of 13 feet, enough with a slight margin to pass vehicles of the 12½-foot height recommended as a maximum. Only 10 of the existing structures provide definitely inadequate vertical clearance.
Combined standards.—In many cases the existing bridges that are substandard in respect to horizontal or vertical clearance, or both, are also substandard in respect to load design. Of the 8,088 bridges that fail to meet the recommended standards of horizontal clearance, 7,445 are inferior in loading design to the H20 standard. Of the 347 bridges that meet the recommended standards of horizontal clearance, only 72 are designed for H20 loading.
These 72 bridges also provide the recommended 14-foot vertical clearance. and are, therefore, the only bridges now existing on the entire mileage of rural roads conforming to the recommended system that closely approximate the standards proposed.
Next in adequacy are the remaining 603 bridges designed for H20 standard loading. All but 2 of these provide the recommended vertical clearance, but are more or less deficient in horizontal clearance. Two hundred and ten of them are long bridges (100 feet or more in length) with deficiencies of horizontal clearance varying from less than 5 to more than 50 feet. Three hundred and ninety-three are short bridges (less than 100 feet long), which are deficient in horizontal clearance by amounts varying from less than 5 to more than 70 feet. One hundred and sixty-six of the long bridges and 376 of the short bridges are of deck-type construction. These, where they are not greatly deficient in width, can be widened with comparative ease.
Desirable Order and Rate of Construction
It will be apparent from the previous section on condition of existing roads, streets, and bridges that there is immediate need for a vast amount of new construction to replace inadequate facilities with the far superior facilities described as appropriate and essential for the interregional system. The need, as has been suggested, arises more from deficiencies in the alinement, width, and access features of the existing roads rather than from inadequacies in the structure of existing surfaces and pavements.
Many of the existing roads are improved with surfaces and pavements of comparatively recent construction and with normal maintenance, these will have a further serviceable life, under the traffic to be expected, of from 10 to 20 years. Where this condition exists, and other features such as curvature, gradient, width, sight distance, and intersection design are not seriously deficient in relation to the traffic carried, the present roads can reasonably be continued in use until either the existing pavement has served out its economic life or the traffic has increased to such a density as to compel improvement.
Principle of minimum rate and indispensable order of construction.—Obsolescence of the existing roads will thus determine a minimum rate at which the interregional system should be constructed, and it may be stated as a general principle, that—
Whenever an existing highway conforming approximately to a route of the interregional highway system shall require reconstruction, by reason of the deterioration of its surface or other incapacity, the highway should be reconstructed only in the location and to the standard
