Any person engaged or employing others in excavation or otherwise or in any manner obstructing a portion of 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 or amber lamps to be securely and conspicuously posted on and at each end of such obstruction, and if the space involved shall exceed fifty feet in extent, then at least one additional lamp for each fifty feet or portion thereof excavated or constructed.