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Traffic Signal Program
Traffic Signal Optimization Program
Managing Competing Interests
Traffic Signal Control
Traffic Management Center
Traffic Signal Glossary
Accessible Pedestrian Signal Locations
Traffic Data & Records
Live Traffic Cameras
Report a Pothole

Traffic Signal Glossary

The following are a set of terms commonly used when discussing traffic signals.

Control Box

The control box is the signal's brain. Basically, it is a computer that contains the signal timing information for every signal at the intersection. Controllers along signalized corridors are interconnected, coordinated, synchronized, and optimized.

Coordination

Where traffic signals are closely spaced (1/2 mile or less) it is recommended that they be operated in a way to move large volumes or "platoons" of traffic in one movement along the corridor. In order to accomplish this, common equipment at each intersection that works together is required, as is communication between each signal (see signal interconnect).

Cycle (cycle length)

The time it takes for a signal (or set of signals) to go from Green to Yellow to Red and back to Green.

Detection Units (loop and video detectors)

Detection units are used to control a traffic signal based on need. When a car pulls up to a signal that is controlled by a detection unit, its presence indicates that the signal should begin to cycle. This method is very effective for cross streets with minimal traffic that do not require a regular traffic signal cycle.

There are two types of detection units used in Seattle. Loop detectors are imbedded in the pavement and detect vehicles. Alternately, video detectors are fixed cameras mounted above intersections to detect auto movement.

Fire pre-emption

The goal of pre-emption is to provide emergency vehicles the ability to control traffic signals at specific intersections and through corridors to allow for safe and fast response times. Once the emergency vehicle has passed through the intersection, the lights will complete the present cycle and return to normal functions. For example, during a pre-emption in downtown, the north/south corridors will remain green while all other lights, including all pedestrian crossing lights, are red. Corriodors with pre-emption capabilities include: Alaskan Way, Northgate Way, 1st Avenue through 5th Avenue in downtown, and James Street. Pre-emption capabilities are one of the main components of many of our capital improvement signal projects.

Intelligent Transportation Systems (ITS)

Intelligent Transportation Systems (ITS) are a collection of wide-ranging technologies that are brought together to make traffic management more "intelligent". Many see ITS as the future backbone of our nation's transportation infrastructure. Components of ITS include real-time traffic information, instant control of traffic signals, variable message signs, information processing, transit signal priority and fire pre-emption.

Seattle's existing signal systems provides a jump start into ITS. However, integration of other ITS technologies into Seattle's transportation system is still in its infancy. However, almost every capital improvement project that involves signals is factoring in the future role of ITS. Whether this involves upgrading systems to future standards or beginning to lay fiber-optic cable that will act as the communication viaduct between the Traffic Management Center and signals throughout the city.

Optimization

The term optimization refers to improvements and adjustments to signalized intersections that add additional reliability and efficiency. Optimizing traffic signals involves the assessment and allocation of green time at intersections and detailed analysis to determine when a green light is given relative to the next intersection. This allows for improved mobility. A balance between parallel streets and their respective cross streets must be present to make sure that mobility is maintained in all directions.

Signal Interconnect

In order for traffic signals to be simultaneously programmed and consistently work together, they must be connected. This connection is usually a physical wire that connects control boxes along a specific corridor. By connecting each control box, a signal electrician only needs to upload new data into one control box to give commands to the entire corridor of signals. Similarly, if a signal is affected by either fire pre-emption or transit signal priority, the entire corridor of signals can respond simultaneously.

Synchronization

Once coordination is established the traffic signals can be synchronized. Traffic signal synchronization refers to the functioning relationship between active signals along a corridor. A common cycle length is established. All intersections in the coordinated system have the same cycle length. By maintaining a constant relationship between the signals at all times, there is a greater likelihood that mobility will be improved. This does not mean that the signals will provide a green light at the same time for the entire length of a corridor; rather, that each signal will quite literally be synchronized with the entire system, allowing for more efficient mobility.

Traffic Management Center

The Traffic Management Center (TMC) is the hub that connects all of the City's traffic control devices. Currently located in the Municipal Building, the TMC allows the various components of traffic management (signals, control boxes, real time video and simulations) to be effectively managed by a team of traffic engineers who monitor and operate signals throughout the city.

Transit Signal Priority

The goal of Transit Signal Priority (TSP) is to improve speed and reliability of transit vehicles. This is achieved by establishing a communicative link between the approaching transit vehicle and the traffic signal. Once both the signal and transit vehicle have been outfitted with TSP equipment, a radio signal from the approaching bus will direct the traffic signal to, if possible, lengthen the green light for the approaching bus, allowing the vehicle "priority" through an intersection. Priority is only allowed in situations when it will not cause a significant impact at the specific intersection.

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

  • Signal Timing Research

  • CCTV Connections to monitor intersections

  • Independent control and real-time monitoring of signalized corridors

  • Control of electronic messaging signs

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

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