Traffic signal preemption
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.
That also allows buses and vehicles transporting dangerous goods to proceed through the intersection without stopping on the railroad tracks.
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.
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.
Unusual circumstances can also occur which can confuse operators of vehicles with traffic preemption equipment who lack proper training.
The LRT operator was viewing right-of-way indications from downstream signals and failed to realize that preemption had occurred at the nearest intersection.
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.
Reflected waves can also create unnecessary collateral preemption events alongside streets near the emergency vehicle's route.
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.
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.Drawbacks of line-of-sight systems include obstructions, lighting and environmental conditions, and undesired activations.
Obstructions may be buildings on a curving road that block visual contact with a traffic signal until very close, or perhaps a large freight truck.
Direct sunlight into a receiver may prevent it from detecting an emitter, and severe environmental conditions, such as heavy rain or snow, may reduce the distance at which a line-of-sight system will function.
Until recently, the major drawback of radio-based traffic signal preemption systems was the possibility of interference from other devices that may be using the same frequency at a given time and location.
The operating range was adjusted by varying the radio signal strength so that traffic lights could be activated only nearby (if desired), or at greater distances.
Extremely heavy cloud cover or severe weather can also adversely impact the ability of the GPS receiver from obtaining the four required satellites.
900 MHz FHSS appears to be the best option, because it is capable of the greatest range (often over 4.8 km or 3 mi for a 1 watt transceiver).
