traffic signal

Traffic signal preemption

Traffic signal preemption is a type of system that allows the normal operation of traffic lights to be preempted, often to assist emergency vehicles. The most common use of these systems is to allow emergency vehicles priority by changing traffic signals in the path of the vehicle to green (or in some cases, flashing green) and stopping conflicting traffic. Sometimes, signal preemption is also used at railroad grade crossings to prevent collisions, and by light-rail and bus rapid transit systems to allow public transportation priority access through intersections to ensure they are able to remain on schedule and improving commute times.

The key advantages of signal preemption include their ability to reduce response time for emergency services and to increase safety on the road.


Older devices generally operate by use of invisible infrared signals or by visible strobe lights. Each emergency vehicle is equipped with an emitter, a device which emits visible flashes of light or invisible infrared pulses at a specified frequency. A unit, mounted next to the lightbar on a fire engine, connects to the control unit mounted in the cab. Receiver devices placed on or near intersection traffic control devices recognize the signal and preempt the normal cycle of traffic lights. Once the emergency vehicle passes through the intersection and the receiving device no longer senses the remote triggering device, 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. There are limitations inherent in the standard signal preemption systems, which rely on line-of-sight communication between the emergency vehicle and the receiver. This leads to some intersections unnecessarily being impacted, and conversely other intersections not receiving the signal in time to clear the intersection for the approaching emergency vehicle. Short-term solutions to these issues have been addressed by the relatively remote placement of receivers (on a pole above the traffic signal, or around a corner that approaches an intersection, for example), and also with physical additions to receiver designs that are less likely to pick up signals unless they are originating from a location certain to be in the path approaching the intersection covered. Research into dynamic route clearance has been made to solve these problems and improve traffic preemption with centrally-based route command and control.

A common implementation of signal preemption systems is a method of communicating to the emergency vehicle operator as well as civilian drivers that a traffic signal is under control of a preemption device. Known as a notifier, this device is almost always an additional light located near the traffic signals. It may be a single light bulb visible to all, which flashes or stays on, or it may be a more sophisticated system such as that available with some systems, where a spotlight is aimed in each direction. The spotlight will either flash or stay on, which communicates 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 emergency vehicle drivers if they have control of the light (especially important when more than one emergency vehicle approaches the same intersection). Typically, a flashing notifier indicates that an emergency vehicle is approaching from ahead or behind, while a solid light indicates the emergency vehicle is approaching laterally.

Often this can be confusing to the motoring public which may not understand its purpose or significance, and requires further education of drivers to react properly. A more prudent approach seems to be notification directly back to the driver of the emergency vehicle in the cab of the vehicle itself, as in the instance of the “EliminatorTM” described below. There are limitations inherent in these standard optical signal preemption systems, which rely on line-of-sight communication between the emergency vehicle and the receiver. Their reliability can be compromised by anything which visually obstructs the emitter’s ability to communicate data to the receiver. Examples of commonly encountered obstacles include large vehicles such as busses, semi-tractors, curves in the roadway, overhanging foliage, bridges, and even fog. In recent years, GPS systems have been introduced to overcome some of these line-of-sight limitations. However they are susceptible to a totally different host of limitations. GPS systems must be “taught” where preemption is desired and require satellite triangulation for the emergency vehicle to “know where it is”. Users of vehicle based GPS navigation systems readily realize that immediate satellite acquision is not always possible. This is all too noticeable in large metropolitan areas where one or more skyscrapers may hinder immediate satellite acquisition. Users of satellite TV also know how satellite reception can be very susceptible to atmospheric phenomena. In these GPS systems, when reliable preemption is required, immediate satellite acquisition is mandatory. However the Eliminator“TM” by Collision Control Communications, Inc. does not have these limitations, and also gives advance warnings of collisions with similarly equipped emergency vehicles; find out more here: Eliminator for Collision Avoidance and Traffic Signal Preemption In one recent Oregon incident (2005) a fire engine pre-empted a signal at a light rail crossing, and proceeded to collide with a light-rail train. A subsequent inquiry determined that the light-rail driver was at fault, falsely believing that once the LRT had obtained the right-of-way across an intersection, it could not be lost until the train had cleared the intersection. Normally, this was the case, but pre-emption by an emergency vehicle was an exception to the rule.

An unusual system prevails in the much of the US state of Washington, where small warning lights mounted on the signal mast illuminate to warn of oncoming emergency vehicles. These lights range from red or white rotating lights or strobes to a simple, enclosed red or white bulb. Since the standard color and shape varies from one locale to another, they are routinely ignored or misunderstood by visitors unfamiliar with the system.

Railroad pre-emption

Another type of preemption is railroad preemption. Traffic-signal-controlled intersections next to railroad crossings on one of 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 (or flashing yellow) to allow traffic on the tracks to clear (in some cases, there are auxiliary traffic signals prior to the railroad crossing which will turn red, keeping new traffic from crossing the tracks. This is in addition to the flashing lights on the crossing gates themselves). After enough time to clear the crossing, the signal will turn. The crossing lights may begin flashing and the gates lower immediately, or this might be delayed until after the traffic light turns red.

The operation of a traffic signal while a train is present may differ from municipality to municipality. 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 green light for the duration of the train while the other directions face a red light for the duration of the train. Examples include the following:

  • The Chicago Drive/Ivanrest Avenue intersection in Grandville, Michigan, gives Chicago Drive traffic (parallel to the tracks) a flashing yellow with fiber-optic lit signs indicating "no right turn" or "no left turn" over the tracks, and Ivanrest traffic faces a solid red light. Similar fiber-optic or LED "no left turn" lights are used along 12th Avenue in Salem, Oregon.
  • The same thing is done to three traffic intersections on Telegraph Rd. between the Ohio state line and Monroe, Michigan. Two of these also include Right turn signals which are solid red when the Telegraph lights are blinking yellow.
  • In Goshen, Indiana, the signals at the intersections on Lincolnway will run normally, with the exception that oncoming traffic (across from the railroad crossing) will face "doghouse" signals with left and right arrows lit: all traffic is required to turn left or right if a train is present, to keep traffic moving.
  • At the intersection of Allen Rd. and Northline Rd. on the border of Southgate and Taylor, Michigan, a railroad track runs diagonally through the intersection from the northeast corner to the southwest. When the lights come on and the arm goes down, all lights turn red and two fiber-optic "no turn on red" signs light up facing Northline, since their right turners cross the tracks. A similar situation is found at the intersection of Oregon Route 10 (Farmington Road) and Lombard Street in Beaverton, Oregon.
  • The Middle Tennessee Blvd./ Church St. intersection in Murfreesboro, Tennessee, gives Church Street (parallel to the tracks) a green with fiber-optic lit signs indicating "no right turn" or "no left turn" over the tracks, and Middle Tennessee Blvd. traffic faces a solid red light.

Unauthorised pre-emption

There have been some concerns that unauthorised people may have obtained devices that can trigger light preemption. The original 3M Opticom pre-emption system was activated by a 14 Hz strobe light added to the light bar of fire trucks, ambulances, and squad cars. When the sensor senses the 14 Hz strobe signal, the pre-emption is activated. The 14 Hz "secret" was eventually discovered, and MIRTs (Mobile InfraRed Transmitters) hit the market, consisting of a 14 Hz strobe with an infrared filter installed on it to make the light invisible to the naked eye. The use or sale of such devices in an unauthorised context was made illegal in the United States in 2005.

In some jurisdictions, traffic lights are set to turn red in all directions when the pre-emption system is activated, rather than holding one direction green to allow an emergency vehicle to proceed with traffic. This stops all traffic except for emergency vehicles, which are permitted to proceed through a red signal anyway, and thus removes much of the incentive for an unauthorised person to manipulate the pre-emption system to their own benefit. However, this can cause a disadvantage for the emergency vehicle because cars in front of it will stop at the intersection, blocking its path.

3M has developed an encrypted Opticom system. However, jurisdictions already using the original system would have to replace the original traffic signal sensors and vehicle-mounted emitters in order to use the encrypted system.

There have been recent concerns about the security of traffic light preemptive systems and the actual underlying network controlling them and traffic lights in general. An article in the hacker E-zine Phrack has outlined flaws in the traffic controlling system that could allow an unauthorised malicious person to abuse it as he sees fit. By issuing valid signal controlling messages from the area traffic control center if access is gained to it, an attacker could essentially control any phase, test phase, preemptive signals, or any function of the traffic system that is controllable remotely. The article also sparked a response by Transport for London where it is reported that a skilled attacker armed with this “step-by-step” guide could in fact cause malicious damage, as reported by a Transport for London spokesman.

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