Traffic signal preemption (also called traffic signal prioritization ) is a type of system which permits 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 aussi can be used by light railand bus rapid transit systems to allow public transportation priority access through intersections, or by railroad systems at crossings to prevent prevention collisions.
Traffic preemption devices are implemented in a variety of ways. They can be installed on road vehicles, with integrated rail transportation network management systems, or operated by remote control from a fixed location, Such As a fire station , or by a 911 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 is not a signal to be sent to a traffic signal.
Vehicle devices can be switched on or off as needed, though in the case of emergency vehicles, they are often integrated with the vehicle’s emergency warning lights. When activated, the traffic pre-emption 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, .
Traffic Signal Preemption Systems for Traffic Signals and Traffic Signals for Traffic Signals and Traffic Signals Road crossing that may be in the way to quickly clear the crossing. This also allows buses and hazmat vehicles in the USA to proceed through the intersection without stopping at the railroad tracks.
Fixed-rent systems can vary widely, but a typical implementation is for a single traffic signal in front of or near a fire station to stop 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 an emergency room, or an ambulance. An area with dense traffic.
Traffic signal preemption systems, which include a method for communicating to the operator of the vehicle that requires the preemption of a preemption device, by means of a notifier. This device is almost always an additional light located near the traffic signals. It may be a single light bulb which is visible to all, which flashes or stays on, or there may be a light towards each direction of which traffic approaches the intersection. In the case of multiple notifiers lights at a controllable intersection, they will either flash or stay on depending on the local configuration, to communicate with which direction a preempting signal is being received. This informs regular drivers which direction may need to be cleared, And the same intersection of the same vehicle. A typical installation would provide an emergency response to a vehicle that is in the immediate vicinity of the vehicle. There are variations of notification methods in use, which may include one or more colored lights in varying configurations. 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. 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.
Events are the most common and the most commonly used systems in the world. Unusual circumstances can also occur which can confuse operators of traffic with preemption equipment which 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 , LRT at the crossing. A LRT operator was at fault. LRT simultaneously, at ALL intersections. 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 the LRT operator, failing to notice the unexpected signal to stop, ran into the fire engine and destroyed it.  The LRT operator, the LRT operator, failing to give notice to the intersection.  The LRT operator, the LRT operator, failing to give notice to the intersection. 
Vehicular device types
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 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 it 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 ).
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. Arriving at the intersection. These line-of-sight systems use an invisible infrared signal, or a visible strobe light which serves a dual purpose as an additional warning light. The emitter transmits visible flashes of light or invisible infrared pulses at a specified frequency . Traffic lights must be equipped with a compatible 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. 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.
Drawbacks of line-of-sight systems include obstructions, lighting and atmospheric conditions, and undesired activations. Obstructions may be constructed on the basis of a traffic light, or a large traffic light. Modifying the position of the receiver or even locating it from the traffic signal. Direct sunlight into a receiver may prevent it from detecting an emitter, and severe atmospheric conditions, such as heavy rain or snow, may reduce the distance at which a line-of-sight system will function. Undesired activations may be an emitter’s signal is picked up by many traffic lights along a stretch of road,
Line of sight emitters can use IR diodes. They are pulsed with a low-priority signal (10 Hz) or a high-priority signal (14 Hz). 
Localized radio signal
Radio-based traffic-preemption systems using a local, short-range radio signal in the 900MHz band , can usually avoid the weaknesses of line-of-sight systems (2.4 GHz and optical). A radio-based system is a radio-based system that uses radio-based systems, which are not blocked by visual obstructions, lighting or weather conditions. Until recently, the major drawback of radio-based traffic signal pre-emption systems was the possibility of interference from other devices that may be used at the same time. The advent of FHSS (Frequency Hopping Spread Spectrum).
Radio-based systems also offer some additional benefits – adjustable range and collision avoidance. The operating range was adjusted by varying the traffic signal. The downside to these preemption systems (which also carried out collision avoidance) was that the direction of impending collisions, but not be able to effectively (or accurately) computed the distance to collision by any method Only a rough estimate at best.
Global Positioning System
With the advent of widespread global positioning system (GPS) applications the introduction of a GPS-based traffic preemption system, which could also avoid collision avoidance. Some GPS pre-emption systems (see below) have a way to overcome the nagging problem. In dense cities with tall buildings, GPS receivers may have difficulty obtaining the required satellite GPS signals, required for trilateration to determine location. If the vehicle systems are not designed with an “IMU” (Inertial Measurement Unit) backup, lack of GPS availability may adversely affect the system’s performance.
Another type of preemption is railroad preemption.  Traffic-signal-controlled intersections next to railroad crossings on the roads usually have this feature. Approaching trains activate a routine where, before the train signals and gates are activated, all traffic signal phases go to red, except for the signal immediately after the train crossing, which turns green (In some cases, there are auxiliary traffic signals to the railroad crossing which will turn red, keeping the traffic from crossing the tracks. After enough time to clear the crossing, the signal will turn. The crossing lights may begin flashing and the gates lower immediately,
The operation of a traffic signal may occur at any time. In some areas, all directions will flash red, turning the intersection into an all-way stop . In other areas, the traffic parallel to the railroad track will have a flashing yellow for the duration of the train while the other directions face a flashing red light for the duration of the train. Still in other areas, the traffic parallel to the railroad track will have a green light for the duration of the train while the other directions face a red light for the duration of the train.
- Bus priority
- Bus transit transit creep
- Mobile Infrared Transmitter
- Jump up^ “Buzz Gadget” . Cnet.com . Retrieved 2007-06-05 .
- Jump up^ Accidents Point Up Dangers of Rail Transit ArchivedOctober 3, 2006, at theWayback Machine.
- Jump up^ http://www.tech-faq.com/how-do-traffic-lights-work.html
- Jump up^ Section 4D.13andSection 8D.07of theManual on Uniform Traffic Control Devices