Width deficiencies
At the extreme of inadequacy are 52 two-way bridges which are less than 18 feet in horizontal clearance. Thirty-one of these bridges, each more than 80 feet long, are classed as long bridges. And, most unfortunately, 14, of which 9 are long bridges, are classed among the strongest of the existing bridges by ratings of H20. Two of these bridges less than 18 feet wide somehow managed to carry an hourly traffic in 1948 of more than 800 vehicles.
The widths of the existing bridges cover such a wide range, the relation of bridge width to the width of existing approach pavement is so various, and, moreover, the relation of the bridge width to the volume of traffic served is so complicated, that it is impossible to summarize in brief statement the many evidences of existing width inadequacy.
Here it may be sufficient to say that of the total of 10,050 bridges carrying the interstate routes in rural areas, only 1,863 conform fully to the standards of width regarded as reasonable for highways of such importance. With some tolerance, 1,517 additional structures may be accepted as substantially adequate; but 6,670 of the existing rural bridges, about two-thirds of the total number, are entirely too narrow.
Underpasses
A similar condition of width inadequacy affects the bridges which carry railroads and other highways, forming underpasses for the interstate highways. Of the 381 bridges which carry railroads over rural sections of the interstate system 316 do not provide sufficient horizontal clearance for the highway as it should be designed; and of 151 bridges carrying other highways over, 41 allow insufficient clearance for a proper width of the interstate highway.
VERTICAL CLEARANCE
In regard to vertical clearance, the situation is much better; in fact, extremely good. Most of the bridges carrying the interstate routes are of the deck type, with no overhead restriction whatever. The commonly accepted standard of vertical clearance for bridges, and the standard which is regarded as adequate for the interstate system, is 14 feet.
There are only 71 of the existing bridges carrying rural routes of the system that provide vertical clearance less than 14 feet. Eighteen of the bridges inadequate in this respect are structures of H20 capacity rating; 53 are of lesser capacity rating.
There are, however, on the rural system 2 highway overpasses, and 70 railway overpasses that provide vertical clearance less than 14 feet; and on the entire system, both urban and rural, there are 320 places where 14 feet of vertical clearance is not available. One hundred and seventy-five of these places are in urban areas and 145 in rural areas.
MATERIALS OF CONSTRUCTION
Of the 12,048 bridges on the system, 2,801, of an aggregate length of 189 miles, are built of steel. Three hundred and ninety-four of these steel bridges are 100 to 200 feet in length; 341 are 200 feet and longer. All but 189 are of H15 load capacity or stronger.
