We take traffic control for granted these days, but in the 1920s, before modern electrical traffic control systems were set up, a scene as (above) was common. The Detroit traffic cop is in a simple booth, with a plain light over his head as he directed traffic by hand signals.
A silent policeman as they were called at the time, as used at less active intersections, can be seen (above and below). The devices came in many variations. Early examples used a kerosene lantern, while some only used a flag.
At about the same time (below) can be seen a much more deluxe enclosed booth with sliding windows, were the officer may have controlled the electric traffic lights above him from the booth as necessary.