The Director of Public Works or any police officer seeing anyone making any excavation in the street, alley or sidewalk, or so near thereto as to endanger the traveling public, shall immediately ascertain whether the person has the authority to do so. The person so making an excavation in or near the street, alley or sidewalk shall show his authority for so doing to the Director or the police officer when demanded. All persons without authority for so excavating shall be stopped by the policeman or the Director.