Although many transportation agencies measure freeway traffic flows using commonly available loop detector data, a similar approach for urban arterial roads doesn't exist. In fact, operational data from urban signal systems are neither stored nor analyzed, preventing traffic engineers from proactively managing their community's arterial streets.

SMART-Signal fills this gap. The tool, which stands for "systematic monitoring of arterial road traffic signals," simultaneously collects event-based high-resolution data from multiple intersections and generates real-time performance measures such as queue length and travel time. By laying the groundwork for better modeling and control strategies, the tool opens new opportunities for managing flows on congested roads.

The tool archives and stores a complete history of signal control, including all vehicle actuation and signal phase change events. An industrial PC with a data acquisition card is installed in the controller cabinet at each intersection, and event data are transmitted to the server in real-time via an Ethernet connection.

The tool uses a newly developed algorithmic approach to queue length estimation based on traffic shockwave theory. Cyclic shockwaves at an intersection can be reconstructed using event-based data, allowing for queue length estimation even when the queue of cars extends beyond the upstream vehicle detector. To measure travel time, the tool simulates the movements of a virtual "probe vehicle" along the road. As the virtual probe moves, it can modify its own state in response to the state of traffic around it by accelerating, decelerating, or maintaining a constant speed at each time step as it encounters queues, signals, and changes in density.

The tool also can optimize signal parameters using the collected high-resolution data. Instead of relying on traditional offset optimization approaches, which are based on manually collected volume data on a typical day, the tool accounts for flow variations by using archived signal data and the derived performance measures.

SMART-Signal's been field-tested on three major arterial corridors in Minnesota, and the state DOT will use it to monitor 100 intersections in the Twin Cities. A demonstration project is also being carried out on Orange Grove Boulevard in Pasadena, Calif. PW

- This article was excerpted with permission from the University of Minnesota Center for Transportation Studies.

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