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 shall install and maintain at least two illuminated warning lamps which shall be securely and conspicuously posted on, at, or near each end of such 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.