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A bridge is a structure built over water or any obstacle or depression to allow the passage of pedestrians or vehicles. Bridges on the Interstate System highways are used by a wide variety of traffic and are designed with certain weights expected to be carried across. However, as trucks grew heavier in the 1950s and 1960s, something had to be done to protect bridges. Therefore, federal and state laws were enacted governing allowable weights to the number and spacing of axles.
The Federal-Aid Highway Amendments of 1974 increased the weights allowed on the Interstate System to 20,000 lbs. on a single axle, 34,000 lbs. on a tandem axle, and 80,000 lbs. gross weight (23 U.S.C. 127). But Congress balanced this concession to productivity by enacting the Bridge Formula. The result is that motor vehicles may be loaded to the maximum weight only if each group of axles on the vehicle and their spacing also satisfy the requirements of the Formula. This prevents the vehicle from overstressing bridges.
Until 1982, Federal law set only upper limits on Interstate System weight limits. A few States retained significantly lower weight limits which hindered long-distance truck traffic. In 1982, Federal law was amended to make Interstate System weight limits, including the bridge formula limits, both the maximum and the minimum weights that States must allow on the Interstate System.