Americans didn't invent outdoor advertising. That distinction would probably have to go to the ancient Egyptians who would put up notices offering rewards for runaway slaves. But Americans certainly moved the outdoor advertising art form forward in our own ostentatious way during the 20th century.
The "flaming" log sign, scanned from the October 1923 issue of Science and Invention magazine
With the rapid adoption of the car, people were increasingly in a hurry during the first few decades of the 20th century. And as such, it became increasingly hard for advertisers of the 1920s to grab the attention of people travelling along at greater speed. Advertisers adopted large and sometimes garish ads on top of their buildings. And sometimes, especially in the Western states, the entire building became the ad.
But one inventor had an idea for something that was not only big but also caught the attention of passersby for the primal fear it might stoke. The idea was to create an advertising sign that appeared as if it was on fire. The "flaming" log sign was patented in 1922 by Jose De Elorza of Madrid, Spain, and it worked by using "logs" of wood that were actually made of amber-coloured glass. The logs, which had slits cut in them, would create quite a smokey red mess.
As you can see from the illustration above, (incidentally drawn by legendary sci-fi illustrator Frank R. Paul), coal was fed into the steam machine below and it would look like the sign was burning red-hot. Exterior lights would also help illuminate the entire display.
As far as I can tell, this idea for the flaming sign of the 1920s never was tried in the real world. Perhaps it would have been a little too terrifying to see the tops of buildings on fire, since Americans were still trying to figure out how to deal with skyscraper fires in the 1920s. But it certainly wouldn't be the worst advertising idea of the century. It probably wasn't even the worst advertising idea of 1923.