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Rh Less critical deficiencies than weak bridges and weak and narrow road surfaces consist of a substantial number of bridges and underpasses of inadequate horizontal and vertical clearance and a considerable mileage of road graded and surfaced to widths less than those called for by the prescribed standards for the traffic they serve.

On 2-lane sections of the rural network, the planning-survey inventories show 819 bridges and underpasses with horizontal clearances less than 18 feet; on 3-lane sections, 46 with horizontal clearances less than 27 feet; and on 4-lane sections, 35 with horizontal clearances less than 36 feet. This makes a total of 900 bridges and underpasses with horizontal clearances less than the acceptable minima for the respective road widths.

Table 3 shows an accumulative numerical distribution of these and additional bridges and underpasses which have various horizontal clearances less than the 30-, 42-, and 54-foot widths recommended as desirable and preferable for structures on 2-, 3-, and 4-lane roads of the strategic network. This table indicates the large extent to which existing bridges and underpasses are inferior in horizontal clearance to tho standards recommended as desirable for new structures on the strategic network, and the smaller extent to which they fall below minimum acceptable standards.

As for vertical clearance, the survey shows 133 bridges on the rural network on which less than the acceptable minimum of 12½ feet is provided.