Do you have to drive in fog with warning lights?

Hello and happy news all times a question on current occasion, because my theoretical examination is already “something” and I am certainly no longer on the latest level of knowledge: Do you have to drive in dense fog with warning flashing light? This morning Dortmund was covered in dense fog. So dense that taxis and night buses were still driving, but did not take anyone with them. (How was that still with the transport obligation? 🙂 Any vehicle on the road that drove had the warning flashers on. The only exception was a police car whose crew, however, obviously had no problem with the fixed lighting. Is that mandatory now or has it only naturalized itself for some reason? Similar perhaps the thought “If everyone in the city 70 drive, I do so too, because otherwise I am a traffic obstacle”? Thank you!