May 17th, 2011
HEBER CITY TO LITE-UPOptimize Mobility, by Catherine Higgins.
This post is second in a series about UDOT and FHWA’s Every Day Counts Innovation Initiative.
A “lite” way to operate signals offers many advantages of a full-blown adaptive system without the high cost.
Adaptive traffic signals will be installed in Heber City, Utah this summer. The new computerized signals will adjust automatically to real-time traffic demand. To understand adaptive signals, it’s helpful to first understand the current signal control system, says Mark Taylor, Signal Systems Engineer for UDOT.
The current state wide traffic management system is operated through a centralized Traffic Operations Center. The signals, usually within one-half mile of each other, are synchronized by time of day with an internal clock, explains Taylor.
Each signal can operate using multiple signal coordination plans that correspond to traffic needs – for example, signals can be programmed to allow more green light time in the peak traffic-direction during morning or afternoon commutes. Signal operators can program several coordination plans per signal that switch from one plan to another automatically throughout the day.
TOC signal operators can also make manual adjustments to help traffic during major incidents or events, such as a crash that blocks travel lanes or a football game.
With 1200 signals to synchronize statewide, creating plans for each signal takes a lot of time and effort. First, an on-site vehicle count is completed at each intersection. Data from that count is analyzed and engineers draw conclusions about traffic flow. From the data and conclusions come basis for each coordination plan.
While the current plan works well, there are some inherent disadvantages:
- Every three to five years, traffic engineers need to review or redo coordination plans. “Time plans can get stale when a new development is built,” says Taylor. When plans go stale, engineers need to manually alter the plans or get new traffic counts, “and that’s a time intensive process.”
- Special events throw another complication into the traffic plan mix. “The current system does not deal with atypical events” like holiday shopping, sports events or an incident like a crash that blocks travel lanes.
- As plans switch from one coordination plan to another, that change is abrupt while actual traffic changes are gradual. “The current system gets the heart of the peak well but not the sides well.”
Adaptive Traffic Control Systems respond to traffic in real-time.
“There’s no library of plans to pick from,” says Taylor. “Actual time plans don’t get stale; they stay current and are optimized on a real time basis.” Changes are made gradually as signals at intersections communicate with each other electronically to provide more green light time or other adjustments as needed.
An ATCS has been operating effectively in Park City for five years. The system works well in the resort town because traffic is seasonal and weather sensitive, so coming up with defined traffic coordination plans is difficult.
As with the current system, ATCSs also have inherent disadvantages. The systems are signal detector heavy, and those detectors need to be working correctly for the system to work. “Some systems are smart enough to ignore some bad data,” but when too many detectors fail, “it sends the system bogus data,” says Taylor. ATCSs are also expensive and sophisticated and require initial calibration, fine tuning and active maintenance. Plus, ATCSs do not link up well with the current system.
Adaptive Control Software Lite
The system being installed in Heber this fall is less complicated than regular ATCSs. Taylor says the algorithms are simpler and the system is less expensive. “Full blown systems are $25 to $30 thousand per intersection.” While ACS Lite doesn’t do as much as its high-end relative, “it should do well to accommodate the needs of Heber City” at half the cost. Plus, the Heber system allows integration with a central management system.
Like Park City, Heber City has “seasonal variations to traffic demand.” With events like Swiss Days and the Cowboy poetry festival, and the draw of Strawberry Reservoir as a popular recreation site, “during the summer, traffic gets busy in Heber.”
Taylor is on an EDC technical advisory committee working to define specifications for ACS Lite. The EDC backed effort aims to “help to create a system that is simpler to use and operate and integrate effectively with the systems currently in place.” Through the EDC initiative, FHWA is sponsoring and funding part of an effort to help the nation’s departments of transportation, cities and towns procure systems that will effectively reduce delay.
Should ACS Lite signals be installed in every intersection? “It’s not a magic bullet,” says Taylor. “Funding and capacity issues can’t be solved easily.” UDOT will evaluate ACS Lite in Heber and look at other appropriate sites for ACS Lite or similar systems in the future as funding allows.
Read more about adaptive signals as part of the EDC Innovation Initiative.