc.? What is the appropriate unit for expressing traffic for various purposes?”[1]
Research on Highway Geometries
In March 1925, Public Roads first reported a number of significant facts concerning the lateral distribution of traffic. These factors included the effect of road width, curves, shoulder conditions, grades, surface crown, and other physical features. Observers recorded the positions of tires in relation to lines painted across the pavement at 1-foot intervals. The results indicated that 18 feet was the minimum pavement width which would permit passenger cars and trucks to pass in safety and with a reasonable amount of clearance. Speeds were not considered in this study.
Four years later, in 1929, when equipment for measuring speeds became more reliable, it was found that the safe passing of “rapidly” moving automobiles and trucks required a surface width of at least 20 feet for two-lane rural highways.
Since 1929, researchers have progressively provided the basic information required to set design standards and operational controls for main roads and freeways. In December 1944, one of the recommendations of a BPR study of the effect of roadway width on traffic operations was that lane widths should be 12 feet for safety and comfort of traffic operations. The AASHO soon adopted this width as a standard for primary highways in the United States. Within a short time this standard was also adopted in many foreign countries. Vehicle widths remain about the same today, and this lane width remains the basic standard.
Horizontal and Vertical Curves
An early design policy statement that is still current says that “In the design of highway curves it is necessary to establish the proper relation between design speed and curvature and also their joint relations with superelevation. While these relations stem from laws of mechanics, the actual values for use in design depend upon practical limits and factors determined more or less empirically over the range of variables involved.”[2] A series of studies have been made to relate the speed of operation on highways to the horizontal and vertical alinement. One of the earliest studies on the effect of speeds on geometric design was conducted in 1929. Even though the legal speed limits in the 1920’s were 35 to 45 miles per hour, A. Bruce, in his article “The Effect of Increased Speed of Vehicles on the Design of Highways,” concluded that there was need for easier curves, greater superelevation of curves, more extensive vertical curves, and greater sight distance on both horizontal and vertical curves.
In 1953 a comprehensive study of driver behavior on vertical and horizontal curves was completed by Public Roads in cooperation with the New York Department of Public Works that showed that the percentage of drivers exceeding a safe speed for the curve radius increased greatly on the sharper curves. The combination of curves over 5 degrees (radius less than 1,200 feet) and grades in excess of 5 percent also were found to result in an especially high accident rate on conventional rural highways.
A comprehensive study of “Driver Performance on Horizontal Curves” was conducted during 1951–1954 in cooperation with five States at 35 horizontal curves. The separate studies were on two-lane, two-directional roads. Among the more important conclusions were the following:
- Drivers did not change their speeds after entering a horizontal curve.
- The existing superelevation had no effect on speeds.
- Operating speeds and the radius of curve are linearly related.
- The curve radius had a greater effect on speeds than sight distance.[3]
In 1969 data in NCHRP[N 1] Report 68 on the application of vehicle operating characteristics to geometric design substantially verified these results.
- ↑ National Cooperative Highway Research Program.
Highway Capacity
Highway capacity has been the subject of careful and painstaking study for nearly six decades. A rational and practical method for the determination of highway capacity was essential for the sound economic and functional design of new highways and for the many existing roads and streets which must continue in use for extended periods of time. Basically, highway capacity concerns the effectiveness of various highways to serve traffic and involves the many elements of highway design, speeds, vehicle and driver performance, and traffic control. In recent years, due to the development of refined traffic study methods, instrumentation and equipment, substantial amounts of reliable field data have been used to develop new insights into problems of and solutions to traffic operations.
In 1934 the BPR set up and took a dominant part in a coordinated series of studies to obtain the basic data on highway capacity. This widespread effort included studies on: methods of counting traffic;
- ↑ W. Hatt, The Field of Highway Research, Public Roads, Vol. 4, No. 5, Sept. 1921, p. 17.
- ↑ Policy On Geometric Design of Rural Highways (American Association of State Highway Officials, Washington, D.C., 1965) p. 152.
- ↑ A. Taragin, Driver Performance on Horizontal Curves, Public Roads, Vol. 28, No. 2, Jun. 1954, pp. 27, 28.
