Not all States were starting from the same status in 1956 either, for a variety of reasons. A “shelf” of acceptable project plans was available in some States but not in others. Some State highway departments were able to staff up to the job rapidly while others took much longer; many had previously acquired little or no right-of-way on their own; some had never before designed or built freeways on a large scale.
Interstate projects are complex undertakings, consuming 3 or 4 years from beginning of route location study to final completion—and often much longer in urban areas. In the early years of the program, a large proportion of effort and expenditure was devoted to preliminary engineering and right-of-way acquisition. Construction followed at some later period.
All of this explains why the Interstate System program did not instantly jump into high gear on the morning of June 30, 1956, producing great stretches of completed pavement. It necessarily would be a number of years before any notable mileage was opened to traffic.
Design Standards
The 1956 Act called for adoption of uniform design standards for the Interstate System in all States. The American Association of State Highway Officials and the Bureau of Public Roads, anticipating the need, began developing geometric standards (those governing curvature, gradient, number and width of lanes, etc.) in May 1956 and by July had completed and adopted a full set of such standards. This rapid accomplishment was possible because the AASHO Committee on Planning and Design Policies had been evolving progressive freeway standards for years and had developed a set of Interstate System standards as early as 1945.
The major items in the adopted standards included: control of access throughout the System; design adequacy for projected 1975 traffic — later changed (in 1963 Act) to a 20-year minimum design period from the date of project approval; 12-foot travel lane width; 10-foot minimum shoulder width; elimination of railroad grade crossings; elimination of highway at-grade intersections; design speeds of 50, 60, and 70 m.p.h respectively for mountainous, rolling, or flat terrain conditions; curvatures and gradients consistent with design speeds; separated traffic lanes with variable median widths on a right-of-way adequate in width to accommodate these standards; and minimum widths for highway bridges.
The 1956 design standards were formulated as minimum rather than fixed levels, with the expectation that minimum levels would be used only where higher ones would result in excessive cost. They have served well, and have required but little modification since their adoption.
Reasonable uniformity prescribed through minimum standards seemed fully appropriate for the Interstate System insofar as geometries were concerned, since these are the features the driver sees and is directly affected by. In contrast, however, structural design features which govern load-carrying capacity and durability are hidden from the driver’s eye. Moreover, structural design of a roadway is necessarily largely guided by localized circumstances—physical conditions, availability of materials, and local experience and practice. Consequently “uniformity” in structural design was not prescribed in detail for the Interstate System—only the requirement that design be soundly and justifiably arrived at and be adequate to support anticipated traffic loads.
While these geometric standards complied with the basic requirements of the law at the time of adoption, they were soon found to be inadequate to meet realistic needs for future traffic service. The statutory design year 1975 was changed by Congress in the 1963 Highway Amendment Act to provide a design requirement for traffic service adequate for a 20-year period commencing on the date of plan approval for the initial construction of the project. The Committee Report stated this action was necessary to prevent premature obsolescence in the System design. And, in 1966, the Congress amended Title 23 to provide that “Such standards shall in all cases provide for at least four lanes of traffic.”
A bicycle path has been built within the right-of-way of I-94 in Minneapolis, Minn., that extends for over half a mile and safely connects two neighborhoods.
Designation of Additional System Mileage
As noted earlier, the 1944 Federal-Aid Highway Act provided for the designation of an Interstate System not to exceed 40,000 miles, and in August 1947 a System totaling 37,700 miles was designated which included routes through and skirting major cities. The remaining mileage authorized (2,300 miles) was designated in September 1955 for additional routes around and through urban areas.
At the time of the initial System designation, there were no detailed location studies and no engineering and economic analyses available for consideration. The locations within each State, therefore, were diagrammetic only, and the estimated mileage of designated System segments was derived from the length of the existing principal highways in the route cor-
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