What does this traffic sign indicate?

You answered correctly. This yellow sign with a "U" followed by a number indicates a numbered diversion route. The "U" stands for "Umleitung," which means diversion in German. These signs guide drivers along a temporary alternate route, usually in cases of roadworks or closures. The number helps drivers follow the correct diversion path. It does not refer to tram lines or underground car parks, which have different signs. Understanding these signs helps you navigate detours safely and avoid confusion during traffic disruptions.