What does this traffic sign indicate?

This traffic sign indicates a diversion route specifically set up for autobahn (motorway) traffic in case it becomes necessary, for example, due to construction or an accident. The letter "U" stands for "Umleitung," which means "detour" or "diversion" in German. The number next to it identifies the specific diversion route.

Such signs help drivers on the autobahn follow a predetermined alternative path to bypass obstacles without confusion. Unlike permanent road signs, these diversions are only used when needed and guide drivers safely back to the autobahn or the intended motorway route.

Remember, this is a blue rectangular sign with a white "U" and a number, not to be confused with the sign for underground stations (which has a white "U" on a blue square) or federal roads (which have a yellow background and black numbers).