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20 RAILROAD GRADE CROSSINGS

A total of 2,047 grade crossings of railroads remain on the roads and streets that presently form the designated system. This is only 17 less than the number of grade crossings that have been eliminated by the construction of grade-separating bridges.

Of the grade crossings remaining, 785 are crossings of railroad sidings or spur tracks only, but 1,262 are crossings of main-line tracks. Of these main-line crossings, 542 are crossings of single-track lines on which there are six or more train movements daily; 218 are crossings of two or more main tracks.

LOAD CAPACITY

With few exceptions the strongest of existing highway bridges are of the design designated as H20-S16. Such bridges are designed to support live loads equivalent in stress effect to those imposed by a 20-ton tractor truck of 14-foot wheel base coupled to a semitrailer with 16 tons on its single axle 14 feet to the rear of the rear axle of the tractor. The gross load of the combination, imposed over a length of 28 feet, is 72,000 pounds. Such bridges will carry, without increase of stress, longer vehicles of much greater gross weight and will support without failure the infrequent passage of heavy military vehicles, if the weight is properly distributed.

Bridges of other design strength are similarly designated as H15-S12, signifying an assumed design loading equivalent to that of a tractor-semitrailer combination of 27 tons gross weight, and as H20, H15, H10, etc., signifying the assumption of design loadings equivalent to those of single trucks of 20, 15, and 10 tons gross weight. In each case the wheel base of the combination assumed is 28 feet, and that of the single truck is 14 feet. All such bridges will safely carry vehicles of gross weight considerably in excess of the design vehicle if the weight is distributed over greater length.

Bridges of adequate capacity

Of the 12,048 bridges presently carrying the interstate routes over streams, railroads, and other highways, 1,607 are designed for or rated as of H20-S16 load capacity. Two hundred and nineteen of these are on urban sections of the system; 1,388 are on rural sections. Bridges rated as H20 number 2,207, of which 763 are on urban, and 1,444 on rural sections of the system. A total of 7,526 of the existing bridges, 887 on urban sections and 6,639 on rural sections of the system, are rated in classes from H15 to H19. These existing bridges on the system will all support with complete safety the frequent passage of vehicles conforming in weight to any of the State laws in current effect. None but the H20-S16 bridges is designed to carry some of the more extreme loads known to be moving over the system.

Bridges of inadequate capacity

A total of 677 of the existing bridges, 110 on urban, and 567 on rural sections of the system, are rated as of less than H15 capacity. All of these must be considered as of inadequate capacity; and 130 of them, 30 in urban, and 100 in rural areas, which are rated below H10 must