Here is a terrific photograph from the Library of Congress, showing a traffic policeman at 14th and Pennsylvania, making use of a rearview mirror. The photo was taken some time in the 1920s.
A Fascinating Photograph from the 1920s of a Traffic Policeman Using a Rearview Mirror
I note the handle of a billy club sticking out of his coat pocket. In those days, when a cop told you to do something, you’d better hop to it. Those billy clubs were pretty handy for administering a ‘wood shampoo’ to anyone who ran afoul of the law.
Maybe he knows Officer Sprinkle!