- Rural sections would be designed for safe travel at a speed of 75 m.p.h. in flat topography; urban sections for 50 m.p.h.
- Traffic lanes would be 12 feet wide.
- Shoulders would be 10 feet wide except in mountainous topography.
- Embankments 10 feet or less in height would have side slopes no steeper than 1 foot vertically to 4 feet horizontally.
- The roadway width on bridges would be at least 6 feet greater than the width of the pavement of the approach roadway; on short bridges the roadway width would be as great as the width of approach roadway, including shoulders.
There were many other details for the design and construction of the system, including such items as signs and markings, lighting and landscaping. A discussion of the principles of landscape design occupied three and one-half pages of the report. It was a complete text within itself.
The design criteria contained in the report were recommendations only. The 1944 legislation authorizing the designation of the National System of Interstate Highways made no reference to the standards to be used in the design and construction of that System. This was to come later.
Enabling legislation for financing and constructing the System was enacted in 1956. This legislation contained the unique provision that the standards for the System would be those approved by the Secretary of Commerce “. . . in cooperation with the State highway departments.” Thus, the continuance of the voluntary cooperative efforts of the States and the Bureau of Public Roads in formulating standards for the other highway systems for the past 20 years was firmly assured and made applicable to the Interstate System.
AASHO responded to the requirement of the law by adopting standards for the System within 3 weeks after passage of the Act. Many of the recommendations of the 1944 Interregional Highways found expression in these standards, which were approved by the Secretary of Commerce in accordance with the law.
Urban Design Policies
The Committee on Planning and Design Policies was well aware, as it always had been, that bare-bone standards are not enough to assure the design of a safe, utilitarian highway that is esthetically pleasing and economical to construct and maintain. They recognized a need for a more casual and philosophical discussion of the principles of freeway location and design. The Committee was also confronted with the challenge of developing guidelines for the utilization of Federal-aid funds in the construction or improvement of city streets and highways.
The Committee chose to combine these objectives and to develop a policy on urban arterial highways, which would, of course, include freeways as well as conventional arterial surface streets. This effort culminated in the publication, in 1957, of A Policy on Arterial Highways in Urban Areas, the “Red Book.” When work began on this policy, there were few freeways in existence, and experience in their design and operation was limited. Nevertheless, the vision and foresight of the Committee members was sufficient to result in a thorough and comprehensive text on this subject, as well as on the more conventional types of streets and urban highways. The discussion of interchange types and configurations was particularly exhaustive.
Experience in the application of the 1957 urban design policy was generally favorable, but after a period of years, operational deficiencies began to develop in many urban freeways built in accordance with the 1957 doctrine. In large measure, these deficiencies were attributable to traffic loads far in excess of those that were anticipated at the design stages. Nevertheless, many of the operational problems would have been alleviated had some of the dimensional values and configurations been more generous. Accordingly, the policy was updated and republished in 1973 as A Policy on Design of Arterial Highways and Urban Streets. In the process of revision, new sections were added on urban transportation planning and on arterial route location.
Design standards and guidelines have undergone only minor modification and upgrading since adoption of the first editions of the two geometric design policies, rural in 1954 and urban in 1957. Such changes as have been made are attributable to two principal factors: (1) Travel speeds have continued to climb, necessitating adjustments for all design features, and (2) vehicular silhouettes have been lowered, requiring natter highway profiles to provide the necessary sight distances for avoidance of accidents.
Clearances to roadside obstacles and the moderately flat side slopes that were entirely adequate for the travel speeds prevalent before the mid-1950’s were found deficient when measured against the speeds and other operational practices of the seventies, as is apparent from an examination of speed trends on main rural highways. Consequently, more liberal dimensions have been incorporated in the standards and above minimum design is the rule rather than the exception.
It may be truthfully said that throughout the history of highway design devekmment, the highway user, in the collective sense, has dictated the character of the highway by his manner of operation on it and by the extent of his willingness to pay, through road user imposts, for roads that would sustain that type of operation. It remained for the design engineer, working in concert with the research engineer, the landscape architect, and the economist, to determine the type of operation demanded by the vehicle operator (now and in the future), the design characteristics of a highway system that would safely support that type of operation without being unnecessarily extravagant, and the probable revenues that would be available. It would not be a gross exaggeration to say that every highway project has represented a compromise between the ideal in design characteristics on the one hand and economic reality on the other. This is likely to remain the case in the future.
