It is understood that the following quantities of work to be done are approximate only, and are intended principally to serve as a guide in calculating the bids. It is further understood that the quantities of work to be done and materials to be furnished may be increased or decreased as may be considered necessary, in the opinion of the Town Engineer, to complete the work as fully planned and contemplated, and that all quantities of work whether increased or decreased, are to be performed at the unit prices set forth below except as provided for in the specifications.