It may be one of the most pervasive gadgets in the country, but few know who they can credit (or blame) as they wait for the light to turn green.
The stoplight has a surprisingly erratic history and has evolved into forms so complex that some engineers describe it as a "hanging PC" Most signals today can detect traffic flow through sensors under the roads and then adjust their timing accordingly. And many can automatically change to green at the flash of a strobe light from an oncoming fire truck.
Cutting-edge and future models can even double as surveillance instruments.
"Some of the new models have all kinds of technology now," said Howard Lamunion of Optisoft, a traffic-light company based in Richardson, Texas. "They have video cards, acoustic sensors and network connections."
Of course, early versions of the stoplight, which appeared not long after cars first hit the road, were much simpler creations.
One of the earliest-known models was a revolving lantern with red and green glass that was installed at an intersection in London in 1868. A police officer operated the unit by turning a lever at its base. The idea was novel, but less than a year after its debut, the kerosene-fueled unit exploded and injured the officer who was operating it.
Born of Necessity
For Garrett Morgan, a black American born of former slaves, the idea sprung from tragedy. While driving around the streets of Cleveland, Morgan witnessed a horrible traffic accident when a car collided with a horse and carriage. The driver of the automobile was knocked unconscious and the horse had to be put down.
Morgan, a successful businessman who had already invented the first version of the gas mask, set to work on a signal that could offer some order on Cleveland's increasingly chaotic and dangerous roads. His hand-cranked invention, patented in 1923, was a T-shaped unit that featured three positions: Stop, Go and an all-directional stop position.
Around the same time, a police officer in Detroit was busy devising his own solution since he hated to see so much police time spent directing traffic. William Potts modeled his red, green and amber light model on automatic controls already being used by railroads. He then made four faces to each light so it could direct four lanes of traffic. His units would prove to be lasting models.
The red, amber and green lights may still look a lot like Potts' original version, but much has changed. One of the most pervasive updates to traditional stoplight is the replacement of incandescent bulbs with light-emitting diodes. Optisoft's Lamunion says LEDs slash energy costs by as much as 95 percent and offer more visible signals.
"About 86 percent of lights out there still use incandescent bulbs," he said. "But the switch is under way."
Signals That See, Hear and Smell
Also under way is the addition of surveillance features to traffic signals. More than 100 cities in 20 states now have traffic video surveillance, including Los Angeles, New York and Chicago. While many systems use separate video systems at intersections, some traffic signals are being designed to house cameras and other sensors right inside the stoplight.
"In the lower left hand corner, there is a small window about the size of a golf ball -- the camera is behind there,"said Lamunion.
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.