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Computers guide traffic lights to reduce congestion for commuters, other drivers

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By Ashley Halsey III
Washington Post Staff Writer
Tuesday, January 5, 2010

There probably are just two times when you think about a traffic light.

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When one just turned yellow. ("Can I make it?")

When one has been red for too long. ("Come on, change!")

Traffic lights, on the other hand, are much more thoughtful about you.

They note the hum of your passing tires. They adjust to meet your needs. They sense your mounting frustration. It won't be long before they also know how many people are in your car and just where you're headed.

Traffic systems -- and the stoplights that are their highway sentinels -- are complex networks that must be synchronized with Balanchine precision or rush hour will descend from the normal headache into commuter hell.

Very few Montgomery County drivers even knew that traffic signals talked behind their backs until a communications glitch in November threw the whole system out of whack and caused almost two days of complete chaos on 3,500 miles of roadway.

Those of you impatient to know precisely how these things work today can scan ahead in this article, but the rest of us will explore how we got to this point.

If your great-great-grandparents lived in a city, they complained about the traffic. That's urban life, and in the mid-19th century cities began putting police at intersections with stop-go signs that could be cranked around to regulate traffic. Electricity came along, lights soon replaced signs, and timers determined when green went to red.

Pins stuck in the timing dial -- much like the one you might use to control a lamp in your living room -- were set to determine how long the light stayed red in each direction. If the individual timers in a whole series of lights could be manually synchronized, then a driver who kept to the speed limit might catch several consecutive greens as he or she drove along.

The control box at each intersection called the shots for its lights. Called "set 'em and forget 'em" signals, each box had a clock. Those clocks regularly "drifted," losing or gaining a few seconds, just like any other clock or watch. Even a few seconds of drift threw off timing between intersections until a human being returned to make adjustments.

Then came computers.


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