No 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 shall fail, at night, to erect or cause to be erected at least two illuminated red or amber lamps to be securely and conspicuously posted on, 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 for each additional fifty feet or portion thereof shall be required.