Whenever a State highway is constructed and the State pays of portion of the cost, and the county, or township, or borough, pays a portion of the cost, or the State pays the entire cost, and it is deemed advisable to widen the State highway beyond the width as laid out and ordered by the secretary or other body having authority over the same, the county and township, or borough, jointly, or the township or borough, may increase the width of the State highway, including the width of right of way, with consent of the secretary or other body having charge of the construction of said State highway. An increase in the width of right of way may be by the adoption of the official plan of the Department of Highways by the township or borough. The cost of such additional width shall be borne jointly by the county and township, or borough, or wholly by the said township or borough, as the case may be. Authority is hereby given the several counties through their commissioners and the several townships and boroughs through their proper officers, to enter into agreement providing for the additional width of the State highway, in accordance herewith. Such additional width shall be constructed under the supervision of the department, and according to its plans and specifications.