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