Bridges and culverts shall be considered in all respects as a part of the road upon which they are, or are proposed to be located. The construction, improvement, repair and maintenance thereof, including adequate approaches and the doing of any act or the performance of any work necessary for the protection thereof, and also including the maintenance and operation of movable span bridges, shall be considered in all respects except as hereinafter otherwise provided, the same as the construction, improving and maintaining of the road upon which any such bridge or culvert is situated, and, except as hereinafter otherwise provided, shall be paid for accordingly.