Approximately 970 of the 8,435 bridges, or more than 11 percent, afford horizontal clearances less than the surfaced widths of the existing roads on which they are located; and on only about 4,340, approximately half of the total number, is the horizontal clearance afforded as much as 4 feet wider than the existing surface of the approach road.
Even in relation to the existing road widths, which, as previously shown, are generally deficient, the clearance of existing bridges will be seen from this analysis to be far from satisfactory. Measured by the standards recommended by the Committee the situation is much worse.
Of the total of 8,435 bridges, 6,466 are located on roads carrying leas than 3,000 vehicles per day, for which the standards proposed provide generally for improvement with two-lane pavements 24 feet wide. Of these 6,466 bridges, 2,289 are 100 feet or more in length and, according to the standards proposed, should have horizontal clearance of 30 feet. Only 202 of the 2,289 bridges meet this requirement. The remainder of 4,177 bridges, which are less than 100 feet in length, to meet the standards proposed should have horizontal clearance of 44 feet; but only 112 of them as presently designed meet this requirement. In sum, therefore, only 314 of the 6,466 bridges now existing on roads approximating sections of the system which should be designed as 24-foot, two-lane highways, conform to the standard of horizontal clearance proposed as desirable by the Committee.
On roads conforming to the system, which now serve volumes of traffic between 3,000 and 15,000 vehicles per day, there are 1,911 of the total of 8,435 bridges. For these roads the recommended standards require four-lane divided highways. Of these 1,911 bridges, 771 are 100 feet or more in length, and for these bridges the proposed standards require a horizontal clearance of at least 58 feet. Only 16 of the 771 bridges meet this requirement. The remainder of 1,140 bridges, which are less than 100 feet in length, require, according to the proposed standards, horizontal clearance of 83 feet, a requirement which is met by only 17 of the existing bridges. Of the total of 1,911 bridges on roads carrying traffic of between 3,000 and 15,000 vehicles per day, therefore, there are only 33 that meet the standards of horizontal clearance proposed by the Committee.
Finally, there are 58 of the 8,435 bridges on roads now carrying traffic in excess of 15,000 vehicles per day, for which six-lane highways arerequired. Thirty-three of the 58 are 100 feet or more in length and, according to the standards proposed, should have horizontal clearance of 82 feet. Twenty-five are less than 100 feet long and should have horizontal clearance of 107 feet to meet the proposed standards. None of these bridges meets either of these requirements.
From the above analysis, therefore, it is apparent that only 347, or 4.1 percent of the total of 8,435 bridges, conform in their present design to the standards of horizontal clearance proposed by the Committee for bridges on the interregional system. To conform to these standards all the rest would have to be widened in amounts varying from a few to more than 70 feet.
Vertical clearance.—The situation in respect to vertical clearance is much better. Of the total of 8,435 structures on all existing rural roads approximating the recommended system, there are 8,41 which, as presently designed, are either unlimited in vertical clearance, or provide at least the 14 feet proposed for structures on the system. Of those that do not meet this standard, 15 provide vertical clearance
