Any person engaged in or employing others in excavating, or otherwise in any manner obstructing a portion or all of any street, sidewalk, alley or other public way, at all times during the night season shall cause at least two illuminated red lamps to be securely and conspicuously posted on, at or near each end of such obstruction or excavation, and if the space involved shall exceed fifty feet in extent, then at least one additional lamp for each added fifty feet or portion thereof excavated or obstructed shall be so posted.