Synchronized signals no match for 'oversaturation'
Jon Hilkevitch – Chicago Tribune| Getting Around
September 24, 2007
CHICAGO - Mike Hartigan's daily driving commute illustrates the frustration over how traffic in the Chicago region, from Algonquin to Yorkville, is descending into gridlock.
If Hartigan leaves his home in Carol Stream before 6:30 a.m., the 6-mile trip to his computer programming job in Lombard is a driving pleasure along busy North Avenue.
"It has become clear that the traffic signals installed eastbound along that route are synchronized, at least during the weekday morning rush," Hartigan said. "If I maintain a speed in the neighborhood of the 45 m.p.h. posted speed limit, I can frequently travel the entire distance from County Farm Road in Carol Stream to Main Street in Lombard without touching the brake pedal."
The effect on traffic is nothing short of fabulous, reducing travel times, fuel consumption and pollution -- but only when there is available capacity on the road.
Those same interconnected traffic signals are "completely useless" against heavy traffic, Hartigan said, if he leaves home any later than 6:30 a.m., and when his workday ends at about 4:30 p.m.
The term that traffic engineers use is "oversaturation." It means there are simply too many vehicles for the available pavement.
Yet the reality of supply and demand isn't stopping transportation officials at the state, county and municipal levels from spending hundreds of millions on interconnecting traffic signals so they "talk" to each other in an attempt to maximize traffic movement.
Northeastern Illinois has about 7,700 traffic signals and about 3,300 of them are on coordinated systems, according to a new inventory conducted by the Chicago Metropolitan Agency for Planning.
Of the total, several thousand signals are merely synchronized, providing less flexibility to manage traffic.
Synchronized signals contain a clock in the controlling mechanism to make the signals in sync with each other. But one signal is not communicating with another signal down the road, and they cannot be adjusted from a remote location.
In addition, such "closed-loop systems" typically operate on timers and handle only five to 10 traffic signals. They max out at about 30 signals per closed loop. And they can be thrown off the correct timing sequence by power outages and other problems, leading to drivers encountering a green light followed by a red light two blocks later.
About 2,000 of the 2,800 signalized intersections in the city of Chicago have synchronized traffic lights, according to the Chicago Traffic Management Authority. Another 476 signals are interconnected citywide. The remaining signals are on less traveled streets and act independently.
The prevalence of a decades-old technology helps explain why commutes like Mike Hartigan's work great early in the morning when traffic loads are light, but not so great in the afternoon.
So, what's the next move?
Traffic signals in most Chicago-area travel corridors have been tweaked and retweaked to the point that little or no additional efficiencies exist to squeeze out more capacity.
"What frustrates motorists is the peak half hour, and in some cases stretching up to three hours depending on the route, where capacity is oversaturated and nothing will work," said Steve Travia, bureau chief of traffic in the Chicago region at the Illinois Department of Transportation.
The situation is receiving renewed attention in light of an authoritative traffic study that the Texas Transportation Institute released last week. It showed that the Chicago region is losing the battle against traffic congestion.
Commuting delays have increased 74 percent since 1995 in the Chicago region and northwest Indiana, the study said.
"Unfortunately, corridor after corridor is hitting the capacity wall and the headway made over the last 20 years in coordinating traffic lights is no longer doing the job," Travia said.
A big part of any solution depends on moving as many people as possible out of cars and onto buses and trains, and using tolls and more creative pricing initiatives to encourage people who continue to drive to travel during nonpeak hours.
"Traffic signals get a lot of attention from the public," Travia said. "But by their very nature, signals are a compromise that stops people going in one direction and lets others go. Signals by themselves do not improve traffic."
Still, cities across the U.S. continue to tap into billions of dollars in federal congestion-mitigation grants to interconnect and synchronize traffic signals, even though it may be a campaign of diminishing returns.
The federal government pays for about 80 percent of the cost to design and build interconnected traffic signals, which cost from $2 million to $10 million per system, officials said.
Meanwhile, the federal government is hoping someone has a better idea for moving traffic through oversaturated intersections.
The Transportation Research Board is offering a $600,000 research grant to develop a guide for operating coordinated traffic signals in oversaturated conditions.
"Urban and suburban intersections operating in oversaturated conditions are now very common and are expected to become more prevalent," said the request for proposals issued by the research board, an arm of the National Academy of Sciences.
"Understanding the underlying cause ... is the critical first step in mitigating it, but there is no coherent method of determining this," it concluded. The deadline for proposals is Oct. 16.
IDOT and the city of Chicago are responsible for more than 5,600 traffic signals in the region. Other municipalities control about 1,200 signals, and county highway departments are in charge of more than 900 signals.
More interconnected signals are being added on major corridors.
Chicago has 25 traffic-signal interconnect systems at 476 intersections, according to the Chicago Office of Emergency Management and Communications, which oversees the city's Traffic Management Authority.
More are scheduled for installation in the next two years. They will be in the Streeterville neighborhood, in the area around Chicago Avenue to the Chicago River, and from Michigan Avenue to Lake Shore Drive; and on the Near West Side, roughly from Madison Street to Lake Street, and the Chicago River to Desplaines Street, said David Zavattero, deputy director of the OEMC.
Ten additional interconnect systems, covering about 300 signals, will follow, Zavattero said.
Travel times are projected to improve 10 percent to 20 percent in each corridor that gets the signal upgrades, he said.
Although much lower traffic-flow improvements can be expected at oversaturated locations, the expansion of interconnected traffic signals is booming in the suburbs too.
IDOT is working with Lake County to produce better results. About 150 interconnected traffic signals in Lake County do more than simply communicate with each other via fiber-optic cables to coordinate strings of red or green lights.
The signals can be adjusted in a dynamic, real-time fashion by centralized traffic-management coordination in the Lake County Traffic Management Center in Libertyville.
Specific traffic signals in the 150-light grid can be altered from the center to help deal with emergencies ranging from accidents to flooded roads, said Tony Khawaja, Lake County traffic engineer.
The county is working with IDOT to develop 18 different incident-response plans for each intersection to deal with just about any possible traffic scenario, Khawaja said.
"This gives us the ability to retime 20 or 30 traffic signals simultaneously, allowing more green time and left-turn movements to reroute traffic around accidents," Khawaja said.
The technology is still being tested, but the goal is to eventually expand the program to cover all 643 signal-controlled intersections in Lake County, he said.
It's part of a comprehensive traffic-moving campaign called Lake County Passage, which uses an array of technology to monitor and manage traffic.
"Until we get the money to widen roads and add lanes, this is a good way to get the most out of our system," Khawaja said.
