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 the obstruction or excavation and, if the space involved shall exceed 50 feet in extent, then at least one additional lamp for each added 50 feet or portion thereof excavated or obstructed.