Traffic signal preemption
Encyclopedia
Traffic signal preemption is a type of system that allows the normal operation of traffic light
s to be preempt
ed, often to assist emergency vehicles. The most common use of these systems is to manipulate traffic signals in the path of an emergency vehicle
, stopping conflicting traffic and allowing the emergency vehicle right-of-way, to help reduce response time
s and enhance traffic safety. Sometimes, signal preemption is also used at railroad grade crossings to prevent collision
s, and by light-rail
and bus rapid transit
systems to allow public transportation priority access through intersections to ensure they remain on schedule and improve commute
time
s.
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, but in the case of emergency vehicles they are frequently integrated with the vehicle's emergency warning lights. When activated, the traffic premption 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.
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 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 there may be a light aimed towards each direction that traffic approaches the intersection from. 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 flashing notifier to indicate that an activating vehicle is approaching from ahead or behind, while a solid notifier would indicate the emergency vehicle is approaching laterally. There are variations of notification methods in use, which may include one or more colored lights in varying configurations.
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 LRT operator had become desensitized to the preemption signals, as the LRT was itself granted right-of-way virtually all of the time, but the system was programmed to grant higher priority to fire apparatus over an LRT. 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.
which typically sends a narrowly-directed signal forward, towards traffic light
s in front of the vehicle, to attempt to obtain right-of-way through controllable intersections before arriving at the intersection. These line-of-sight systems generally utilize 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 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.
Drawbacks of line-of-sight systems include obstructions, lighting and atmospheric 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 in front of a police car blocking the traffic signal from receiving the emitter's signal from the police car. Modifying the position of the receiver or even locating it separate from the traffic signal equipment can sometimes correct this problem. 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 occur if an emitter's signal is picked up by many traffic lights along a stretch of road, all directed to change to green in that direction, prior to the activating vehicle turning off the road, or being parked without its emitter being deactivated.
(GPS) applications came the introduction of a GPS-based traffic preemption system. These systems require software and a communications platform to determine where the activating vehicle is located, in which direction it is headed, which traffic lights should be preempted, and the ability for the central application to activate the desired traffic lights promptly.
Drawbacks of GPS systems include obstructions, single point of failure
exposure, atmospheric conditions, and GPS satellite availability. In dense cities with tall buildings, GPS receivers may have difficulty obtaining at least three GPS satellite signals, required for triangulation
to determine location. If the primary application is not installed with redundant
hardware, a single failure on the primary system controller can disable all traffic preemption functions within the entire traffic network covered by the GPS-based system. Extremely heavy cloud cover or severe weather can also adversely impact the ability of the GPS receiver from obtaining the three required satellites. Additionally, recent concerns about the health of the GPS satellite network raise the possibility that the reliability of the GPS satellite system may degrade in the future.
Radio-based systems can offer some additional benefits: adjustable range and collision avoidance. The operating range can be adjusted by varying the radio signal strength so that traffic lights are activated only nearby, or at greater distances. The hardware utilized by radio-based systems and installed on a vehicle is also capable of interacting with other equipped vehicles, primarily for the purpose of providing collision avoidance warnings when two or more vehicles approach each other while operating their preemption systems.
Compared to GPS systems, the cost difference may or may not be significant, as the single expense of the GPS primary application hardware and software comprises the bulk of the expense, which decreases per vehicle as more vehicles are added to the system.
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:
Traffic light
Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic...
s to be preempt
Preempt
Preempt is a bid in contract bridge whose primary objectives are to thwart opponents ability to bid to their best contract, with some safety, and to fully describe one's hand to one's partner in a single bid. A preemptive bid is usually made by jumping, i.e. skipping one or more bidding levels...
ed, often to assist emergency vehicles. The most common use of these systems is to manipulate traffic signals in the path of an emergency vehicle
Emergency vehicle
An emergency vehicle is any vehicle that is designated and authorized to respond to an emergency. These vehicles are usually operated by designated agencies, often part of the government, but also run by charities, non-governmental organizations and some commercial companies...
, stopping conflicting traffic and allowing the emergency vehicle right-of-way, to help reduce response time
Response time
In technology, response time is the time a system or functional unit takes to react to a given input.- Data processing :In data processing, the response time perceived by the end user is the interval between the instant at which an operator at a terminal enters a request for a response from a...
s and enhance traffic safety. Sometimes, signal preemption is also used at railroad grade crossings to prevent collision
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
s, and by light-rail
Light rail
Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...
and bus rapid transit
Bus rapid transit
Bus rapid transit is a term applied to a variety of public transportation systems using buses to provide faster, more efficient service than an ordinary bus line. Often this is achieved by making improvements to existing infrastructure, vehicles and scheduling...
systems to allow public transportation priority access through intersections to ensure they remain on schedule and improve commute
Commute
Commute, commutation or commutative may refer to:* Commuting, the process of travelling between a place of residence and a place of work* Commutative property, a property of a mathematical operation...
time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
s.
Implementation
Traffic preemption devices are implemented in a variety of ways. They can be installed on road vehicles, integrated with trainTrain
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
transportation network management systems, or operated by remote control from a fixed location, such as a fire station
Fire station
A fire station is a structure or other area set aside for storage of firefighting apparatus , personal protective equipment, fire hose, fire extinguishers, and other fire extinguishing equipment...
, or by a 9-1-1
9-1-1
9-1-1 is the emergency telephone number for the North American Numbering Plan .It is one of eight N11 codes.The use of this number is for emergency circumstances only, and to use it for any other purpose can be a crime.-History:In the earliest days of telephone technology, prior to the...
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, but in the case of emergency vehicles they are frequently integrated with the vehicle's emergency warning lights. When activated, the traffic premption 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.
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 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 there may be a light aimed towards each direction that traffic approaches the intersection from. 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 flashing notifier to indicate that an activating vehicle is approaching from ahead or behind, while a solid notifier would indicate the emergency vehicle is approaching laterally. There are variations of notification methods in use, which may include one or more colored lights in varying configurations.
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
Light rail
Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...
train (LRT) crossing in Hillsboro, Oregon
Hillsboro, Oregon
Hillsboro is the fifth-largest city in the U.S. state of Oregon and is the county seat of Washington County. Lying in the Tualatin Valley on the west side of the Portland metropolitan area, the city is home to many high-technology companies, such as Intel, that compose what has become known as the...
, yet the fire engine was hit by an LRT at the crossing. A subsequent inquiry determined that the LRT operator was at fault. The LRT operator had become desensitized to the preemption signals, as the LRT was itself granted right-of-way virtually all of the time, but the system was programmed to grant higher priority to fire apparatus over an LRT. 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.
Acoustic
Some systems use an acoustic sensor linked to the preemption system. This can be used by itself 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 picked up. 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 along side 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.Line-of-Sight
A vehicle that utilizes a line-of-sight traffic signal preemption system is equipped with an emitterEmitter
-In general:*A device used to exude any signal, beacon, light, odor, liquid, fragrance, ionizing particles or any other type of signal.-In horticulture:*A device used in drip irrigation.-In electronics and instrument physics:...
which typically sends a narrowly-directed signal forward, towards traffic light
Traffic light
Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic...
s in front of the vehicle, to attempt to obtain right-of-way through controllable intersections before arriving at the intersection. These line-of-sight systems generally utilize an invisible infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
signal, or a visible strobe light
Strobe light
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...
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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. Traffic lights must be equipped with a compatible traffic signal preemption receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
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.
Global Positioning System
With the advent of widespread Global Positioning SystemGlobal Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
(GPS) applications came the introduction of a GPS-based traffic preemption system. These systems require software and a communications platform to determine where the activating vehicle is located, in which direction it is headed, which traffic lights should be preempted, and the ability for the central application to activate the desired traffic lights promptly.
Drawbacks of GPS systems include obstructions, single point of failure
Single point of failure
A single point of failure is a part of a system that, if it fails, will stop the entire system from working. They are undesirable in any system with a goal of high availability or reliability, be it a business practice, software application, or other industrial system.-Overview:Systems can be made...
exposure, atmospheric conditions, and GPS satellite availability. In dense cities with tall buildings, GPS receivers may have difficulty obtaining at least three GPS satellite signals, required for triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
to determine location. If the primary application is not installed with redundant
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
hardware, a single failure on the primary system controller can disable all traffic preemption functions within the entire traffic network covered by the GPS-based system. Extremely heavy cloud cover or severe weather can also adversely impact the ability of the GPS receiver from obtaining the three required satellites. Additionally, recent concerns about the health of the GPS satellite network raise the possibility that the reliability of the GPS satellite system may degrade in the future.
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 as well as GPS systems. A radio-based system still utilizes a directional signal transmitted from an emitter, but being radio-based, its signal is not blocked by visual obstructions, lighting or weather conditions.Radio-based systems can offer some additional benefits: adjustable range and collision avoidance. The operating range can be adjusted by varying the radio signal strength so that traffic lights are activated only nearby, or at greater distances. The hardware utilized by radio-based systems and installed on a vehicle is also capable of interacting with other equipped vehicles, primarily for the purpose of providing collision avoidance warnings when two or more vehicles approach each other while operating their preemption systems.
Compared to GPS systems, the cost difference may or may not be significant, as the single expense of the GPS primary application hardware and software comprises the bulk of the expense, which decreases per vehicle as more vehicles are added to the system.
Railroad preemption
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 SalemSalem, OregonSalem is the capital of the U.S. state of Oregon, and the county seat of Marion County. It is located in the center of the Willamette Valley alongside the Willamette River, which runs north through the city. The river forms the boundary between Marion and Polk counties, and the city neighborhood...
, OregonOregonOregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
. - The same thing is done to three traffic intersections on Telegraph Rd. between the OhioOhioOhio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
state line and Monroe, MichiganMonroe, MichiganMonroe is a city in the U.S. state of Michigan. The population was 20,733 at the 2010 census. It is the largest city and county seat of Monroe County. The city is bordered on the south by Monroe Charter Township, but both are politically independent. The city is located approximately 14 miles ...
. Two of these also include Right turn signals which are solid red when the Telegraph lights are blinking yellow. - In Goshen, IndianaGoshen, IndianaGoshen is a city in and the county seat of Elkhart County, Indiana, United States. It is the smaller of the two principal cities of the Elkhart-Goshen Metropolitan Statistical Area, which in turn is part of the South Bend-Elkhart-Mishawaka Combined Statistical Area. It is located in the northern...
, 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 SouthgateSouthgate, MichiganSouthgate is a city in Wayne County in the U.S. state of Michigan. The population was 30,047 at the 2010 census.Southgate was the last city to incorporate from the former Ecorse Township, gaining city status in October 1958...
and TaylorTaylor, MichiganTaylor is a city in Wayne County in the U.S. state of Michigan. The population was 63,131 at the 2010 census. Taylor was originally known as Taylor Township and residents of the township voted to incorporate as the City of Taylor in May 1968...
, MichiganMichiganMichigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....
, 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 illuminate 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, OregonBeaverton, OregonBeaverton is a city in Washington County, Oregon, United States, seven miles west of Portland in the Tualatin River Valley.As of the 2010 census, the population is 90,267. This makes it the second-largest city in the county and Oregon's sixth-largest city...
. - 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.
External links
- Why is Traffic Signal Preemption So Important?
- What Do Public Safety Officials Say About Preemption?
- Frequently Asked Questions(FAQ) About Traffic Signal Preemption
- Guide for Traffic Signal Preemption near Railroad Grade Crossing (PDF) (from the Texas Transportation InstituteTexas Transportation InstituteThe Texas Transportation Institute in College Station, Texas is the largest transportation research agency in the United States. Created in 1950, primarily in response to the needs of the Texas Highway Department , TTI has since broadened its focus to address all modes of transportation–highway,...
) - Section 4D.13 of the U.S. Manual on Uniform Traffic Control DevicesManual on Uniform Traffic Control DevicesThe Manual on Uniform Traffic Control Devices is a document issued by the Federal Highway Administration of the United States Department of Transportation to specify the standards by which traffic signs, road surface markings, and signals are designed, installed, and used...
- University of MN Intelligent Transportation Systems Institute report on Dynamic route clearance
- "Hackers target traffic lights" at TMCnet