Traffic signal preemption

Traffic signal preemption (also called traffic signal prioritization) is a type of system that allows the normal operation of traffic lights to be preempted. The most common use of these systems is to manipulate traffic signals in the path of an emergency vehicle, halting conflicting traffic and allowing the emergency vehicle right-of-way, to help reduce response times and enhance traffic safety. Signal preemption can also be used by light-rail and bus rapid transit systems to allow public transportation priority access through intersections, or by railroad systems at crossings to prevent collisions. Traffic signal preemption (also called traffic signal prioritization) is a type of system that allows the normal operation of traffic lights to be preempted. The most common use of these systems is to manipulate traffic signals in the path of an emergency vehicle, halting conflicting traffic and allowing the emergency vehicle right-of-way, to help reduce response times and enhance traffic safety. Signal preemption can also be used by light-rail and bus rapid transit systems to allow public transportation priority access through intersections, or by railroad systems at crossings to prevent collisions. Traffic preemption devices are implemented in a variety of ways. They can be installed on road vehicles, integrated with train transportation network management systems, or operated by remote control from a fixed location, such as a fire station, or by a 9-1-1 dispatcher at an emergency call center. Traffic lights must be equipped to receive an activation signal to be controlled by any system intended for use in that area. A traffic signal not equipped to receive a traffic preemption signal will not recognize an activation, and will continue to operate in its normal cycle. Vehicular devices can be switched on or off as needed, though 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 properly equipped traffic lights in the path of the vehicle to cycle immediately, to grant right-of-way in the desired direction, after allowing for normal programmed time delays for signal changes and 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 that may be in the way to quickly clear the crossing. This also allows buses and hazmat vehicles in the United States to proceed through the intersection without stopping at 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 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. Some systems use an acoustic sensor linked to the preemption system. This can be used alone or in conjunction with other systems. Systems of this type override the traffic signal when a specific pattern of tweets or wails from the siren of an emergency vehicle is detected. Advantages of a system like this are that they are fairly inexpensive to integrate into existing traffic signals and the ability to use siren equipment already installed in emergency vehicles – thus dispensing with the need for special equipment. A major disadvantage is that sound waves can easily be reflected by buildings or other large vehicles present at or near an intersection, causing the 'reflected' wave to trigger a preemption event in the wrong direction. Reflected waves can also create unnecessary collateral preemption events alongside streets near the emergency vehicle's route. Yet another disadvantage is that the acoustic sensors can sometimes be sensitive enough to activate the preemption in response to a siren from too far away, or from an unauthorized vehicle with a horn exceeding 120 dB (many truck and bus horns exceed this threshold at close range). A vehicle that uses a line-of-sight traffic signal preemption system is equipped with an emitter which typically sends a narrowly directed signal forward, towards traffic lights in front of the vehicle, in an attempt to obtain right-of-way through a controllable intersection prior to arrival. These line-of-sight systems will generally use an invisible infrared signal, or a visible strobe light as an emitter. In the case of a strobe light, it may also serve a dual purpose as an additional warning light. To communicate to the traffic light, the emitter transmits visible flashes of light or invisible infrared pulses at a specified frequency. Traffic lights must be equipped with a compatible traffic signal preemption receiver to respond. Once the vehicle with the active emitter has passed the intersection, the receiving device no longer senses the emitter's signal, and normal operation resumes. Some systems can be implemented with varying frequencies assigned to specific types of uses, which would then allow an intersection's preemption equipment to differentiate between a fire engine and a bus sending a signal simultaneously, and then grant priority access first to the fire engine.