Every person who digs any excavation in or adjoining to any public way of the City and every person who places on any public street any obstruction or building material, shall cause two yellow flashing lights to be securely and conspicuously placed in or near the excavation, building material or obstruction. One light shall be placed at each end of the space so excavated or occupied and the excavated area shall be barricaded in such a way as provided by the Safety- Service Director. If the space occupied by the obstructions exceeds twenty-five feet, then the person shall cause an additional light to be posted for every additional twenty-five feet thereof so excavated or occupied and shall keep the light burning the entire night.