If, in the opinion of the commissioner of the department of highways, the design, construction and safety of any highway, or portion thereof, are such that the length limitations prescribed in section four (c) of this article can be increased without undue risk of damage to other vehicles lawfully using such highway or portion thereof, to bridges or other road structures, and to municipal and utility company facilities, wires, traffic devices or other structures, the commissioner may, by order, increase the length limitations of vehicles which may be operated upon any such highway, or portion thereof, designated by him in such order and may establish therein the maximum length limitations which shall thereafter be applicable to the highway or portion thereof so designated by him Provided, That the maximum length of any combination of vehicles including any load thereon shall not exceed sixty feet, except as otherwise provided in this article with respect to the size of vehicles: Provided, however, That no such order of the commissioner shall establish any height or length limitation in excess of or in conflict with any height or length limitation prescribed by or pursuant to acts of Congress with respect to the national system of interstate defense highways.