https://www.scientificamerican.com/article/honking-cars-are-the-best-warning-for-air-raids/

“An organization of automobile owners in London has rendered valuable service to the public in connection with the raids of the German air pirates. When warning of an air raid is received in the city, explosive sky rockets are fired from various points. Automobiles then drive through the city streets, honking their horns to attract attention. On each side of the car, a large sign is carried, on which is printed the warning, ‘Take Cover.’ When the danger is passed the reverse side of the sign is shown, which reads, ‘All Clear.’ A Boy Scout bugler who rides in the car also assists to inform the citizens that the Huns have departed.”

—Scientific American, June 1918