Traffic signal priority & preemption

Priority and preemption are used at intersections with traffic signals to give the green light to buses, light rail, streetcar, trucks, emergency vehicles, or trains. Its primary purpose is to reduce travel times, improve safety, and clear a path so that the priority vehicle can arrive at its next stop or destination faster.

• Signal Priority
  is used to provide extra green time at intersections to larger and street-running transit vehicles, such as trucks, buses, light rail, and streetcar. By helping the bus or truck avoid a stop at a red light, this additional time can help the bus arrive at stops on-time, and can prevent traffic delays and congestion. It also improves safety by helping these vehicles clear the intersection on a green light when it may be difficult to stop.
   
• Signal Preemption
  is used to provide a green light for emergency vehicles, trains, and light rail. Preemption is different from priority because it takes control of an intersection and immediately turns all directions red except for the direction with the emergency vehicle, train, or light rail. Preemption overrides any signal priority and will terminate an extended green light in favor of a green light for the vehicle requesting preemption. This helps emergency vehicles reduce the time it takes them to get to their destination by clearing a path through traffic signals. It also improves safety by providing a red light to all vehicles, pedestrians, and bicyclists that are expected to stop and allow the emergency vehicle, train or light rail to proceed.
   

Both priority and preemption require equipment on the vehicle and at the intersection to communicate to the traffic signal and request preemption or priority. Many detection technologies exist and selecting the technology should be determined based on the specific project needs.

Typically, the same equipment is used to detect a bus and an emergency vehicle, but detection for freight (trains and trucks) is usually different. For trucks, the detection technology normally relies on intersection detection only because it’s unlikely to expect every truck to install the same equipment to request priority. For trains, the detection must be reliable and therefore typically uses train-to-wayside (TWC) communications.
 

Three distinct categories of detection exist to detect the approaching vehicle and transfer the request (point, zone, and area):

• A point detection system detects a vehicle at a single point and transfers the request for priority to the traffic signal. Multiple detectors are generally considered for point detection systems to provide redundancy and and verify when the priority vehicle has not only entered but exited. Since the point detection system only detects the vehicle at a specific location, it does not recognize if the vehicle changes speed or stops before the intersection.
   
• A zone detection system detects the presence of a vehicle within a zone near the intersection and transfers the request for priority to the traffic signal. The range of the zone varies depending on the detection technology and specific intersection characteristics, but generally, can detect an approaching vehicle within 3,000 feet of an intersection. These systems only recognize that the vehicle is within the zone, and cannot determine the location or speed of the vehicle.
   
• An area detection system is different from a point or zone detection system because it tracks the location of the vehicle within an area. The area could be significant and cover an entire stretch of roadway or urbanized area. Area detectors improve the ability to predict the arrival time of the vehicle at the intersection because they can tell the vehicle’s speed. Area detectors depend on a vehicle location system and a communications network to share the vehicle location and request for priority over a wide area. Typically the communications network is the local cellular network.