Right of Passage: The Controversy Over Vertical Clearance on the Interstate System
When the American Association of State Highway Officials (AASHO) issued design standards for the Interstate System, the minimum design value of 14 feet was included for vertical clearance (that is, the distance from the Interstate pavement to the bottom of overpasses). Although 14 feet was sufficient for civilian needs, the Department of Defense informed the U.S. Bureau of Public Roads, which had adopted the AASHO design standards, that a vertical clearance of 17 feet was needed for defense purposes on the National System of Interstate and Defense Highways.
On January 27, 1960, Administrator Bert Tallamy signed Instructional Memorandum 20-2-60 on "Vertical Clearance on the National System of Interstate and Defense Highways." It indicated that the Secretary of Commerce had approved a revision in the minimum vertical clearance to a minimum value of 16 feet, superseding the minimum value of 14 feet approved July 17, 1956 (i.e., in the AASHO design standards for the Interstate System - AASHO modified its standards on vertical clearance in 1963). "The Secretary's action is in accordance with requests made by the Department of Defense and with AASHO recommendations incident thereto."
The revised 16-foot vertical clearance applied to all portions of the Interstate System in rural areas. In urban areas, "application of the 16-foot vertical clearance shall be limited to a single routing where the revised vertical clearance can be developed most economically, even though that single route is indirect." All projects under design or construction were to be revised according to the new requirement. The IM concluded, "No decision has been reached with respect to the treatment of structures already built which provide clearances of less than 16 feet." (The first highway bridge modified was on I-94 near Lansing, Michigan. An article in AASHO's American Highways, April 1960, said, "Using hydraulic jacks, workmen raised the partially completed deck of a bridge under construction to increase the separation for Clear Lake Road east of Jackson. Additional cost: $18,000. The bridge was raised in March 1960.)
Several years passed without a decision on existing bridges. During this time, a 1967 survey revealed that some 2,650 Interstate overpasses did not meet the 16-foot vertical clearance standard. In 1969, the FHWA, AASHO, and the Military Traffic Management Command agreed that correcting all these deficiencies would be uneconomical. However, they mutually agreed on a 26,000-mile priority network that had 350 deficient structures and served about 95 percent of the major military installations. On the priority network, the States were encouraged to implement, as rapidly as practical, those modifications necessary to obtain a 16-foot clearance-"when a clear need arises or when reconstruction work is undertaken." Off the priority network, the 16-foot clearance would be implemented only in conjunction with other construction work. As explained in an FHWA notice issued on June 16, 1978, "If the work necessary to obtain the clearance is a logical addition and the cost is not excessive, it should be incorporated into the overall project."