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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: Managing Competing Interests

Every signalized intersection in Seattle addresses a wide variety of needs. The signal must be effective and functional for pedestrians, automobiles, bicyclists, transit, and large trucks. In order to be effective, each intersection must factor real-life information into their timing. How many people use the intersection to walk across the street? Is the intersection on a bus route? Do large trucks frequent the corridor or intersection?

The simple fact is that not everyone can have a green light or a walk signal all the time. For one group to have a green, another must have a red. By figuring out the popular usage of each intersection, engineers are able to figure out the best possible timing scenario at each intersection for different times of the day and week.

Because traffic patterns at different intersections vary significantly, engineers use a variety of tools to manage them:

  • Semi-actuated signals are used at intersections with minimal cross traffic. They are only activated when a car rests above a sensor in the road or a pedestrian pushes the crosswalk button.

  • Multiple timing cycles are used in all of the City's interconnected signals. New controllers allow for up to 16 different cycles to be programmed, depending on the traffic flow at any given time of day. The result is more effective signal control, responsive to traffic.

  • Left turn signals are used at many intersections to move large numbers of cars across the path of oncoming traffic. Depending on the specific intersection and time of day, the timing of these signals can vary significantly.

  • Transit signal priority gives transit vehicles a time advantage when approaching a signal. A sensor placed on the transit vehicle informs the signal of the approaching vehicle and holds the light green to allow it to pass.

  • Fire Pre-emption allows emergency vehicles to control traffic signals along major corridors to ensure safe and fast passage.

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

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