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96 of 40 feet suggested for three-axle busses is desirable for the encouragement of improvements in the design of bus bodies and equipment. It will not require any alteration of highway design consistent with the 35-foot length recommended for trucks, provided that, as recommended, the longer bus is carried on three axles.

Length of 60 feet for two-unit combinations other than tractor-semitrailers is consistent with the 35- and 50-foot lengths proposed for the other classes of vehicles in the fact that it involves approximately the same amount of off-tracking. It is essential to permit a distribution of the heavier of probable highway freight loadings consistent with reasonable standards of road, bridge, and vehicle design. There is substantial warrant for the belief that such a length will be required and efficiently used in future civil transport operation, as it is already equaled and exceeded in the design of military vehicles. No confirmation has been found in extensive traffic observations of the fear that vehicles of such length will be unduly difficult or dangerous to pass.

Considerations in regard to loads

An axle load of 18,000 pounds is the heaviest that can be allowed. Such a limit should be rigidly enforced for the protection of existing road surfaces, most of which will not carry heavier loads without damage and unreasonable shortening of their normal service lives. If future roads were designed to support heavier axle loads it would be years before the replacement of existing surfaces would permit vehicle loading to the higher axle limit. The 18,000-pound axle-load limit will not unduly restrict the design and efficient loading of vehicles if laws are amended to permit the lengths and group axle loads recommended. By encouraging the provision and use of multiple-axle vehicles for movement of the heavier loads, the 18,000-pound limit in combination with the length and group axle-load limits recommended will permit more effective braking and make for greater safety in vehicle operation.

The tabular values of group axle loading recommended are safe for H15 bridges which, with few exceptions, are the strongest of existing bridges. Fixed gross-load limits corresponding to classes of vehicles, still provided in the laws of some States, should be abolished as rapidly as possible and group axle-load limitation equivalent to that recommended should be substituted. Existing roads and bridges will be equally adequate for gross vehicle weight greater than the highest value allowed in the recommended table if vehicle length and number of axles are correspondingly increased and the 18,000-pound axle-load limit is faithfully observed.

The limitations placed on the design of military motor vehicles are prescribed in Army Regulations No. 700-105, section II. The limitations imposed upon the design of Air Force and Navy motor vehicles are essentially the same as those included in the Army regulations.

In general, there are two sets of limitations on the size and weight of military motor vehicles, one applying to general-purpose and special-equipment vehicles and the other to special-purpose and combat vehicles. The first group includes the bulk of the military vehicles.