Section 31. The registrar may make rules and regulations governing the use and operation of motor vehicles or trailers and the conduct of operators and chauffeurs, and may alter, rescind or add to any rules and regulations previously made by him. A copy of such rules and regulations attested by the registrar shall be prima facie evidence that they have been made as provided by law. This section shall not be construed as giving the registrar power to regulate the speed at which motor vehicles may be operated on the public ways.