23 CFR § 658.17 - Weight.
(a) The provisions of the section are applicable to the National System of Interstate and Defense Highways and reasonable access thereto.
(b) The maximum gross vehicle weight shall be 80,000 pounds except where lower gross vehicle weight is dictated by the bridge formula.
(c) The maximum gross weight upon any one axle, including any one axle of a group of axles, or a vehicle is 20,000 pounds.
(d) The maximum gross weight on tandem axles is 34,000 pounds.
(e) No vehicle or combination of vehicles shall be moved or operated on any Interstate highway when the gross weight on two or more consecutive axles exceeds the limitations prescribed by the following formula, referred to as the Bridge Gross Weight Formula:
(f) Except as provided herein, States may not enforce on the Interstate System vehicle weight limits of less than 20,000 pounds on a single axle, 34,000 pounds on a tandem axle, or the weights derived from the Bridge Formula, up to a maximum of 80,000 pounds, including all enforcement tolerances. States may not limit tire loads to less than 500 pounds per inch of tire or tread width, except that such limits may not be applied to tires on the steering axle. States may not limit steering axle weights to less than 20,000 pounds or the axle rating established by the manufacturer, whichever is lower.
(g) The weights in paragraphs (b), (c), (d), and (e) of this section shall be inclusive of all tolerances, enforcement or otherwise, with the exception of a scale allowance factor when using portable scales (wheel-load weighers). The current accuracy of such scales is generally within 2 or 3 percent of actual weight, but in no case shall an allowance in excess of 5 percent be applied. Penalty or fine schedules which impose no fine up to a specified threshold, i.e., 1,000 pounds, will be considered as tolerance provisions not authorized by 23 U.S.C. 127.
(h) States may issue special permits without regard to the axle, gross, or Federal Bridge Formula requirements for nondivisible vehicles or loads.
(i) The provisions of paragraphs (b), (c), and (d) of this section shall not apply to single-, or tandem-axle weights, or gross weights legally authorized under State law on July 1, 1956. The group of axles requirement established in this section shall not apply to vehicles legally grandfathered under State groups of axles tables or formulas on January 4, 1975. Grandfathered weight limits are vested on the date specified by Congress and remain available to a State even if it chooses to adopt a lower weight limit for a time.
(j) The provisions of paragraphs (c) through (e) of this section shall not apply to the operation on Interstate Route 68 in Allegany and Garrett Counties, Maryland, of any specialized vehicle equipped with a steering axle and a tridem axle and used for hauling coal, logs, and pulpwood if such vehicle is of a type of vehicle as was operating in such counties on U.S. Routes 40 or 48 for such purposes on August 1, 1991.
(k) Any over-the-road bus, or any vehicle which is regularly and exclusively used as an intrastate public agency transit passenger bus, is excluded from the axle weight limits in paragraphs (c) through (e) of this section until October 1, 2009. Any State that has enforced, in the period beginning October 6, 1992, and ending November 30, 2005, a single axle weight limitation of 20,000 pounds or greater but less than 24,000 pounds may not enforce a single axle weight limit on these vehicles of less than 24,000 lbs.
(m) The provisions of paragraphs (b) through (e) of this section shall not apply to the operation, on I–99 between Bedford and Bald Eagle, Pennsylvania, of any vehicle that could legally operate on this highway section before December 29, 1995.
(n) Any vehicle subject to this subpart that utilizes an auxiliary power or idle reduction technology unit in order to promote reduction of fuel use and emissions because of engine idling, may be allowed up to an additional 400 lbs. total in gross, axle, tandem, or bridge formula weight limits.
(1) To be eligible for this exception, the operator of the vehicle must be able to prove:
(i) By written certification, the weight of the APU; and
(ii) By demonstration or certification, that the idle reduction technology is fully functional at all times.
(2) Certification of the weight of the APU must be available to law enforcement officers if the vehicle is found in violation of applicable weight laws. The additional weight allowed cannot exceed 400 lbs. or the weight certified, whichever is less.