No vehicle shall make a U-turn upon any curve, or upon the approach to or near the crest of a grade where such vehicle cannot be seen by the driver of any other vehicle approaching from either direction within 500 feet, nor at any place where the City Council, whenever it shall by resolution determine that the public safety, convenience, or welfare require or warrant, by resolution, may designate streets or parts of streets where such turns are prohibited and prohibit such turns by signs.