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Concept
Bus priority signal
Summary
Bus priority or transit signal priority (TSP) is a name for various techniques to improve service and reduce delay for mass transit vehicles at intersections (or junctions) controlled by traffic signals. TSP techniques are most commonly associated with buses, but can also be used along tram/streetcar or light rail lines, especially those that mix with or conflict with general vehicular traffic.
Transit signal priority techniques can generally be classified as "active" or "passive". Passive TSP techniques typically involve optimizing signal timing or coordinating successive signals to create a “green wave” for traffic along the transit line's route. Passive techniques require no specialized hardware (such as bus detectors and specialized traffic signal controllers) and rely on simply improving traffic for all vehicles along the transit vehicle's route.
Active TSP techniques rely on detecting transit vehicles as they approach an intersection and adjusting the signal timing dynamically to improve service for the transit vehicle. Unlike passive techniques, active TSP requires specialized hardware: the detection system typically involves a transmitter on the transit vehicle and one or more receivers (detectors), and the signal controller must be “TSP capable”, i.e. sophisticated enough to perform the required timing adjustments. Active strategies include:
Green Extension: This strategy is used to extend the green interval by up to a preset maximum value if a transit vehicle is approaching. Detectors are located so that any transit vehicle that would just miss the green light ("just" meaning by no more than the specified maximum green extension time) extends the green and is able to clear the intersection rather than waiting through an entire red interval cycle. Green Extension provides a benefit to a relatively small percentage of buses (only buses that arrive during a short window each cycle benefit), but the reduction in delay for those buses that do benefit is large (an entire red interval).
Official source
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