Before letting contracts for the construction and improvement of State highways and bridges in any county, the Department of Highways shall consult the county commissioners, and ascertain their preference as to the State highways and bridges, or sections of State highways, which are to be constructed and improved by the use of the moneys allocated for expenditure in such county as in this act provided; and the county commissioners shall indicate, prior to June first of each year, their first, second and third choices; and the Department of Highways may select the State highways, bridges, or sections of State highways to be improved in such county from such choices, or, in its discretion, may select other State highways, bridges, or sections in such county for improvement: Provided, That where the Department of Highways has issued or is about to issue authorizations for work to be performed by department forces, or has heretofore prepared plans and was about to let contracts for the improvement of any State highway, or section thereof, in any county, the department may proceed with such improvement, but no such improvement requiring moneys in excess of the amount allocated to the county shall be made from moneys allocated to other counties under the provisions of this act.