Section 514. Bridges on State Highways in Boroughs and Incorporated Towns.--The department shall construct, reconstruct, and maintain any bridge over a stream or mill race which the borough or incorporated town is obligated to maintain, and which bridge is located on or forms a part of a State highway within the limits of any borough or incorporated town: Provided, That nothing herein contained shall authorize any assessment to be made against the Commonwealth by reason of, or to assist in the elimination of, any grade crossing, or in the reconstruction of any structure eliminating a grade crossing, on any highway within the limits of a borough or incorporated town, and no such assessment shall hereafter be made under any act of Assembly heretofore enacted.