Traffic signals in Ann Arbor fall into two general categories: coordinated and non-coordinated. There are benefits to each type of operation.
Coordinated Signals. When intersections are close together, coordinating traffic signals is beneficial so that traffic can move through several intersections without being stopped.
Fully actuated Signals. When intersections are further apart, the group of cars starting from the first signal as a closely spaced "platoon" spreads apart. It becomes very difficult to say for which cars the green should be displayed on the next signal: for the few fastest that first arrive, or for the bigger group in the middle. In this case the actuated controller works the best, adjusting signal phases to the volume of traffic approaching the intersection.
Coordination of traffic signals is maintained in our city using two methods: a central computer and master controllers.
The central computer system was rebuilt in 1997 and upgraded in 2003. A brand new fiber optic communication network connects intersection controllers with two computers; one in the Signal Shop at 415 W. Washington St., the second in City Hall. The computers use Pentium processors running ACTRA software from Eagle Systems. The central computer communicates with individual intersections, checking its status, comparing its clock with the central computer's clock, collecting data and changing timing plans. We can monitor the performance of the system on the computer screen, change the settings of the controllers from the office, and get information about signal malfunctions within seconds.
The ACTRA system controls the downtown, from approximately Stadium Blvd. to Depot St., and from Observatory to First St. Three major arteries are also added to this system - Washtenaw Ave, Eisenhower Parkway, and Plymouth Road, and an expansion is planned by building communication lines which will extend the system to include the entire Maple / Stadium corridor from Miller to Washtenaw.
In 2004 we started implementation of the most modern “off the shelf” signal control system on the market, namely SCOOT (Split Cycle Offset Optimization Technique) by Siemens. This Traffic Adaptive system optimizes signal operation in real-time, constantly minimizing delay. This is the system Toronto and Minneapolis are using in North America, and many other big cities in the world. SCOOT is operational since April 2005 on three of our arteries: Washtenaw from US-23 to S. University, Plymouth from US-23 to Swift, and Eisenhower/ Packard from Ann Arbor – Saline to Turnberry.
SCOOT is very efficient minimizing delays across the system. It is clearly working very well in unforeseen situations. We could see several events when US-23 was blocked by an accident, traffic volumes were much higher than normal on Plymouth or Washtenaw, but the delay per vehicle was still in a reasonable range, thanks to signal timings adapting to traffic volumes.
On-street MARC (Monitor and Report Console) Master controllers are controllers installed on arterial streets. They have similar functions to ACTRA, with some limitations. We used to have five of them. Now, all but one are replaced by fiber connecting signals to central computer. The last one controls signals on Fuller Road from Cedar Bend to Fuller Court.
The remaining intersections are operating “solo”. The majority of signals are traffic actuated and some are pre-timed, following the same pattern during the same hours.
All of the intersection controllers within the city are modern microprocessors.