In all cases where any county is authorized by law to appropriate and expend moneys for the improvement or maintenance of any public highways, either in full payment thereof or in payment thereof in conjunction with any township, borough, or incorporated town, the county commissioners of such county may make, execute, and deliver all necessary agreements and contracts for such improvement or maintenance with the lowest responsible bidder, and to that end the secretary shall be authorized to make all necessary surveys and plans, and to advertise for, receive, and open bids therefor in the manner now provided in the case of the improvement of State highways.
36 P.S. § 670-903