It is unlawful for any person in authority, having control of the running or drawing of any car or cars by means of a steam engine, or the control or running of any steam engine alone, within the city limits of the city of Aberdeen, to allow or permit the same to go or move at a greater rate of speed than ten miles per hour over the street crossings which are used by the public, or at a greater rate of speed than twenty (20) miles per hour where there are no street crossings within the city limits of the city of Aberdeen. Provided, however, that the speed provided for at the crossing on Boone Street in the city of Aberdeen shall be reduced to five miles per hour until such time as traffic signals are installed at the crossings.