ment and shoulder widths, wider bridges, and flatter slopes with better rounding at intersections of slope planes. The need for a more open type of design was occasioned, in part, by the continued increase in travel speeds, but in most cases, these liberal practices were as much a result of a greater flow of money into the highway coffers as to any sudden awareness of need for more generous dimensions. It would be difficult to say which is cause and which is effect—increases in speed or improvements in design standards—but historically they have gone hand in hand.
AASHO prepared concise, abbreviated design standards for the several classes of highways, starting with those for primary highways in 1941. Design and construction standards for secondary and feeder roads and for the Interstate System were published in 1945. These have since been expanded and upgraded, as necessary, for the three general categories of roads. The classification terminology has been changed somewhat with the passage of time.[N 1]
These standards have since been approved by the Federal Highway Administrator for application on Federal-aid highways and are the specific controls for the design of such highways. Many States have also adopted them in their exact form for application on highways that are off the Federal-aid systems. Other States have promulgated standards of their own for application on their various classes of State highways. Where found to be in reasonable conformity with AASHO standards, these have been approved by the Federal Highway Administrator for use on the Federal-aid systems.
Getting AASHO standards approved has not always been accomplished without dissension and debate. When a pavement width of 24 feet was proposed for heavily traveled two-lane roads in 1941, for example, some officials, accustomed to building lanes only 9 or 10 feet wide and having been stung by the bad accident experience on three-lane roads, were fearful that drivers would mistake the 24-foot roadway for a three-lane road. Other States were deeply concerned by the added costs for higher standards.
- ↑ Current versions prepared by the Committee on Planning and Design Policies are Geometric Design Standards for the National System of Interstate and Defense Highways—1967; Geometric Design, Standards for Highways Other Than Freeways—1969; and Geometric Design Guide for Local Roads and Streets—1970.
Rural Design Policies
Highway design policies are general procedures and controls which are less specific than design standards, often with a range of acceptable values, and which are officially adopted or accepted for application in the design of highways. In 1950, the seven separately published design policies were reprinted and bound as a single volume under the title, Policies on Geometric Highway Design. Parenthetically, the final one of the policies, published in 1944, dealt with interchanges and grade separations and should properly be identified with freeway development to a greater extent than with conventional highways. The policies were updated and republished in 1954 as A Policy on Geometric Design of Rural Highways, the “Blue Book.” This publication broadened considerably the earlier work, largely as a result of research efforts in the field of traffic operation. The Highway Capacity Manual, published by the Highway Research Board in 1950, supplied material on the relation between highway capacity and roadway characteristics. This information was incorporated in the 1954 rural design policy and has been invaluable in aiding the designer to better fit the highway to traffic requirements.
The results of studies of truck speeds on grades as related to motive power and load carried also became available after 1940 and were used in selecting values for control gradients for the design of the several classes of highways in different types of terrain. One solution to the problem created by slow moving trucks on grades was the provision of an added lane in the uphill direction for use by trucks. These have become known as “climbing lanes.” Criteria for climbing lanes were included in the 1954 policy.
The 1954 design policy also enlarged upon the design speed concept and included tables showing the relation between design speed, degree of curvature, rate of superelevation, and needed length of spiral transition. The subjects of freeway design and interchange geometries were discussed to the extent that the state-of-the-art permitted.
In 1965 the rural design policy was again brought up to date and republished under the same title.
A Need to Expand the Highway Network
As efforts to improve and expand the highway network continued, it became increasingly apparent to highway administrators and lawmakers alike that portions of the predominantly two-lane rural system of highways were becoming severely strained by the ever increasing traffic burden. Urban arterials were becoming choked by intensified commercial development coupled with rapid traffic growth. Inadequacies in the form of low travel speeds, low capacities, and high accident rates were clearly evident to those at even the highest level of government. At the same time, there was an awareness that transportation within cities was a national matter rather than a local problem. As a means of defining the scope of the problem and of developing remedial measures, President Roosevelt, on April 14, 1941, appointed the National Interregional Highway Committee to “. . . investigate the need for a limited system of national highways to improve the facilities now available for interregional transportation, and to advise . . . as to the desirable character of such improvement. . . .”[1]
The product of the Committee’s efforts was the celebrated report Interregional Highways, submitted to the President on January 5, 1944. The ultimate accomplishment was the incorporation in the Federal-Aid Highway Act of 1944 of a provision for designating a national system of highways and a further provision for the expenditure of Federal-aid highway funds in urban areas.
Highlights of some of the recommendations of the report as to locating and designing the system were:
- The system would be both urban and rural in extent.
- Roadways and structures would be designed to serve vehicles of the types and numbers to be expected 20 years from the date of construction.
- Intersections with crossroads and railroads would be separated in grade.
- ↑ Interregional Highways, H. Doc. 379, 78th Cong., 2d Sess., p. III.
