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LED traffic signals get the green light
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Ted Cohn
(right) looks on as VDL staff members Daniel Greenhouse (center)
and Kent Christianson, check brightness comparison data. The
amber incandescent light (on the right) emits a different
pattern of intensity of light – a bright center, surrounded
by a less bright circle, surrounded by another brighter circle.
The amber LED
source (on the left), emits a more uniform punctuate pattern.
The Berkeley Visual Detection Lab study verified
that despite the different patterns, drivers would find the
LED lights equivalent in visibility. Photo: Ben Ailes |
By Susan Piper
When Caltrans needed state-of-the-art capabilities to test the
visual effectiveness of new light emitting diode (LED) traffic
lights, the state’s Department of Transportation (Caltrans)
called upon Berkeley’s Theodore Cohn, professor of vision
science and bioengineering with dual appointments to the College
of Engineering and the School of Optometry.
Cohn runs the Visual Detection Laboratory at Berkeley, which has
pioneered instruments and methods to investigate factors that
limit the sensitivity of the visual sensory system in health and
disease, and to explore the importance of those factors in applied
settings. Most recently, the lab has directed its attention to
understanding the role of vision in transportation
problems.
"Caltrans and a number of local municipalities were interested
in using the new LED technology in traffic lights because it offered
significant energy savings – up to 75 percent – as
well as significant maintenance cost savings," says Cohn.
But the agency needed to know if drivers would respond to the
new LED lights as effectively as they do to incandescent traffic
signals. "Our job was to determine if the LED light would
do to the eye what the incandescent light does to the eye,"
says Cohn.
The Visual Detection Lab applied technology known as heterochromatic
flicker photometry (HFP) to help measure the usability and ultimate
viability of LED light sources. Basically, an instrument takes
one light and rapidly replaces it with another, repeating this
sequence continuously about 15 times per second, giving the appearance
of a slight flicker. In their tests, the researchers adjust the
intensity of the LED source until the viewer no longer sees the
flicker – in effect calibrating the LED source to match
the subjective brightness of the incandescent source.
Cohn’s lab used HFP three years ago in a study that measured
the effectiveness of LED traffic lights that were red –
at that time, the only color of LED traffic light available. Since
then, industry has developed the capability to produce cost-effective
green and amber lights, and it was these new LED lights that Caltrans
wanted to document.
Cohn and his team were interested in two factors. The first was
whether for normal observers, green and amber LED lamps have equal
physical intensities when seen at the same subjective brightness.
This step was taken to study whether these LED lamps should have
different recommended intensities compared to those of incandescent
signals. LEDs proved suitable replacements provided they meet
applicable intensity standards. Second, the team measured reaction
times to the transition between green (off ) and amber (on) for
both LEDs and incandescents, finding a marked advantage due to
very fast onset and offset times of LEDs compared to incandescent
lamps.
As a result of the study, Caltrans and other agencies determined
that the amber and green lamps could not only be substituted for
the more traditional incandescent lights, but offered significant
dollar savings, as well. "While the LEDs cost more to purchase
than the standard incandescent lamps, the capital cost can be
recouped in about three years," says Cohn, "due to reduced
energy and maintenance costs.
"These lights last many years," says Cohn, "freeing
up maintenance workers and avoiding the many injuries workers
sustain while high up in the cherry pickers used to repair broken
or burned out lights. And this study couldn’t have come
at a more appropriate time," he adds. "Prior to our
study, only a few municipalities, in addition to Caltrans, were
experimenting with the new LED traffic lights. But two years ago,
as the energy crisis began rearing its head, more agencies began
considering LEDs. Once we did the test and proved that there were
many benefits and no risks, it created an impetus to install these
lights."
So, the next time you find yourself stopped at a traffic light,
take a good look. If the red, green, or amber light looks like
a pincushion rather than a central bright area of light surrounded
by slightly darker and lighter concentric circles, you are viewing
the new LED traffic light.
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