The mayor is hereby authorized to determine those street intersections at which drivers of vehicles shall not make right, left or U-turns, and shall have proper signs placed at intersections. The making of the turns may be prohibited between certain hours of any day and permitted at other hours. Where turns are restricted during certain hours pursuant to this section, the same shall be plainly indicated on the signs, or such signs may be removed when turns are permitted. (1990 Code § 15-902)