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Delivering a first-rate transportation system for Seattle Scott Kubly, Director







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Traffic Signal Program
Traffic Signal Optimization Program
Managing Competing Interests
Traffic Signal Control
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Traffic Signal Optimization Program

Seattle has over 975 signalized intersections, three quarters of which are on major transportation corridors such as Aurora Avenue North, Delridge Way Southwest, Rainier Avenue South, and in the entire downtown area.

Seattle's Signal Optimization Program is a coordinated effort designed to make the most efficient use of our city's traffic signals by improving traffic signals, gathering up-to-date traffic data, and taking advantage of new technologies. "Optimization" in this context refers to all maintenance, upgrades, timing adjustments, and miscellaneous efforts to improve our signals.

This year, the Seattle Department of Transportation (SDOT) will re-time, or synchronize, a record number of traffic signals in 17 corridors throughout the city. The result will be smoother and quicker trips for platoons of traffic, with maximum green-light time, especially through corridors that handle heavy volumes of freight. Work has already begun on the 150 signals to be analyzed and re-timed as part of this synchronization effort.

The 2004 plan includes work in South Seattle, downtown, Ballard, West Seattle, and Beacon Hill. Seattle has over 975 signalized intersections, many of which have been re-timed during the last five years.

To get the most out of our network of signals, engineers must constantly make sure that equipment, such as signal heads, control boxes, and sensors, are functioning optimally; and that the data, which include traffic volumes and intersection specific traffic movements, are current and reliable. Assuring these two things enables the Traffic Management Signal Operations team to help manage our increasing number of cars, buses, pedestrians, and bicyclists.

On high-volume signalized routes, for example, engineers want to make sure that our signals are optimized by synchronizing and coordinating each intersection throughout the entire corridor. This allows for platoons of traffic to move smoothly and quickly, with maximum green-light time, through the corridor.

SDOT engineers start by gathering real-time data at each intersection along the corridor to find out the most up-to-date travel patterns. Since our City's transportation demographics are constantly shifting, it is important to gather data every few years to ensure the best use of our signals. Once the data have been gathered, engineers use a computer program to develop the best timing scenarios for each intersection on both an individual and a collective basis.

More than half of all signalized intersections in Seattle have the newest control boxes which allow for up to 16 cycle options (which differ due to changing traffic patterns over the course of a day) and are interconnected to allow for coordinated communication between the intersections.

2004 Signal Synchronization-Optimization Plan (PDF format)

How Traffic Signals Are Timed

Managing Competing Interests

Traffic Signal Control

If you have questions or comments about the City's Traffic Signal Program, call (206) 684-ROAD or send an e-mail to

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