these lanes appeared uncomfortably narrow to most motorists, especially when passing trucks. The lane lines also caused trucks to run closer to the shoulder where they caused increased breakage of slab edges and corners. To provide greater safety and reduce edge damage, the State highway departments built wider pavements, and they also made new roads straighter. These improvements along with mechanical advances in vehicles, such as more powerful engines and four-wheel brakes, in turn encouraged even higher road speeds.
Thus, after 1918, highway design followed a spiral of cause and effect, resulting in higher and higher speeds and wider and wider pavements. The motivating force behind this spiral was the driving speed preferences of the great mass of vehicle operators, and the public authorities were never able for very long to impose or enforce speed limits that this great mass of operators believed to be unreasonably low.[N 1]
Balanced Design for Safety
In the 1920’s and 1930’s, it was good engineering practice to locate new highways as much as possible in long straight lines or “tangents.” When it became necessary to change direction, the locator laid out a circular curve, the radius of which he selected to fit the ground with the least construction cost but which could not be less than a certain minimum fixed by department policy.[N 2] In practice, locators made the curves natter than this minimum when it was cheaper to do so, but with little consistency. Motorists driving these roads were expected to adjust their speeds to the varying radii; and on the sharper curves safe speeds might be considerably lower than the posted speed limit.
In their increasing concern for highway safety, many highway engineers worried about this inconsistency between speed limits and safe speeds on curves. One of these was Joseph Barnett of the BPR, and in 1935 he proposed that all new rural roads be designed according to an “assumed design speed.” This, he said, should be “the maximum reasonably uniform speed which would be adopted by the faster driving group of vehicle operators, once clear of urban areas.”[3] All features of geometric design—curve radii, sight distance, superelevation, even gradients—should then be made consistent with the chosen design speed so that a motorist traveling at that speed would not have to slow down to round any of the curves or ascend any of the hills.[N 3]
Barnett’s “balanced design” concept became a permanent feature of American design policy with its adoption by the American Association of State High-way Officials in 1938. In its Policy on Highway Classification, AASHO declared,
A principal factor affecting the choice of a design speed is the character of the terrain. In general, rolling terrain will justify a higher design speed than mountainous country since the cost of constructing almost every highway detail will be less. An important highway carrying a large volume of traffic may justify a higher design speed than a less important highway in similar topography due to the fact that the increased expenditure for right of way and construction will be offset by the savings in vehicle operation, highway maintenance, and other operating costs.[4]
The essential data needed to implement the balanced design concept came from a series of research studies on driver reactions, curve dynamics and vehicle capabilities which began in the middle 1920’s. By 1936 the fruits of this research were so abundantly available that AASHO appointed a special high-level committee of senior State design engineers to review the available information on highway design, bring it up to date and publish the results in usable form. Chief MacDonald assigned a small task force of BPR experts to work under this committee. Between 1937 and 1944 this Special Committee on Administrative Design Policy summarized and published all that was known about motor highway design in seven “policies.” In 1954 the Committee combined these policies into a single manual which, with later revisions, is still the final authority in the United States on rural highway design.[5]
The Parkway—A New Idea In Highways
In 1907, the New York Legislature created the Bronx River Commission and authorized it to preserve the waters of the Bronx River from the pollution of encroaching trash dumps and land fills. The Commissioners acquired broad strips of land on both sides of the river, then built a highway through the resulting elongated park. They planned this parkway as a four-lane, low-speed recreational road connecting the public parks of northern New York City with city reservoirs in Westchester County.
Originally Herman Merkel, the consulting landscape architect, recommended that the parkway be planned as two widely separated one-way roads at different levels with a wide belt of undisturbed land between them—a very advanced concept for the year 1917. He was, however, overruled by the Commission, and only two short divided sections were built.[6][N 4]
- ↑ In 1933, Maryland studies showed that in 40-mile per hour speed zones the average speed of all traffic was 37 m.p.h., and 87 percent of the drivers were traveling less than 45 miles per hour. Only a minuscule fraction were traveling as fast as 60 m.p.h. The Maryland authorities construed these findings as good public acceptance of the posted limit.[1]
- ↑ In 1912, the minimum radius in New York was 200 feet, but it was considered good practice to use 300- or 400-foot radii for curves on steep grades or at the foot of such grades.[2]
- ↑ This idea was developed concurrently by the German highway engineers in their designs for the Reichsautobahnen.
- ↑ This unfortunate result was probably caused by a popular engineering misconception that was not dispelled until the 1930’s. E. W. James, Chief of Design of the BPR, stated in 1929 that, “Two 20-foot pavements segregating traffic, with a parking between, are not adequate. Apparently two 30-foot pavements are needed to equal a single 40-foot pavement in capacity.”[7]
In practically every respect, the Bronx Parkway was different from other highways of its time. It had an unlimited right-of-way, and this relieved the builders from the need to confine their construction within a narrow band of fixed width. Furthermore, this right-of-way was also parkland which insulated the roadway from the adjoining street and highway
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- ↑ A. Johnson, Notes on Traffic Speeds, Proceedings, 13th Annual Meeting (Highway Research Board, Washington, D.C., 1934) p. 353.
- ↑ W. Harger & E. Bonney, Handbook For Highway Engineers (McGraw-Hill, New York, 1919) p. 33.
- ↑ J. Barnett, Safe Side Friction Factors and Superelevation Design, Proceedings, 16th Annual Meeting, Vol. 16 (Highway Research Board, Washington, D.C., 1936) p. 75.
- ↑ A Policy on Highway Classification, Approved September 16, 1938, Policies On Geometric Design (American Association of State Highway Officials, Washington, D.C., 1950) p. 8.
- ↑ A Policy On Geometric Design of Rural Highways (American Association of State Highway Officials, Washington, D.C., 1954).
- ↑ G. D. Clarke, The Parkway Idea, The Highway and The Landscape, W. B. Snow, ed. (Rutgers Univ. Press, New Brunswick, N.J., 1959) pp. 39, 40.
- ↑ E. James, Parkway Features of Interest to the Highway Engineer, Public Roads, Vol. 10, No. 2, Apr. 1929, p. 22.
