Page:Highway Needs of the National Defense.pdf/115

Rh Factors considered

In deciding upon these recommendations the highway officials, members of the association, were influenced by several considerations. Foremost in their consideration was the essential protection of existing road and bridge structures against undue damage by vehicles excessively large and heavy, and the necessary avoidance of highway usage by vehicles unduly hazardous to other traffic. A further consideration was the possibility of deciding upon a single system of limitation that would have reasonable prospect of early uniform adoption by all States. A third consideration, taken in recognition of the tendency of efficient practice in vehicle design and operation to move toward the supply and usage of vehicles of greater load-carrying capacity, was the adoption of limits which, while affording sufficient protection for existing roads and bridges, would allow for a reasonable enlargement of the vehicles in prevalent use, and serve as the basis of the advanced standards desirable of adoption for highways and bridges to be built in the future.

PUBLIC ROADS ENDORSEMENT OF POLICY

The Public Roads Administration endorses the limits recommended by the association. Without regard to their incorporation in the laws of the States, it recommends their adoption as the safe and reasonable basis of highway and bridge design standards to be employed in the future construction of all States. It discounts the probability of an future change in regulatory laws or in transportation practice that will cause the undue obsolescence of roads and bridges adequately designed for the accommodation of traffic including a probable frequency of occurrence of vehicles of the maximum sizes and weights provided for.

Considerations in regard to size

This endorsement is based upon the following considerations with reference to each of the proposed limits:

The height of 12 feet 6 inches is the maximum safely consistent with vertical clearance of 14 feet. The difficulties of providing at bridges and structures, particularly in cities, clearance in excess of 14 feet warrant the expectation that such clearance will be the greatest that can be generally provided.

The width of 96 inches is the greatest that can be accommodated on the narrower thousands of miles of existing highways. A width of 102 inches, proposed for eventual adoption, is sufficient to permit desirable improvements in the mechanical and body design of vehicles, and is consistent with lane widths of 11 and 12 feet for highways of light and heavy traffic density, which are regarded as reasonable highway design standards.

Lengths of 35 fect for single vehicles and 50 feet for tractor-semi-trailer combinations are consistent as limits for the two classes of vehicles from the viewpoint of the off-tracking of rear from front wheels that occurs in turning movements, the amount of such off- tracking being substantially the same for vehicles of each class at the limiting lengths. There is no observable tendency of truck design or truck operational practice to excced these limits. The limit