A 'No U-Turn' sign means:

A circular sign showing a U-shaped arrow with a red line through it prohibits U-turns at that location. U-turns are dangerous in many places: at busy intersections, on highways, on hills with limited sight distance, where the road is too narrow to complete the maneuver safely. Where U-turns are prohibited, look for an alternative — driving past the intersection and using a side street, or going around the block. Violating a no-U-turn sign is a traffic violation. When U-turns are permitted, complete them carefully: signal, yield to all approaching traffic, and ensure you have a complete view of the road in both directions before beginning the turn.