

by / April 25, 2007

Just as a music conductor guides an orchestra, making interpretative decisions as to the tempo of a music passage, real-time intelligent traffic systems now help cities conduct light-rail and side-street traffic, resulting in a harmonious flow of transportation through bustling city streets.

Such technology can be used to regulate car commute times, increase the viability of light-rail systems and avoid the ever-growing problem of congested roadways. 

A group of Arizona cities is working toward this end and plans to implement a "predictive priority" system for the Central Phoenix/East Valley Light Rail Transit Project - now under construction and set to open in December 2008.

Valley Metro is the local agency responsible for public transportation in the area. Phoenix, Tempe, Mesa and Glendale created the nonprofit METRO light rail, under Arizona statute, to construct, design and maintain Valley Metro's light rail.

Predictive priority is meant to synchronize traffic lights to increase the smooth circulation of car and light-rail traffic. Phoenix's system balances the need to give priority to approaching light-rail trains - ensuring the fewest red lights for public transportation - without disrupting traffic flow, said Pat Fuller, deputy project manager of design and construction at METRO light rail.

"Without the system, the train wouldn't be competitive in regard to travel times with vehicles," Fuller said. "And we had to prove that we were able to compete with vehicles, otherwise people just wouldn't ride it."

The localized traffic intelligence system is based on complex communication networks - sensor networks to interpret characteristics of oncoming traffic, and mathematical and predictive algorithms that compute optimum settings for traffic light cycles.

Houston and Salt Lake City recently built light-rail predictive priority systems, but Phoenix's system will be more extensive because its 20-mile track snakes through major streets in Phoenix, Tempe and Mesa, requiring more intersections to be equipped with the technology.

System Design
Construction for the Central Phoenix/East Valley Light Rail Transit Project began in 2003 and is one of the largest infrastructure projects in Arizona's history. Fuller said engineers at the Traffic Signal Test Center in Phoenix have worked to perfect the predictive priority system since 2005. The system is 50 percent complete and was tested on live trains in March 2006.

The predictive technology gives light-rail traffic an edge, but not an automatic green light, because this would halt normal traffic flow, he said. Also, added side-street gridlock from more red lights would be counterproductive by encumbering the drive to the light-rail train station.

The system is built with a gigabyte Ethernet network running along the 20-mile corridor, and will allow quick, clear communications to the on-site traffic controllers.

Intersections will be equipped with check-in and check-out detectors that trip when the train speeds over them. The detectors will broadcast the rail's real-time position to upcoming intersections, giving them several minutes to prepare, Fuller said. Then, sophisticated algorithms analyze the time it will take the train to reach each station and decide where the traffic light will be in its cycle upon the train's arrival.

"The theory is we can get far enough ahead of the train in arrival time, that that's enough to facilitate assuring the train a green on arrival," Fuller said.

At its core, the predictive priority system allows for coordination between adjacent traffic light signals - whereas most traffic lights operate independently, said Larry Head, interim department head of Systems and Industrial Engineering in the Engineering College at the University of Arizona.

System communication is organized in groups of five or six intersections that talk to four midlevel switches, located five miles apart, Fuller said. Then, a network backbone collects and disperses the information to three traffic management centers, which provide central control, observation and dissemination of


The traffic signal controller is a field-hardened computer that makes decisions about the timing of red, green and yellow lights and responds to needed programming changes at each intersection, such as a light-rail car approaching an intersection and needing a green light to get through. Traffic controllers, detectors and switches are all located in the cabinet, or large metal box, at each traffic light, Head said.

Houston, Salt Lake City and Phoenix use NextPhase, a traffic control software solution designed by Siemens Intelligent Transportation Systems (ITS).

To react to traffic variables - such as unforeseen congestion and passenger load and unload times - the system manipulates traffic lights in several ways, Fuller said. Traffic controllers can lengthen the time a light is red or green or shift the left-turn signal from the beginning to the end of the traffic light cycle. Already, future northwest extensions in Phoenix and Glendale have been planned for the light rail.

Cultivating the Idea

While predictive priority is still new, the idea of traffic signal communication originated more than two decades ago, Head said.

Baltimore's light-rail system, which opened in 1992, is an early example of a "wired interconnect system," where sensors on the tracks alert traffic light signals downstream of the train's timing, Head said.

Head, an expert on intelligent traffic systems, worked at Siemens ITS and helped design Salt Lake City's predictive priority system for the 2002 Olympics before returning to academia.

It was the light rail's placement amid Arizona's largest cities that first led authorities to believe an intelligent traffic system was needed, Fuller said.

"No one's really done a system quite this size," he said. "And that's why [METRO light rail] and the cities felt like we needed something innovative that gives us a high probability, but not derogate the side-street traffic."

In fact, drivers will hardly notice the light rail's impact since the carefully choreographed traffic lights should let drivers reach their destinations at about the same times as they did before the light rail was built, Fuller said.  

Fuller said METRO first looked at other options for the light-rail system - like coordinating its traffic signals or giving the light rail an automatic green light - but settled on predictive priority because of good results in Salt Lake City and Houston.

"We needed an operation that the cycle time would still function but be adaptive," he explained. "Detector inputs several minutes in advance of arrival of the light rail at the intersection allow for this minor manipulation with minimal impact on other traffic."

The Salt Lake City light-rail system of trip switches and backbones is much like Phoenix's setup, but differs in its size and use of a central controller, Fuller said.

"Salt Lake City [light rail] is much smaller," he explained. "Most of their system is a dedicated guideway - trains go 55 mph. The difference is their intelligence is done at a central controller versus a controller on the street."

Houston's light rail encounters 90 signals while Salt Lake City's light rail bypasses fewer than 20 signals, Head said.

Cost-wise, predictive priority makes sense because it has a big impact on light rail effectiveness, Head and Fuller agreed. 

"Proportionally to the cost of the rest of the system," Head said, "it's a low-cost item, but can have a big impact on performance."

Predictive priority also reaps dividends by getting more people to ride light rail.

"To get the mode shift to people on the train, you have to have good, reliable and efficient transportation service with good travel time," Head said.

The Federal Highway Administration (FWHA) estimates the price of real-time traffic control systems at $10,000 to $40,000 per intersection, and

about $1,000 per intersection for subsequent years. Normal retiming of traditional signals costs $5,000 biannually, according to the FWHA.

Intelligent Buses, Cars

The same predictive priority technology used for light-rail transit can be applied to other forms of public transportation, such as buses, Head said.

Buses, however, present a different challenge since they share the roadways with cars.

"Buses are a little bit harder," Head said, "because they're mixed in with the traffic and their arrival times are harder to predict."

Head said cars may soon become "smarter" as they too communicate with their environment - the symphony of lane signals, traffic lights and ramp meters that surround them.

The Collision Avoidance Systems initiative - a national partnership between automotive manufacturers and state and local departments of transportation - is working to develop applications for in-car safety mechanisms. One such application, Head said, involves a system that warns drivers when they're about to violate traffic signals.

"Someday," Head said, "if the car's going to violate, we may hold the traffic signal red a little longer."

While mainstream uses of the technology might be a while off, predictive priority - which can be integrated into existing traffic systems - has already arrived.

"The industry and the technology are moving in this direction anyway," Head said. "I certainly wouldn't not do it today - I think the cost of not doing it is higher than the cost of doing it."

Jessica Weidling Staff Writer