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ON CAMPUS
All in good time
Traffic-sensitive signals developed at the Robotics Institute
are saving fuel, time and drivers’ nerves
by linda k. schmitmeyer
Gregory Barlow knows a lot about traffic, and not just how
long it takes to commute from his home in Squirrel Hill
to his office in Newell-Simon Hall, where he works with
CMU research professor Stephen Smith on tackling traffic
congestion in urban areas.
Barlow (CS’11), a post-doctoral researcher in
the Robotics Institute, knows traffic problems
are nothing new: The early Romans wrestled
with them two millennia ago by banning
wagons on their roads during certain times
of the day. He also knows they’re costly: In
2009, drivers in 439 urban areas in the United
States traveled 4.8 billion hours longer and
purchased 3.9 billion more gallons of fuel
because of traffic snarls, for a total congestion
cost of $115 billion.
And all this comes from Barlow, a man who admits,
“I don’t really like to drive.”
Maybe that’s why he’s working to improve the driving
experience in Pittsburgh, along with CMU colleagues
Xiao-Feng Xie, a research associate, and Zachary B.
Rubinstein, a senior systems scientist. They are part of a
team led by Smith, director of the Intelligent Coordination
and Logistics Laboratory in the Robotics Institute, where
they developed a “smart” traffic signal that works in realtime.
Smith has dubbed the project Scalable Urban Traffic
Control (SURTRAC).
Today, most signals operate by pre-setting the timing on
the green light for several different periods throughout
the day, depending on the number of vehicles expected
to travel through an intersection. As part of the Traffic21
Initiative in Carnegie Mellon’s H. John Heinz III College,
the researchers developed software that allows a signal
to respond to traffic as it is happening. Last spring, they
installed adaptive signaling systems at nine intersections
in Pittsburgh’s East Liberty neighborhood.
“Our system watches actual traffic flow through the
cameras at each intersection and dynamically adjusts
the green timing periods on a second-by-second basis,”
Smith says.
According to Smith, similar software is already being
used to manage traffic flow onto along arterial roadways—
surface roads with a strong traffic flow in one direction
and some side streets. With these systems, though,
information is typically collected and sent to a central
location, where data are analyzed and adjustments to the
signaling are determined.
“It can take anywhere from five to 15 minutes
(to reprogram the signals), depending on
the system being used,” Smith says.
“The beauty of our approach is that it is
decentralized, which makes it inherently
scalable, in principle.”
There also are decentralized systems used on
arterial roads, says Smith, but they operate on
the assumption that there is a dominant flow
of traffic that does not change.
SURTRAC, in contrast, is designed to discover the
dominant flow of vehicles through an intersection and
automatically adjust the signaling. It is designed to operate
within urban grids, where the volume and direction of
traffic can change throughout the day.
At the East Liberty intersections, cameras take continuous
shots of the traffic, which allows SURTRAC to create a
schedule for moving vehicles through the intersections
in the most efficient way possible. Each intersection also
communicates via a fiber optic cable (or, in the case of
one East Liberty intersection, a wireless signal) to its
downstream neighbors as to what the projected outflow
from its signal will be. The neighboring signals do the same
thing, and together they create a communications network
that’s akin to having the watchful eyes of traffic police at
every intersection.
More impressive, though, are the net results. At the East
Liberty intersections, the research team found that the
wait time for people driving through the grid was reduced
by 40 percent. Travel time was reduced by 26 percent and
projected vehicle emission by 21 percent. The researchers
obtained the results by completing “before” and “after”
drives along 12 predetermined routes through the nine
intersections. They did the drives during four set times
throughout the day; GPS tracers on their cell phones
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