Stoplights Have Long Evolution

Other options include acoustic sensors that automatically alert 911 dispatchers when a loud noise from, say, a car crash or gunshot sounds in the vicinity of an intersection. An alert is sent through a wireless connection hooked up within the traffic signal. Then dispatchers can tune into live video at the location to gauge an appropriate response.

Michael Meyer, a professor of civil and environmental engineering at Georgia Tech University, points out that many signal systems are also being fitted to respond to terrorism threats. Some house nuclear sensors and can track a vehicle that may be carrying nuclear material from intersection to intersection. And, thanks to automatic and manual timing controls, most stoplights can be adjusted with the flip of a switch to accommodate a sudden change in traffic.

In Washington, D.C., for example, after the 9/11 attack on the Pentagon, workers at the city's traffic management center "flipped the switch" to synchronize the lights to favor outgoing, rather than incoming traffic. This allowed a swifter evacuation as people flooded out of the city.

"Traffic signals have become extremely complex units," said Meyer. "And traffic control involves a lot of very complicated math."

Today's technology may be a far leap from Morgan's early hand-cranked model, but Meyer points out stoplights' main and most important function -- telling drivers (or horses or pedestrians) when to stop and go -- has persisted through the decades.

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