Via Shorpy. There are two in the photo, click on it for the full size, one is right behind the unbrella holder. Notice the antique 2 bulb stop light as well.
Sally Halterman, 1st woman to get Wash DC motorcycle permit
That's a machine gun mounted up there... remember, this was when prohibition was happening, tommy guns were being used by bank robbers, and "G Men" were trying to hunt down Bonnie and Clyde
Notice int he above photo the woman in the lower right corner is crossing through traffic, and the two vehicles she's walking toward are both signalling with their arms to indicate slow/caution / stop by hanging their arms out the window and down