Traffic lights are so ubiquitous that we hardly give them a second thought, except to curse their existence when they impede us on our daily drive. But no matter how much it seems like traffic lights have the ability to read our minds and tell when we’re running late, they’re really not much more than a set of lights and a programmable controller.
Simple in practice, but as usual, the devil is in the details, and for a system that needs to work as close to 100% of the time as possible, the details are important. Let’s explore the inner workings of traffic signals.Electromechanical Timing
The traffic lights and crosswalk signals at an intersection are only the public user interface, of course. The interesting stuff is going on in the control box. There’s at least one at every intersection, usually a plain metal cabinet set back from the road, sometimes camouflaged with public bills or graffiti. But inside are the guts of what makes an intersection work and keeps vehicle and foot traffic moving smoothly and safely.
Unsurprisingly, most traffic signal controls started out as purely electromechanical devices. Cabinets were chock full of synchronous motors turning timing wheels with cams to cycle the intersection’s lights through the proper sequence. One old time controller that was common up until recently was made by Econolite, and the insides are a paragon of sturdy design.
The controller shown in the video is a pretty simple one for a straightforward intersection that only needs to run a fixed program or two plus allow for a timed switch to a “flash only” mode after a certain time of night when traffic drops off.
More complicated electromechanical controllers implemented crosswalk signals coordinated with the traffic signals, and provided for magnetometers or pressure switches installed in the pavement to detect the presence of vehicles and alter the timing of the signals.A System of Interconnected Things
Electromechanical controllers served municipalities well for decades, and some intersections are likely still controlled by one of these simple but robust devices. As technology advanced and traffic engineers came up with more and more complicated intersections to support increased demand, the old electromechanical boxes were retired in favor of solid-state controls.
The first generation…
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