This arrow-shaped sign lurking under the Broadway Brooklyn el on Broadway and Aberdeen Street is a remaining specimen of the previous generation of one-way signs. They were in use prior to 1965 or so, and were white with the words ONE-WAY in black. Some of the signs carried the words “POLICE DEPT.” on them. This one, of course, has been painted over for some time. Leftover signage and ancient street lighting can sometimes be found hiding under elevated trains.
In 1965, the modern one-way signs, black rectangles with the white arrows painted on them, began to appear and quickly took over.