(a) This subchapter does not authorize the operation on the national system of interstate and defense highways in this state of a vehicle of a size or weight greater than that authorized by 23 U.S.C. § 127, as amended.
(b) If the United States authorizes the operation on the national system of interstate and defense highways of a vehicle of a size or weight greater than that authorized by 23 U.S.C. § 127 on August 29, 1977, the new limit takes effect on the national system of interstate and defense highways in this state.