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