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36 and 13,091 miles of roads not included in the interregional system. Second priority routes total 23,072 miles, and third priority routes 9,142. No part of the mileage designated as of second and third priority was included in the interregional system recommended by the Public Roads Administration.

Conditions of bridges.—Included in the total mileage there are upwards of 18,000 bridges and underpasses more than 20 feet in length. In the larger rural portion the number of such structures is 16,692, consisting of 15,813 bridges and 879 underpasses.

The condition studies thus far made by the Public Roads Administration and the State highway departments h ave been confined to the rural portion of the network. On this portion the more extreme deficiencies consist of 5,095 miles of surface less than 18 feet in width, approximately 14,000 miles of surface rated as incapable of supporting the frequent, year-round application of 9,000-pound wheel loads on low-pressure tires, and 2,436 bridges of load capacity inferior to the H–15 standard.

In table 2, these 2,436 bridges have been grouped into rated capacity classes-rated capacity is the gross vehicle load for which a bridge is designed. This table also shows the average gross weight of the heaviest vehicles passing over the bridges of each capacity class with an average frequency of 1 a day, and indicates the large degree to which some of the structures are overloaded by normal civil traffic. This overloading is evidenced by the ratio of the gross weights of such vehicles to the respective rated capacities of the bridges, and varies all the way from gross loads only a fifth greater than the bridges in the 25,000- and 28,000-pound rated-capacity classes were designed to carry, to loads nearly 22 times as heavy as those for which the bridge in the 2,000-pound capacity class was designed.