Seeing Red

School of Optometry's Tests Give Tennis Shoe Blinker
Technology a Green Light for Use in Traffic Signals

by Patricia McBroom

The red lights you see in kid's tennis shoes will soon be controlling traffic in California, thanks to a test of their visual comparability with traditional stop lights, done by vision scientists here.

Not only are groups of the tiny lights as bright as a fixture using incandescent bulbs, but they are cooler and will save the state millions of dollars in energy costs, not to mention thousands of hours of manpower time changing the bulbs. The new red lights, called light emitting diodes (LEDs), last 10 times as long as incandescent stop lights.

These and other results of the visual tests at the School of Optometry are being forwarded soon in a final report to Caltrans, which commissioned the study through the campus's Institute for Transportation Studies.

Caltrans, already aware of the results, has decided to move ahead immediately with a statewide changeover of its 60,000 red light and pedestrian signals from incandescent to LEDs.

The new lights should be in place within a year, said Stephen Prey, coordinator of Caltrans Energy Conservation Projects in Sacramento. He estimates that Caltrans can save $3 million per year in energy costs alone with the new red LEDs and perhaps twice that if the yellow and green were also changed.

If all the traffic lights in the United States were converted to LEDs, Prey estimates the country could save the energy equivalent of two or three major electric power plants.

"This is light without heat," said Ted Cohn, professor of vision science who headed the visual testing.

He said the tiny lights are grouped in an array of some 300 in one stop light, taking on the appearance of glistening jewels at close range. At stopping distance, however, the LEDs look like regular red lights and are seen by the eye to be as bright as necessary, said Cohn.

"We've tested the lights in the laboratory and in the field and our judgment is there is no difference between the visual effectiveness of LEDs and incandescent bulbs. As far as vision is concerned, they are equivalent," said Cohn.

California was the first state to experiment with the new signals by installing 2,300 units in a field test in Fresno in 1992. Since then, Oregon has begun using LEDs, as have Philadelphia and Denver.

This is, however, the first testing of LEDs to prove they are visually equivalent to incandescent bulbs. Such tests could become crucial in the case of accidents or litigation.

Cohn's study evaluated all illuminants in present or proposed use in street lights in the United States, including neon and halogen lights.

The tests also revealed that LEDs were visually superior to incandescent light when the sun was shining directly into the stop light. Under those conditions, because of the sun's reflection, traditional red lights can appear to be on, but LEDs are less likely to appear that way.

Light emitting diodes grow dimmer, however, in hot weather, losing 1 percent of their output for every one-half degree above 70 degrees-which could be a problem in hot central California counties.

But studies have shown that the lights are still within acceptable brightness range, even with 85 percent degradation in output. Also, Caltrans is insisting that any manufacturer who wants to market its LED traffic lights in California fit them out with an energy regulator that boosts the energy input as the lights lose power.

Caltrans operates about 7 percent of the signaled intersections throughout the state. Cities and counties, which operate the vast majority of signal lights, are expected to follow Caltrans and install LEDs.

One major advantage of the cool lights, which rely on relatively new semi-conductor technology, is that they save thousands of hours in manpower time changing the lightbulbs.

Even more important, they save lives.

Prey said that Caltrans loses more highway workers per year than does the California Highway Patrol because its workers are often suspended over an intersection servicing a red light while traffic flows below, among other dangerous work environments.

"Soon we will not have to send someone out at 3 am in the morning to change a lightbulb and run the risk of death by some impaired driver," he said.



Copyright 1997, The Regents of the University of California.
Produced and maintained by the Office of Public Affairs at UC Berkeley.
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