Traffic signal preemption (also called traffic signal prioritisation) is a system that allows an operator to override the normal operation of traffic lights. The most common use of these systems manipulates traffic signals in the path of an emergency vehicle, halting conflicting traffic and allowing the emergency vehicle right-of-way, thereby reducing response times and enhancing traffic safety. Signal preemption can also be used on tram, light-rail and bus rapid transit systems, to allow public transportation priority access through intersections, and by railroad systems at crossings to prevent collisions.
Implementation Traffic preemption is implemented in a variety of ways. Traffic light activation devices can be installed on road vehicles, integrated with transport network management systems, or operated by remote control from a fixed location, such as a fire station, or by an emergency call dispatcher. Traffic lights are equipped to receive an activation signal which interrupts the normal cycle. Traffic signals not equipped to receive a traffic preemption signal will not recognize an activation, and will continue to operate on the normal cycle. Vehicular devices can be switched on or off as needed, but in the case of emergency vehicles, they are frequently integrated with the vehicle's emergency warning lights. When activated, the traffic preemption device will cause appropriately equipped traffic lights in the path of the vehicle to cycle immediately, granting right-of-way in the desired direction, after allowing programmed time delays for the necessary signal changes and for pedestrian crosswalks to clear. Traffic signal preemption systems integrated with train transportation networks typically extend their control of traffic from the typical crossarms and warning lights to one or more nearby traffic intersections, to prevent excessive road traffic from approaching the crossing, while also obtaining the right-of-way for road traffic to quickly clear the crossing. That also allows buses and vehicles transporting dangerous goods to proceed through the intersection without stopping on the railroad tracks. Fixed-location systems can vary widely, but a typical implementation is for a single traffic signal in front of or near a fire station to stop traffic and allow emergency vehicles to exit the station unimpeded. Alternatively, an entire corridor of traffic signals along a street may be operated from a fixed location, such as to allow fire apparatus to quickly respond through a crowded downtown area, or to allow an ambulance faster access when transporting a critical patient to a hospital in an area with dense traffic. Traffic signal preemption systems sometimes include a method for communicating to the operator of the vehicle that requested the preemption (as well as other drivers) that a traffic signal is under control of a preemption device, by means of a notifier. This device is commonly referred to in the industry as a "confirmation beacon". It is usually an additional light located near the traffic signals. It may be a single LED light bulb visible to all, which flashes or stays on, or there may be a light aimed towards each direction from which traffic approaches the intersection. In the case of multiple notifier lights at a controllable intersection, they will either flash or stay on depending on the local configuration, to communicate to all drivers from which direction a preempting signal is being received. This informs regular drivers which direction may need to be cleared, and informs activating vehicle drivers if they have control of the light (especially important when more than one activating vehicle approaches the same intersection). A typical installation would provide a solid notifier to indicate that an activating vehicle is approaching from behind, while a flashing notifier would indicate the emergency vehicle is approaching laterally or oncoming. There are variations of notification methods in use, which may include one or more colored lights in varying configurations. Some of the newer high tech systems have a display in the cab, which will eliminate the necessity of a confirmation beacon. This can also reduce the cost of a preemption project considerably. Events leading up to an activation and notification are not experienced by drivers on a daily basis, and driver education and awareness of these systems can play a role in how effective the systems are in speeding response times. Unusual circumstances can also occur which can confuse operators of vehicles with traffic preemption equipment who lack proper training. For example, on January 2, 2005, a fire engine successfully preempted a traffic light at an intersection which included a light rail train (LRT) crossing in Hillsboro, Oregon, yet the fire engine was hit by an LRT at the crossing. A subsequent inquiry determined that the LRT operator was at fault. The accident occurred in the middle of a network of closely spaced signalized intersections where the signs and signals granted right-of-way to the LRT simultaneously, at ALL intersections. The LRT operator was viewing right-of-way indications from downstream signals and failed to realize that preemption had occurred at the nearest intersection. The fire engine, granted the green light before it arrived at the intersection, proceeded through while the LRT operator, failing to notice the unexpected signal to stop, ran into the fire engine and destroyed it.
Vehicular device types
… excerpt ends here. Continue reading the full article.
