New responsive traffic-signal system aims to ease Chicago congestion
Last month the city of Chicago introduced a state-of-the-art traffic system
designed to control signal lights based on real-time traffic conditions — the first test of the emerging technology in a major city.
If it works, the system could be marketed nationally by next year, making
it possible for state and local governments to replace the inflexible timing
systems used today. Those systems set lights based on historical data —
how traffic usually flows at a given time of day — not on actual traffic
The new technology has the power to change signal times in less than
two seconds in response to traffic accidents or gridlock. If traffic is
backing up at one intersection, for example, the system can slow the flow
of traffic through preceding lights.
The Real-Time Traffic Adaptive Control System (RT-TRACS), developed
by the Federal Highway Administration and PB Farradyne Inc., uses more than
30 closed-circuit video cameras. They're mounted on streetlights at 12 intersections
just north of the bustling Chicago Loop.
The cameras, or detectors, note approaching traffic then send those
images to a computerized control box at each intersection. Using an optimization
algorithm, the computer calculates the length of the green signal that will
maximize traffic flow at a given intersection. Or the system can control
and coordinate the timing of lights at a series of intersections.
"It's going to be a good boon for the city if we can ease up some of
the congestion,'' said Carl Byrd, deputy commissioner of the Bureau of
Traffic in Chicago's Department of Transportation.
The timing systems widely used at intersections throughout the country
rely on baseline data that predicts when traffic will be heavy or light
to determine when the signals will change. This approach means someone must
sit at a terminal and monitor the traffic flow to gauge when adjustments
need to be made, and a technician actually has to go to the intersection
to adjust the signal times.
Chicago's streets are set up on a grid system. Signals are usually timed
for 75 seconds from red to red, based on the premise that cars should be
able travel from one end of the city to the other at 29 miles per hour and
hit green signals, Byrd said.
But as the population has grown, so has the number of cars. And the
premise of moving through green lights by maintaining a certain speed rarely
happens, he said. Unlike suburban areas or even some other major cities,
Chicago does not have the option of widening its downtown streets.
"We're pretty much locked in, infrastructure-wise, to the streets as
they are," Byrd said. "When you talk about traffic management, we're pretty
limited to the tools at our disposal."
The $1.8 million traffic-signal system will be tested through the end of
the year. And it will be connected to the traffic management system in City
Hall so that commuters eventually will be able to tap into real-time traffic
data via the Internet before they leave their driveways.
"How do you give people accurate, up-to-date, real-time information
so they can make decisions on what they're going to do?" Byrd asked. "We're
ultimately working toward that goal."
Keeping traffic moving steadily also helps increase fuel efficiency,
reduces pollution and eases some of the frustrations that tempt drivers
to run red lights, officials say.
Chicago grades its intersections on a scale from A to F, with A being
a free flow of traffic and F meaning cars are stuck at an intersection for
a minute or more. Most of the intersections that will be using RT-TRACS
average a D level of service or worse during peak traffic times. The goal
is to improve traffic flow at those intersections to a C or better, Byrd
Chicago is one of three cities in the United States to test the adaptive
control technology. The first field test began in March 1998 with 14 intersections
along a four-mile stretch in Reston, Va. That resulted in a 6 percent to
12 percent improvement in traffic flow, said Farhad Pooran, manager of intelligent
transportation systems research at PB Farradyne. Another field test at 10
intersections in Seattle is now being developed.
The technology contains three algorithms that can be used for different
types of traffic corridors. At slower traffic times, such as early morning,
or to handle heavy traffic from special events, the system can be switched
back to use fixed times.
RT-TRACS can control up to 250 intersections with just two standard
PCs. The company also has a wireless system that can reduce deployment costs
by 70 percent to $3,000 per intersection, Pooran said. With this drop in
expense, even smaller cities and towns will be able to afford the technology
to smooth the flow of traffic.
In addition, Pooran said that state and local transportation shops would
need to devote far fewer traffic engineers to handling flow because signals
would be updated automatically in real time — remotely.
"That will definitely reduce the cost of operations," he said. "You
don't have to travel along the corridor. A traffic engineer can just use
his laptop to dial in from home. You don't need to be present at specific
intersections to do the job."
The adaptive control technology is likely to be used throughout the
country once it is perfected, said James Cheeks, standards development manager
at the Institute of Transportation Engineers in Washington, D.C. However,
he said that the technology is still in its infancy and that no standards
have been developed to commercialize the system. Industry will begin moving
to market the technology once the federal government gives its seal of approval
to the system, he said.
But other concerns remain. Despite being more flexible than existing
systems, an automated system could cause its own problems. For example,
if the system is programmed to revert to baseline data at certain times,
rather than monitor real traffic, the city may get caught short.
"If it relies on some baseline information, and that baseline information
says every Thursday at 3:00 we get a rush of people leaving town, but one
Thursday a large group of people are coming into the city for some event,
adaptive control is going to deal with real-time traffic," he said. "Commuters
[leaving town] would then have to sit through longer cycles."
In addition, there has not been enough published research detailing
the particulars of which intersections would benefit most from the technology,
said Michael Pietrzyk, intelligent transportation systems program director
at the Center for Urban Transportation Research at the University of South
"It's not a cookie-cutter type of thing," Pietrzyk said. "Many factors
come into play on it. You look at how big of a network you're trying to
put on an adaptive control signal. What's the volume on the principal street
vs. the cross streets?"
However, Pietrzyk said that RT-TRACS is a systematic approach that takes
into account traffic conditions at multiple traffic corridors vs. one intersection
or a single corridor. "It just takes into account a larger area of impact,"
he said. "It implies it can better manage the overall traffic flow. It balances
a larger network and a larger number of movements."Harreld is a freelance writer based in Cary, N.C.
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