In the big city,
collisions between buses and cars are all too common. One third
of these accidents occur when a car slams into the back of a bus.
Why? A common answer is that the driver simply didn't see the slowing
or stopped bus until it was too late. Combining his knowledge of
the visual nervous system with some ingenious engineering, Berkeley
professor of vision science and bioengineering Theodore E. Cohn
built an improved signal light that warns drivers when to back off.
"Our approach saves the driver on average one-tenth of
a second in reaction time from when they see the light and hit
the brake," Cohn says. "If you're going 30 miles per
hour, that's a little more than four feet which is just enough
to avoid hitting the bus."
Cohn's light bar is a five-foot wide piece of aluminum outfitted
with eight individually-controlled groups of light-emitting diodes
(LEDs). As part of a Federal Transit Administration project, the
light bar will be mounted on the back of a bus along with a radar
system. When the radar detects that a car is too close, approaching
too rapidly, or both, it will trigger the light bar warning system.
The FTA plans to put the entire apparatus through its paces later
this year.
With the assistance of Bioengineering undergraduate student
Khoi Nguyen and other collaborators, Cohn modified and optimized
an off-the-shelf light bar provided by the FTA. Harkening back
to earlier research to build a better brake light for cars, Cohn
knew to take advantage of the visual nervous system's millions
of years of evolution. There are two major pathways between the
eye and the brain, he explains. The detail and color we see when
reading or looking at art, for example, travels along the parvocellular
(P) pathway. But it's the magnocellular (M) pathway that codes
for motion.
Professor
Theodore Cohn and student Khoi Nguyen in front of their
bus light bar with a photo of a bus's rear for scale. (Click
for larger image.)
David
Pescovitz photo
|
"It's the fastest pathway because it deals with information
that's time-sensitive, like when you stalk prey or flee a predator,"
Cohn says.
To enable the light bar to benefit from the M pathway's shortcut
to the brain, he modified the off-the-shelf light bar so the groups
of bulbs are illuminated two at a time instead of all at once.
First the inside pair light up, followed by the other pairs moving
outward at 50 millisecond intervals.
"What you see is something that appears to be getting big
so it seems to be coming toward you," Cohn says. "And
that signal takes a shortcut to the part of the brain that tells
you something is going on and you better do something about it."
In addition to introducing motion into the warning system to
improve its effectiveness, Cohn proved that replacing the light
bar's incandescent bulbs with LEDs also helps alert the driver
more quickly. Incandescent bulbs suffer from a 30 millisecond
delay between the time they're switched on and the moment when
they first begin to glow, he says. Worsening the problem, incandescent
illumination is gradual—it can take up to another quarter
of a second before a bulb's light is bright enough to be seen.
On the other hand, LEDs flash on almost instantaneously, shaving
precious milliseconds off the warning process.
Cohn expects the results of his bus light bar experiments
to impact current research in his Visual Detection Laboratory. He's
now studying what part of an oncoming speeding train first grabs
an observer's attention. Understanding this visual act could inform
placement of light bars on buses or locomotives. Potentially, Cohn
says, it could even lead to markings or lighting systems on trains
that more clearly convey speed and help prevent collision.
"Right now, people don't perceive that a train is going
80 miles per hour when they're at a crossing," he says. "It
looks like it's lumbering along and you have months to get out
of the way. Because of that misperception, many people don't get
out of the way in time."
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