When is it illegal to make a U-turn?

Making a U-turn is illegal at intersections where U-turns are prohibited by signage because such signs indicate specific regulations that must be followed for safety reasons. These prohibitions are in place to prevent hazardous situations that could arise from potentially conflicting traffic patterns or limited visibility. The signage communicates the rules of the road to all drivers, helping to ensure a smooth flow of traffic and reduce the risk of accidents.

The other options do not appropriately capture the reasons behind U-turn regulations. At intersections where U-turns are allowed, it is perfectly acceptable to make one, while on any highway, the legality of U-turns may be more complex, often depending on specific signs or local laws, rather than being outright illegal everywhere. Lastly, legality is not determined by the presence of oncoming vehicles, as a U-turn could still pose danger regardless of traffic conditions. Hence, understanding signage and following it is crucial for safe driving practices.