In preparing the cost estimates of needed improvements for this study the highest design standards were, of course, used for the interstate system. Progressively lower standards were used for each road system of lesser importance. In general, the design standards used in the cost estimates are in accord with those adopted by the States.
For the interstate system the concept is one of improvement so that the entire system will, by 1964, be structurally and functionally adequate, with all the eseential features of a well-designed road capable of handling 1974 traffic. This implies substantial rebuilding during the first 10 years.
For other principal road and street systems, determination of sections needing rebuilding by 1964 was based on the acceptability of tolerable conditions—under which roads and streets can be considered acceptable even though they may not measure up to the design standard. Such roads and streets would be kept in service until they were no longer judged as being adequate even by the measure of tolerable conditions.
For example, an adequate design for an anticipated traffic volume of 3,000 vehicles per day in 1964 might call for a 24-foot width of surface, 10-foot shoulders, a maximum grade of 5 percent, and a maximum curvature of 5 degrees; but the existing road would be considered tolerable with respect to these features if it had a 20-foot surfaced width, 6-foot shoulders, a maximum grade of 6 percent, and a maximum curvature of 9 degrees.
Similar criteria with respect to other features were employed as an aid in determining the extent of needed work to overcome deficiencies, The procedure is in general conformity with the practices employed by the States in recent years in estimating highway needs.
For local roads and city streets, the conditions of tolerability were much more flexible. Considerable weight was given to established local practices and policies. For example, there were many cases where no estimate of needs was made as long as a road was considered passable and as long as the users would accept the condition as tolerable.
CONSTRUCTION NEEDS, 1955–64
Estimates of construction needs (including right-of-way costs), summarized herein for the 10-year period 1955–64, were prepared by each State, the District of Columbia, Hawaii, and Puerto Rico. A number of States submitted revised estimates, a few of which were received too late to include in the final compilations of summary data. The totals reported by each State for each road system are listed in table 1, and nationwide totals are summarized in table 2. A graphic comparison of the 10-year construction needs, 1955–64, and the estimated 1965 mileages, by system, is shown in figure 1.
