The Department may, when it deems it necessary for the performance of its duties, and the Chief of the Fire Department may, when necessary for fire-fighting purposes, close any street or part thereof to travel. If it shall be necessary to close a street which is being constructed, improved, rebuilt or repaired so as to permit a proper completion of such work, the Department shall post a notice or notices with the words "Street Closed" in letters of sufficient size to be visible at a distance of 100 feet therefrom, at such place or places on said street or at the ends thereof or at the intersection of other streets leading thereto as may be necessary to give warning to the public travel thereon that such street is so closed. No person shall go or attempt to go, on foot, by animal or in any vehicle, upon any such street or part thereof so closed, except such persons as shall be engaged in work thereon.