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46 Of the H20 bridges, 18 in rural areas and 4 in urban areas are deficient in vertical clearance; and 1,084 of the bridges of this capacity in rural areas are inadequate in horizontal clearance, 222 of them only moderately so.

It is impossible to rate the adequacy of horizontal clearance for many bridges in urban areas because of uncertainty as to the hourly traffic volume they carried. It is believed that nearly all existing H20-S16 bridges in urban areas are of substantially adequate horizontal clearance. However, more than half of the urban bridges of H20 capacity are seriously inadequate in width.

Of the bridges of capacity inferior to H20, only 53 in rural areas and 10 in urban areas are deficient in vertical clearance. But these bridges, deficient in load-carrying capacity, are also generally deficient in horizontal clearance. Of 7,218 such bridges in rural areas 6,198 are thus deficient, 5,181 of them substantially so.

Deficiency of tunnel clearance

Of the 13 rural tunnels all but one are substantially adequate in vertical clearance, but all are in some degree inadequate in horizontal clearance.

Of the eight tunnels in urban areas, three are deficient in vertical clearance and all are inadequate in horizontal clearance.

In the light of the conditions found by the survey to exist on the roads and bridges now constituting the designated interstate highway system, and in consideration of the standards agreed upon as necessary for adequate service of the 1948 traffic on the system, the several State highway departments were asked to determine the kind of improvement that would be required to bring the entire system up to a desirable degree of adequacy, as of the present time, and to estimate the cost of so doing.

As a result of this analysis only 1,900 miles of the system in rural areas and 398 miles in urban areas, as they presently exist, are found to require no improvement. The mileage here classified as urban includes 98 miles in towns of less than 5,000 population. For purposes of the estimate of costs of required improvements, only those sections of the system included within the limits of the urban areas of cities of 5,000 population and more, as defined by the Federal-aid Highway Act of 1944, have been classified as urban. On this basis, the mileage requiring no improvement is 1,998 miles rural and 300 miles urban, a total of 2,298 miles, or about 6 percent of the system as it now exists.

CHARACTER OF ROAD IMPROVEMENT REQUIRED

All other parts of the system are found to require some measure of improvement. The essential improvement of 8,687 miles, about 23 percent of the system as it now exists, consists of widening only. Of this total, 8,306 miles are rural, 381 urban.

Improvements classified as reconstruction, which can be effected with no more than minor relocation of the existing roads, are required on 14,283 miles, nearly 38 percent of the existing total mileage. Of these roads 13,467 miles are in rural areas and 816 miles in urban areas.