(a) During the progress of building operations, a sidewalk not less than five feet in width, or an equal width of a clean, hard, paved roadway, made available and safe for a footway, and with adequate guard railings, shall at all times be kept open, clean and unobstructed for the purpose and uses of a passageway in front of such lot.
(b) It is not intended by any of the provisions of this article to prohibit the maintenance of a safe driveway for the delivery of material across a sidewalk from the curb line to the building site; provided, however, that such curb and sidewalk across which such driveway shall extend shall be protected with wooden planks in a manner satisfactory to the director of public works.