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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-06-139
Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II


AC (also written ac): Alternating Current. See definition of alternating current.

ACTIVE SENSOR: A sensor that transmits energy, a portion of which is reflected or scattered from a vehicle or other objects and surfaces in its detection zone back toward the receiving aperture of the sensor.

ACTUATED CONTROLLER: A traffic signal controller that receives information from vehicle and/or pedestrian sensors and provides appropriate signal timing.

ACTUATION: The operation of any type of sensor (NEMA). The word operation means an output from the sensor system to the controller.

ADVISORY SENSOR: A device that detects vehicles on one or more intersection approaches solely for the purpose of modifying the phase sequence and/or length for other approaches to the intersection (NEMA).

ALTERNATING CURRENT (ac): A current that reverses its magnitude and direction of flow at regular intervals. The rate of reversal is expressed in hertz (cycles per second).

AMPERE: The unit expressing the rate of flow of electrons through a conductor. One ampere is the current flowing through a 1-ohm resistance with 1-volt pressure.

AMPLIFIER: See electronics unit.

ANALOG: An electronic design that uses continuously varying voltages, rather than discrete digital values.

ANSI: American National Standards Institute.

ANTENNA: The radiating or receiving elements utilized in transmitting or receiving electromagnetic waves (NEMA).

AREA DETECTION: The continuous detection of vehicles over a length of roadway wherein the call of a vehicle in the detection area is intended to be held for as long as the vehicle remains in the area of detection. (Most sensors cannot hold the call indefinitely.) Sensors used for this are frequently referred to as large-area detectors, long-loop detectors, or presence detectors.

AREA OF DETECTION: See Detection Zone.

AREA DETECTOR: See Large Area Detector.

ARTERIAL: A major urban roadway usually having coordinated signals along its length.

ASTM: American Society for Testing and Materials.

ATC: Advanced Transportation Controller.

AUXILIARY EQUIPMENT: Separate devices used to add supplementary features to a controller assembly (NEMA).

AWG: American Wire Gauge. The standard measurement of wire size based on the circular mil system (1 mil equals 0.001 inch).

CABLE: A group of separately insulated conductors wrapped together and covered with an outer jacket.

CALL: A registration of a demand for right-of-way by traffic at a controller unit (NEMA). The call comes to the controller from a traffic flow sensor that is outputting an actuation.

CALLING DETECTOR (CALLING SENSOR): A detector (sensor) that is installed in a selected location to detect vehicles which may not otherwise be detected and whose output may be modified by the controller unit (NEMA). This traditionally has meant a small area detector near the Stop Line to detect vehicles entering the roadway from a driveway during the red or yellow signal. The detector is disconnected when the green signal is displayed so that extensions of the green can only come from the appropriate Extension Detector.

CAPACITANCE: That property of dielectric materials that permits the storage of electrically separated charges when potential differences exist between the surfaces of the dielectric. Capacitance is expressed as the ratio of an electric charge to a potential difference.

CARD-RACK MOUNTED ELECTRONICS UNIT: See Rack Mounted Electronics Unit.

CARRYOVER OUTPUT: The ability of a sensor to continue its output for a predetermined length of time following an actuation (NEMA). See Extended Call Detector.

CHAMFER: Diagonal saw slots at the corners of square or rectangular inductive loops to reduce the angle of bend of the loop wires at the intersecting saw slots.

CHANGE INTERVAL: The yellow interval following the green signal indicating the change to a conflicting phase.

CHANNEL: Electronic circuitry that processes information from an inductive-loop detector or other sensor.

CIRCUIT: A closed path followed by an electric current.

CLEARANCE INTERVAL: See Red Clearance Interval.

COIL: A coiled conductor, wound on a form or core, which uses electromagnetic induction to cause changes in a current.

CONDUCTANCE: The measure of ability to conduct electricity expressed in units of inverse ohms-meter.

CONDUCTOR: A medium for transmitting electrical current. A conductor usually consists of copper or other electrically conductive materials.

CONDUIT: A tube for protecting electrical wires or cables.

CONFLICTING CALL: See Serviceable Conflicting Call.

CONFLICTING PHASES: Two or more traffic phases which will cause interfering (i.e., conflicting) traffic movements if operated concurrently.

CONTINUOUS PRESENCE MODE: Detector (sensor) output continues if any vehicle (first or last remaining) remains in the field of influence (NEMA). This definition does not imply that the use of this mode guarantees that the output will continue indefinitely, as most detectors are incapable of holding a call beyond a finite interval. See Detector Mode.

CONTROLLED OUTPUT: The ability of a detector (sensor) to produce a pulse that has a predetermined duration regardless of the length of time a vehicle is in the field of influence (NEMA). See Detector Mode.

CONTROLLER ASSEMBLY: A complete electrical mechanism mounted in a cabinet for controlling the operation of a traffic control signal (NEMA).

  1. Traffic-Actuated Controller Assembly: A controller assembly for supervising the operation of traffic control signals in accordance with the varying demands of traffic as registered with the controller by detectors (NEMA).
  2. Semi-Traffic-Actuated Controller Assembly: A type of traffic actuated controller assembly in which means are provided for traffic actuation on one or more, but not all approaches to the intersection (NEMA).
  3. Full-Traffic-Actuated Controller Assembly: A type of traffic actuated controller assembly in which means are provided for traffic actuation on all approaches to the intersection (NEMA).
  4. Pedestrian-Actuated Controller Assembly: A controller assembly in which intervals, such as pedestrian WALK and clearance intervals, can be added to or included in the controller cycle by the actuation of a pedestrian detector (NEMA). Pedestrian actuation may be part of an intersection controller or be used to control a midblock crosswalk.

CONTROLLER UNIT: A controller unit is that potion of a controller assembly that is devoted to the selection and timing of signal displays (NEMA). Also referred to as the Dispatcher or Timer. Another controller class (i.e., Type 170, 179, and 2070) standardizes hardware modules and uses specialized software to implement traffic management functions.

CRITICAL INTERSECTION CONTROL (CIC): An algorithm employed in traffic systems to dynamically control the split at signalized locations where the traffic patterns are such that special control, responsive to changing conditions, is needed.

CRITICAL LANE DETECTION: A system of hardware and software designed to provide data on traffic flow for a selected lane, usually the heaviest volume lane on an approach to a signalized intersection.

CROSSTALK: The adverse interaction of any channel of a sensor or sensor electronics unit with any other channel in that or another device. Crosstalk can occur via mutual coupling of magnetic fields in nearby inductive loops. The effect is an interaction between two or more electronics units in the same cabinet when the units operate at the same or nearby frequencies. Crosstalk results in a sensor output actuation in the absence of a vehicle.

CYCLE: A complete sequence of all signal indications at an intersection. In an actuated controller, a complete cycle is dependent on the presence of sensor calls on all phases.

CYCLE LENGTH: The time period in seconds required for a complete cycle. Cycle length is normally variable for actuated intersections unless they are part of a coordinated system.

DELAYED CALL DETECTOR (ELECTRONICS UNIT): A detector (electronics unit) that does not issue an output until the detection zone has been occupied for a period of time that exceeds the time preset on an adjustable timer incorporated into the electronics unit.

DELAYED OUTPUT: The ability of an electronics unit to delay its output for a predetermined length of time during an extended actuation (NEMA).

DELTA L (ΔL): Change in inductance.

DEMAND: The request for service, e.g., one or more vehicles desiring to use a given segment of roadway during a specified unit of time.

DEMAND CONTROL: See Loop Occupancy Control.

DEMAND OPERATION: A mode of operation whereby the service provided at an intersection reflects the presence of demand for that service without regard to background cycles.

DENSITY: A measure of the concentration of vehicles, stated as the number of vehicles per mile per lane.

DENSITY CONTROLLER: Actuated controller that has timing adjustments for the selection of the allowable gap independent of the passage time. A volume-density controller and a modified density controller are each a type of density controller.

DESIGN SPEED: The speed used as typical by the designer of the sensor/- controller system in the kinematic analysis of the scheme under free traffic flow conditions.

DETECTION ZONE: That area of the roadway within which a vehicle will be detected by a vehicle sensor (NEMA). Also called zone of detection, sensing zone, area of detection, detection area, effective loop area, field of influence, field of view, or footprint.

DETECTOR or SENSOR: A device for indicating the presence or passage of automotive vehicles, light and heavy rail, buses, or pedestrians. This general term is usually supplemented with a modifier indicating type (e.g., inductive-loop detector, magnetic detector, radar sensor, and video image processor); operation (e.g., point detector or sensor, presence detector or sensor, and large area sensor); or function (e.g., calling detector and extension detector). See also Traffic Detector or Sensor.

DETECTOR AMPLIFIER: See Electronics Unit.

DETECTOR (ELECRONICS UNIT) FAILURES: The occurrence of detector malfunctions including nonoperation, chattering, or other intermittently erroneous detections.

DETECTOR (ELECTRONICS UNIT) MEMORY: The retention of an actuation for future utilization by the controller assembly. The phrase might better be detection memory to make it clearer that the memory is within the controller, not the sensor.

DETECTOR MODE: See Electronics Unit Mode.

DETECTOR SETBACK: See Sensor Setback.

DETECTOR SYSTEM: See Sensor System.

DIELECTRIC: Any insulating material that is a nonconductor of electricity.

DILEMMA ZONE: A distance or time interval related to the onset of the yellow interval. Originally the term was used to describe that portion of the roadway in advance of the intersection within which a driver can neither stop prior to the stop line nor clear the intersection before conflicting traffic is released. That usage pertained to insufficient length of timing of the yellow and/or all-red intervals. More recently the term has been used to describe that portion of the roadway in advance of the intersection within which a driver is indecisive regarding stopping prior to the stop line or proceeding into or through the intersection. May also be expressed as the increment of time corresponding to the dilemma zone distance.

DILEMMA-ZONE PROTECTION: Any method or procedure that attempts to control the end of the green interval so that no vehicle will be caught in the dilemma zone when the signal turns yellow.

DIRECT CURRENT (dc): An electrical current that travels in one direction.

DISPATCHER: Obsolete Name. See Controller Unit

DRIFT: Change in the electrical properties of the sensor system or a portion of it due to environmental changes, particularly temperature variations and rain water.

EDDY CURRENT: An electric current induced within the body of a conductor when that conductor moves through a time-varying magnetic field.

EFFECTIVE LOOP AREA: See Detection Zone.

EIA: Electronic Industries Association.

ELECTRONICS UNIT: An electronic device that energizes the wire loop(s), monitors the loop(s) inductance by filtering and amplifying the signals it receives, and responds to a predetermined decrease in inductance with an output that indicates the passage or presence of vehicles in the detection zone. It is the electronics element in a loop detector system, i.e., it does not include the loop(s), lead-in wire, or lead-in cable. Sometimes called an amplifier or detector, although it performs other functions as well, e.g., sensitivity adjustment, failure indication, and delayed actuation. Electronics units are also used with magnetic detectors and magnetometers.

ELECTRONICS UNIT MODE: A term used to describe the operation of an electronics unit channel output when a presence (or passage) detection occurs (NEMA). Also referred to as Detector Mode. See pulse mode, controlled output, continuous presence mode, and limited presence mode.

ENCAPSULATION: The process of filling the saw slot with sealant to surround the wires in the slot and protect them from traffic, weather, and debris.

ENCASEMENT: Material used to enclose the loop wire for protection, such as a polyvinyl or polyethylene tube. Often referred to as Detect-a-duct or other similar commercial names.

EPOXY: A resin used in bonding.

EXTENDED CALL DETECTOR (ELECTRONICS UNIT): When selected, this detector extends the output (See Carryover Output) after the vehicle departs the detection zone for a preset time.

EXTENDED CALL TIMING: An electronics unit with carryover output. It holds or stretches the call of a vehicle for a period of seconds that has been set on an adjustable timer incorporated into the electronics unit. It can be designed to begin the timing of that period when the vehicle enters the detection area, or when it leaves. The latter is specified by NEMA. Also referred to as a Stretch Detector.

EXTENSION DETECTOR (ELECTRONICS UNIT): An electronics unit that is configured to register actuations at the controller only during the green interval for a given approach, so as to extend the green time for that approach to accommodate the actuating vehicles. It is not active during the red or yellow intervals for that approach.

EXTENSION LIMIT: The maximum length of time that the actuations on any traffic phase may retain the right-of-way after an actuation on an opposing traffic phase. Also known as maximum green.

FAILSAFE (as in output relay design): A type of output-relay design that produces a constant call, thereby keeping traffic moving in the event that the electronics unit fails.

FARAD: A unit of capacitance, usually expressed in microfarads (µF) (one millionth of a farad).

FEEDER CABLE: See lead-in cable.

FIELD OF INFLUENCE: See Detection Zone.

FIXED TIMING PLAN: Timing plan in which the cycle length, split, offset, and actuated settings are not adjusted between switching of timing plans. These are used in TOD/DOW and traffic adjusted systems.

FLEXIBLE TIMING PLAN: A dynamic timing plan in which the cycle length, split, offset, and actuated settings may be adjusted once per cycle between switching of timing plans. These are used in Traffic Responsive Systems such as SCOOT.

FMCW: Frequency modulated, continuous wave. A waveform Capable of detecting presence and measuring the relative speed of vehicles within the range measurement interval.

FREEWAY SURVEILLANCE: Process or method of monitoring freeway traffic performance and control system operation.

FREQUENCY: The number of times an alternating current repeats its cycle in 1 second.

FULL TRAFFIC-ACTUATED CONTROLLER ASSEMBLY: A controller assembly for supervising the operation of traffic control signals in which means are provided for traffic actuation on all approaches to the intersection.

GAP: The time interval between the end of one vehicle sensor actuation and the beginning of the next actuation.

GAP OUT: Terminating of a green phase due to an excessive time interval between the actuations of vehicles arriving on the green, allowing the green to be served to a competing phase.

GAP REDUCTION: Å feature whereby the unit extension or allowed time spacing between successive vehicle actuations on the phase displaying the green in the extensible portion of the interval is reduced (NEMA).

GREEN EXTENSION SYSTEM: Hardware assembly of extended call electronics units and auxiliary logic. The logic can monitor the signal display, enable or disable the selected extended call sensors, and hold the controller in artery green.

GROUND: The earth and all parts conductively connected to the earth.

HAND HOLE: See Pull Box.

HENRY (H): The measure of inductance, defined as the inductance of a circuit in which a counter electromotive force of one volt is generated when the current is changing at the rate of one ampere per second.

HERTZ (Hz): A term replacing cycles per second as the measurement unit for frequency.

HOLD: A command that retains the existing right-of-way. A command to the controller, which causes it to retain the existing right-of-way. A momentary release of the hold command allows the controller to yield to other conflicting phases requesting service (often referred to as the yield command or yield point).

HOLD ON LINE: A connection that modifies a controller from fully actuated, isolated operation to semiactuated, system-controlled operation. It is used as the basic tie from the local intersection to the system master.

HOME-RUN CABLE: See Lead-In Cable.

HONDA 100 cc MOTORCYCLE: A small motorcycle commonly considered to be the smallest registered motor vehicle. It is used as the baseline for motorized vehicle detection.

µH (MICROHENRY): A measure of inductance. See Henry and Inductance.

IMSA: International Municipal Signal Association.

INDUCTANCE: That property of an electric circuit or of two neighboring circuits, whereby an electromotive force is generated in one circuit by a change of current in itself or in the other circuit. The ratio of the electromotive force to the rate of change of the current. Measured in microhenrys (µH).


INDUCTIVE REACTANCE: The reactance (ohms) of an ideal (lossless) inductor is the product of the voltage across the inductor and the sine of the phase angle (0 to 90 degrees) between inductor voltage and current divided by the inductor current assuming sinusoidal excitation.

INFRARED SENSOR: See passive infrared sensor.

INITIAL PORTION: The first timed portion actuated controller unit:

  1. Fixed initial portion: A preset initial portion that does not change.
  2. Computed initial portion: An initial portion that is traffic adjusted.
  3. Maximum initial portion: The limit of the computed initial portion.
  4. Minimum initial portion: See fixed initial portion.
  5. Added initial portion: An increment of time added to the minimum initial portion in response to vehicle actuations.

IN-ROADWAY SENSOR: A sensor that is embedded in the pavement of the roadway, embedded in the subgrade of the roadway, or taped or otherwise attached to the surface of the roadway.

INTERCONNECTED CONTROL: Control systems in which basic coordination is provided by wireline or wireless interconnect. This information is used to determine the local signal timing of offset and cycle/split or actuated timing based on the Time-of-Day/Day-of-Week. The operator can select timing and download timing plans and changes and monitor system status. There are no system detectors in Interconnected Control systems.

INTERVAL: The part of parts of the signal cycle during which signal indications do not change (NEMA).

ISOLATED INTERSECTION CONTROL: The form of signal control for a single signalized intersection that does not have control relationships between intersections nor any data flow between intersections. The system of intersections is not responsive to traffic in real time. The individual isolated intersection may have local actuated control or Time of Day/Day of Week changes based on historical traffic flows. Also called uncoordinated signal control. See also totally isolated systems.

ITE: Institute of Transportation Engineers.



KILOHERTZ (kHz): Thousands of hertz. A measure of frequency.

L: Symbol for inductance.

ΔL: Symbol for change in inductance.

LARGE AREA SENSOR: One or more sensors or combination of sensors connected together in series, parallel, or series/parallel covering an area in the approach to an intersection or extended freeway segment. Large area sensors that monitor intersection approaches have detection areas that vary from 6 x 40 ft (1.8 x 12 m) to 6 x 100 ft (1.8 x 30 m) or larger. A common configuration is four 6- x 6-ft (1.8- x 1.8-m) loops spaced 10 ft (3 m) apart for a length of 54 ft (16.5 m). Also known as large area detector, area detector, or area sensor.

LASER RADAR: An active sensor that transmits energy usually in the near infrared spectrum for traffic management applications. Single and twolane models are manufactured.

LAST CAR PASSAGE: A selected feature of a density controller that, upon gap-out, will cause the green to complete the timing of the passage time. The last vehicle to have been detected, known as the Last Car, will therefore retain the green until it reaches the stop line. Thus, it is assured of avoiding the dilemma zone problem and of clearing the intersection. This feature is not defined by NEMA; however, a number of manufacturers provide it.

LEAD-IN CABLE: The electrical cable that is spliced to the lead-in wire in the pull box and connects to the input of the electronics unit in the controller cabinet (NEMA). Sometimes called home-run cable, transmission line, or feeder cable.

LEAD-IN WIRE: That portion of the inductive-loop wire between the physical edge of the loop and the pull box, which should be twisted a specified number of turns per foot. The lead-in wire is located in the saw slot connecting the loop to the edge of the roadway. From there, it is placed in conduit to the controller or to a pull box, where it is spliced to the lead-in cable. For a magnetic detector and magnetometer, it is the wire that runs from the sensor (probe) to the pull box.

LIGHT-SENSITIVE VEHICLE SENSOR: A sensor consisting of a light source and photoelectric cell or cells, capable of being actuated by the passage of a vehicle interrupting the beam or beams of light. When properly equipped, directional characteristics are present.

LIMITED PRESENCE MODE: Electronics unit output continues for a limited period of time if vehicles remain in the field of influence. See electronics unit mode.

LINK: The length of roadway between two signalized locations.

LIP: See local intersection program.

LOCAL CONTROLLER: A controller supervising the operating of traffic signals at a single intersection. Also see controller assembly and controller unit.

LOCAL INTERSECTION PROGRAM (LIP): A Type 170 software program developed by Caltrans that incorporates electronics unit timing features within the program.

LOCKING DETECTION MEMORY: A selectable feature of the circuit design for a controller phase, whereby the call of a vehicle arriving on the red (or yellow) is remembered or held by the controller after the vehicle leaves the detection area until it has been satisfied by the display of a green interval to that phase.

LOOP DETECTOR: An active sensor composed, in part, of one or more loops of wire whose inductance decreases in response to the passage or presence of a vehicle in the detection zone of the loops. See also Loop Detector System.

  1. Motion loop detector, nondirectional: An inductive-loop detector that produces a signal when a vehicle passes over any portion of the loop.
  2. Motion loop detector, directional: An inductive-loop detector consisting of two separate loops either closely spaced or partially overlapping that is not affected by the passage of a vehicle except in the desired direction.
  3. Presence loop detector: An inductive-loop detector that is capable of detecting the presence of a standing or moving vehicle in any portion of the effective loop area.

LOOP DETECTOR SYSTEM: A vehicle detection system that senses a decrease in inductance of the wire loop(s) during the passage or presence of a vehicle in the detection zone of the loop(s) (NEMA). Has the same meaning as loop detector, but is clearer in its inclusion of the wire loop(s), lead-in wire, lead-in cable, and the electronics unit.

LOOP ELECTRONICS UNIT: See Electronics Unit.

LOOP OCCUPANCY CONTROL: A detector/controller design using long detection loop(s) [normally 30 ft (9 m) or longer], and a controller operated in the nonlocking mode. A loop occupancy controller may, but need not necessarily, be designed to rest in all red in the absence of any traffic demand. Loop occupancy control can utilize magnetometer and other sensors, as well as inductive-loop detectors.

LOOP SYSTEM: The wire loops and lead-in wire that are connected through lead-in cable to the input terminals of the electronics unit.

LOOP LEAD-IN WIRE: See Lead-In Wire.

LSI CHIP: An electrical component with more than 1,000 logic elements. This large-scale integration of miniature elements is used in some digital loop electronics units.

MAGNETIC DETECTOR: A passive device that senses changes in the Earth’s magnetic field caused by the movement of a ferrous-metal vehicle in or near its detection zone. It is placed under or in the roadway to detect the passage of a vehicle over the sensor. These sensors generally detect only moving vehicles. Also known as induction and search coil magnetometers. Their output is connected to an electronics unit.

  1. Compensated magnetic vehicle detector: Any magnetic detector that is designed and structured to not be affected by changing electromagnetic influence other than that resulting from the passage of a vehicle. When properly equipped, a compensated magnetic detector has strong directional characteristics.
  2. Noncompensated magnetic detector: Any magnetic detector other than a compensated magnetic detector.

MAGNETIC SHADOW: The distortion of the flux lines of the Earth’s magnetic field as they pass through a ferrous vehicle, due to the greater permeability of the vehicle to these flux lines as compared to air.

MAGNETOMETER SENSOR: A passive device that detects the passage or presence of a ferrous metal object through the perturbation (know as a magnetic anomaly or magnetic shadow) it causes in the Earth’s magnetic field. Its output is connected to an electronics unit. Not to be confused with a magnetic detector. Also known as a fluxgate magnetometer. Twoaxis fluxgate magnetometers respond to changes in the vertical and horizontal components of the Earth’s magnetic field.

MAXIMUM GREEN LIMIT: The maximum green time after an opposing actuation, which may start in the initial portion.

MEGGER: A device used by power companies to measure very high resistance to earth ground.

MEGOHM: One million ohms, which is the unit of electrical resistance.

MEMORY OFF: A selectable feature of an actuated controller, synonymous with nonlocking detection memory.

MICROHENRY: One millionth of a henry (µH), from the unit of measurement of inductance.

MICROWAVE RADAR SENSOR: An active vehicle sensor installed above or adjacent (side-mounted) to the roadway that transmits and receives energy in the microwave portion of the electromagnetic spectrum. Two types of microwave radar sensors exist. The Doppler or continuous wave radar is a passage sensor and detects only vehicles moving faster than some minimum speed. The FMCW presence radar detects stopped and moving vehicles. Models are manufactured for single and multilane applications.

MINIMUM GREEN INTERVAL: The shortest green time of a phase. If a time setting control is designated as "minimum green," the green time shall not be less than that setting.

MINIMUM VEHICLE STANDARD: A test unit that produces the minimum change in input for which the sensor system must detect and indicate passage or presence. NEMA specifies a Class 1 vehicle (a small motorcycle).

MOTION SENSOR: A sensor that detects the motion of a vehicle passing through the detection zone of the sensor at some minimum speed, usually 2 to 3 mi/h (3.2 to 4.8 km/h). Vehicles traveling slower than the minimum speed or stopped in the detection zone are not detected.

NANOHENRY: One billionth of a henry, from the unit of measurement of inductance.

NEMA: National Electrical Manufacturers Association.

NETWORK: A series of intersecting arterials or streets that are a part of a coordinated signal system.

  1. Open Network: A network where the arterials do not intersect more than once (i.e., there are no closed loops in the system).
  2. Closed Network: A network that contains closed loops.

NOMINAL INDUCTANCE: A design value of inductance where the actual value can vary from the specified value within a range that permits satisfactory equipment operation.

NONCONFLICTING PHASES: Two or more traffic phases that will not cause interfacing traffic movements if they are operated concurrently.

NON-DIRECTIONAL SENSOR: A sensor capable of being actuated by vehicles proceeding in any direction.

NON-LOCKING MEMORY: A mode of actuated-controller unit operation that does not require memory (NEMA). In this mode of operation, the call of a vehicle arriving on the red (or yellow) is forgotten or dropped by the controller as soon as the vehicle leaves the detection area.

OCCUPANCY: The percent of time a sensor’s detection zone is occupied. Occupancy is a pseudomeasure of density on a roadway.

OHM: The unit of electrical resistance equal to the resistance through which a current of one ampere will flow when there is a potential difference of 1 volt across it.

OVER-ROADWAY SENSOR: A sensor that is mounted above the surface of the roadway either above the roadway itself or alongside the roadway, offset from the nearest traffic lane by some distance.

PASSAGE DETECTION: The ability of a vehicle sensor to detect the passage of a vehicle moving through the detection zone and to ignore the presence of a vehicle stopped within the detection zone (NEMA).

PASSAGE SENSOR: A traffic flow sensor that detects the passage of a vehicle moving through the detection zone and ignores the presence of a vehicle stopped within the detection zone (NEMA). Also known as a motion sensor, dynamic detector or sensor, and movement detector or sensor.

PASSAGE PERIOD: The time allowed for a vehicle to travel at a selected speed from the sensor to the nearest point of conflicting traffic (NEMA).

PASSAGE TIME: The timing interval during the extensible portion that is resetable by each electronics unit actuation. The green right-of-way of the phase may terminate on expiration of the unit extension time. Also known as vehicle interval or preset gap.

PASSIVE SENSOR: A sensor that transmits no energy of its own, but rather receives energy emitted from objects in its detection zone or energy transmitted from other sources that is reflected from objects in the detection zone.

PASSIVE ACOUSTIC SENSOR: A sensor that detects acoustic energy or audible sounds produced by vehicular traffic from a variety of sources within each vehicle and from the interaction of vehicle’s tires with the road. Also known as passive acoustic array sensors because they consist of arrays of small microphones.

PASSIVE INFRARED SENSOR: A sensor utilized in traffic management applications, which operates in the far infrared spectrum. It transmits no energy of its own. Rather it detects energy from two sources: (1) energy emitted from vehicles, road surfaces, and other objects in their field of view and (2) energy emitted by the atmosphere and reflected by vehicles, road surfaces, or other objects into the sensor aperture.

PEDESTRIAN ACTUATED CONTROLLER ASSEMBLY: A controller assembly in which intervals, such as pedestrian WALK and clearance intervals, can be added to or included in the controller cycle by the actuation of a pedestrian sensor.

PEDESTRIAN CLEARANCE INTERVAL: The first clearance interval following the pedestrian WALK indication, normally flashing DON’T WALK.

PEDESTRIAN SENSOR: A sensor that is responsive to operation by or the presence of a pedestrian (NEMA). This traditionally has been of the push-button type, installed near the roadway and operated by hand. Preferably it should have some form of pilot light to indicate upon actuation that the unit is operating, but this is rarely provided because of susceptibility to vandalism. Also, NEMA does not provide an output to illuminate this indicator.

PEDESTRIAN PHASE: A traffic phase allocated to pedestrian traffic, which may provide a right-of-way indication either concurrently with one or more vehicular phases, or to the exclusion of all vehicular phases.


PHASE: A traffic signal phase has two different meanings in traffic signal terminology, as follows:

  1. NEMA: A vehicular phase is a phase that is allocated to one specific vehicular traffic movement (e.g., eastbound through traffic as timed by a dual ring controller unit). See Conflicting Phases and Non-Conflicting Phases.
  2. TRADITIONAL: A part of the cycle allocated to any specific traffic movement receiving the right-of-way or to any combination of traffic movements receiving the right-of-way simultaneously during one or more intervals.

PHASE SEQUENCE: A predetermined order in which the phases of a cycle occur.

POINT DETECTION: The detection of vehicles as they pass a specific point on the roadway. Frequently referred to as small area detection.

POINT SENSOR: A sensor that measures the passage of vehicles past a point [i.e., a small area usually not exceeding 6 x 6 ft (1.8 x 1.8 m)]. Also referred to as a small area detector or small area sensor.

POLYCHLOROPRENE: (Chemical name for Neoprene.) Used for jacketing wire and cable that will be subject to rough usage, moisture, oil, greases, solvents, or chemicals. May also be used as a low-voltage insulating material.

POLYETHYLENE: A family of insulating materials derived from the polymerization of ethylene gas. All members of the family are excellent dielectrics. Electrically they are superior to other extruded dielectrics in use today. They have high insulation resistance, high dielectric strength, and good abrasion resistance. Polyethylene is widely used for insulation on signal and loop detector wire and cable. It is suitable for direct burial.

POLYOLEFINS: A family of plastics including cross-linked polyethylene and various ethylene copolymers, one that shrinks when heat is applied. Commonly used in splicing.

POLYPROPYLENE: A thermoplastic with good electrical characteristics, high tensile strength, and resistance to heat.

POLYSULFONE: A polymer highly resistant to mineral acid, alkali, and salt solutions.

POLYURETHANE: Enamel that has excellent moisture resistance, easily soldered, and excellent winding properties.

POLYVINYL CHLORIDE (PVC): A family of insulating compounds whose basic ingredient is either polyvinyl chloride or its copolymer with vinyl acetate. Can be either rigid as used in conduit for preformed detector loops or flexible as used in duct tubing to encase detector wire. It is also widely used as insulation material on wire known as types T and TW in detector applications. Known as PVC or Vinyl.

POWERHEAD: A small 3- x 6-ft (0.9- x 1.8-m) loop installed at the stop line end of a long loop in order to improve the detection of small vehicles. It usually has multiple turns of wire and may have an angled configuration.

PRESENCE DETECTION: The ability of a vehicle sensor to detect a moving or stopped vehicle in its detection zone.

PRESENCE SENSOR: A traffic flow sensor that detects the presence of a vehicle within its detection zone and holds the call for a specified minimum time. The sensor may cover a large area or be a series of small sensors wired together in series, parallel, or series/parallel.

PRESENCE HOLDING TIME: The time that a sensor system will continue to indicate the presence of a vehicle over one of its loops without adjusting to vehicle movement to a new position. When the adjustment occurs, the actuation is terminated. NEMA requires that a Class 2 vehicle (large motorcycle) be detected for a minimum of 3 minutes.

PRESENCE LOOP DETECTOR: An inductive-loop detector that is capable of detecting the presence of a standing or moving vehicle in any portion of the effective loop area (ITE).

PRESET GAP: See Passage Time.

PRESSURE DETECTOR OR SENSOR: A sensor that consists of two metal plates separated by spacers. When a vehicle tire passes over the sensor, it compresses the spacers and allows the two plates to make contact, thereby closing an electrical circuit. This type of sensor normally produces two or more pulses per vehicle because each axle creates a contact.

PROBE: The type of sensor commonly used with a magnetometer sensor. A magnetic detector is also referred to as a probe.

PROGRESSIVE FLOW: Coordinated movement along an arterial at a given speed.

PULL BOX: A container usually at least 1 cubic foot (0.028 cubic meter) in size that is placed underground with a removable cover flush with the ground surface. Splices between the loop lead-in wires (or magnetometer or magnetic detector cables) and the lead-in cable to the controller cabinet are located in the pull box.

PULSE MODE: The electronics unit output that produces a short pulse (between 100 and 150 ms) when a vehicle enters the sensor detection zone. This occurs even though the vehicle remains in the detection zone for a longer time. See Electronics Unit Mode.

Q: See Quality Factor.

QUADRUPOLE: A loop configuration that adds a longitudinal saw slot along the center of the rectangle, so that the wire can be installed in a figure-eight pattern, thereby producing four electromagnetic poles instead of the normal two. The design improves the sensitivity to small vehicles and also minimizes splashover.

QUALITY FACTOR: A numerical index for rating the quality of a resonant circuit. A higher number indicates lower loss and increased detection sensitivity in a resonant type sensor system such as is typical of inductive-loop detectors.

QUEUE DETECTOR OR SENSOR: Component of a traffic control system that senses the presence (or number) of vehicles waiting in a queue at an intersection or on a freeway ramp.

QUEUE LENGTH: Number of vehicles stopped or slowly moving in a line, where the movement of each vehicle is constrained by that of the lead vehicle.

RACK-MOUNTED ELECTRONICS UNITS: Electronics units that have no enclosing case and are connected to the sensor system by inserting its printed circuit board into a wired receptacle or "rack" located in the controller cabinet. Compare with shelf mounted electronics units.

RADIO-FREQUENCY DETECTOR: A vehicle detector consisting of a loop of wire embedded in the roadway that is tuned to receive a preselected radio frequency from a transmitter located on a vehicle.

REACTANCE: The reactance (ohms) of a circuit component is the product of the voltage across the component and the sine of the phase angle between the voltage and the current divided by the current through the component assuming sinusoidal operation.

RED CLEARANCE INTERVAL: A clearance interval that may follow the yellow interval during which both the terminating phase and the next right-of-way phase display red (NEMA).

REJECTION (Adjacent Lane): The ability of a sensor not to output an unwanted actuation caused by a vehicle in a lane adjacent to that in which the sensor is located.

RELUCTANCE: The opposition that a magnetic material or magnetic circuit offers to the passage of magnetic lines of force. Reluctance is the reciprocal of permeability (the ability of a metal to conduct lines of force) and permeance (the measure of that conductivity). Therefore, reluctance is related to permeability and permeance in much the same way as resistance is related to conductance in electrical circuits.

REPHASE: The process of resetting, after a pulse actuation, to enable another pulse actuation should another vehicle enter the detection zone. Can also be used as a verb: "The sensor shall rephase 2 seconds after initiating an output pulse."

RESIN: An organic substance that is a nonconductor of electricity. Resins are widely used for insulation and encapsulation.

RESISTANCE: The opposition that a device or material offers to the flow of current, equal to the voltage drop across the element divided by the current through the element.

RESONANT FREQUENCY: The oscillation frequency of a loop and its leadin wire, to which the electronics unit must tune.

RESPONSE TIME:The time a sensor system takes to respond to the initiation of a detection by a vehicle.

REST-IN-RED: A controller operational mode intended to display red to all movements, in the absence of any traffic demand.

RMS: Root-Mean-Square.

SAMPLING SENSOR: Any type of vehicle sensor used to obtain representative traffic flow information (NEMA).

SCANNING ELECTRONICS UNIT: A multichannel electronics unit in which the loop(s) of each channel are energized in sequence, one at a time, in quick succession.

SCATS: The Sydney Coordinated Adaptive Traffic System (SCATS). A realtime system developed by the Roads and Traffic Authority (RTA) of New South Wales, Australia. SCATS uses two levels of control. Strategic control determines timing plans for areas and sub-areas based on average traffic conditions. Tactical control applies at the individual intersection. This system is considered to fall under the Traffic Responsive category of real time traffic signal system control categories.

SCOOT: The Split, Cycle and Offset Optimization Technique (SCOOT) is a real-time system developed by the Transport and Road Research Laboratory (TRRL) in Great Britain. SCOOT computes the cyclic flow profile for every traffic link every four seconds. It projects these profiles downstream using the TRANSYT dispersion model.

SDLC: Synchronous Data Link Control.

SEALANT: The material used in a saw slot of an inductive-loop detector to seal the wires in the slot.

SELF-POWERED VEHICLE DETECTOR (SPVD): One type of magnetometer sensor buried in the pavement, which uses a selfcontained battery for power and transmits vehicle flow information to the controller without the need for direct connection (lead-in cable).

SELF-TRACKING ELECTRONICS UNIT (DETECTOR): A loop electronics unit, not necessarily self-tuning, which includes circuits that compensate for environmental drift.

SELF-TUNING LOOP ELECTRONICS UNIT (DETECTOR): One that is capable of adapting its operation to the resonant frequency of the loop and lead-in wire without any manual adjustment required. The term applies particularly to the startup of the electronic unit’s operation, upon turn-on. Compare Self-Tracking Electronics Unit.

SEMI-TRAFFIC-ACTUATED CONTROLLER ASSEMBLY: A controller assembly for supervising the operation of traffic control signals in which means are provided for traffic actuation on one or more but not all approaches to the intersection.

SENSING ZONE: See Detection Zone.

SENSITIVITY: As it relates to an inductive-loop system, the change in total inductance of a system caused by a minimum-sized vehicle at one loop, expressed as a percentage of the total inductance. As it relates to an electronics unit, it is the minimum inductance change in percent required at the input terminals to the electronics unit that cause it to actuate.

SENSOR (SYSTEM AND LOCAL): Traffic detection devices that permit the system master or a local controller to obtain information about the traffic flow characteristics in the detection zone of the sensor. (See Detector.) NEMA limits the meaning of "sensor" to the sensing element of a detection system.

SENSOR LOOP: An electrical conductor arranged in a square, rectangular, circular, or other configuration to encompass a portion of the roadway. It provides a detection zone designed such that the passage or presence of a vehicle in the zone causes a decrease in the inductance of the loop that can be measured for detection purposes (NEMA). Also known as wire loop or inductive-loop detector.

SENSOR OFFSET: For an over-roadway sensor, the horizontal distance between the sensor mounting location and the edge of the closest portion of the detection zone.

SERIES-PARALLEL: Type of electrical interconnection of four 6- x 6-ft (1.8- x 1.8-m) loops, usually 9 ft (2.7 m) apart and installed in a line in one lane to give a 51 ft (15.5 m) length of detection area. This interconnection scheme gives a combined inductance close to the optimum value.

SENSOR SETBACK: The longitudinal distance between stop line and traffic flow sensor.

SENSOR SYSTEM: The complete sensing and actuation group of components, consisting of the electronics unit in the controller, the leadin cable, the lead-in wires, and the sensor.

SENSOR WEB DENSITY: The density of the configuration of sensors required for different kinds of arterial and network traffic control and surveillance systems by link and by lane.


  1. Occurs on a conflicting phase not having the right-of-way at the time the call is placed.
  2. Occurs on a conflicting phase that is capable of responding to the call.
  3. When occurring on a conflicting phase operating in an occupancy mode, remains present until given its right-of-way (NEMA).

SHEATH: The outer covering or jacket over the insulated conductors to provide mechanical protection for the conductors.

SHELF-MOUNTED ELECTRONICS UNITS: Electronics units that have an enclosing case and, therefore, can be mounted in the cabinet by placing them on a shelf. They can stand alone. Compare Rack-Mounted Electronics Units.

SHIELD: A conductive material surrounding the twisted pair(s) of wires in the lead-in cable of an inductive-loop detector installation, so that outside electrical interference will not induce noise onto the twisted pair(s).

SHORTED-TURN MODEL: A lossless shorted, single-turn loop with a size and shape equal to the perimeter of the vehicle located above the loop at the average vehicle undercarriage height.

SIDE-FIRE SENSOR: A vehicle sensor located to one side of the roadway, such as on a pole, rather than directly over the roadway.

SIU: Serial interface unit which provides communication, interface and message protocol between the ATC and the Input and Output units but does no processing of the data and control messages.

SMALL-AREA SENSOR: A sensor that detects vehicles at a spot location, i.e., a small area usually not exceeding 6 x 6 ft (1.8 x 1.8 m). It may detect vehicles in more than one lane. The 6- x 6-ft (1.8- x 1.8-m) inductive-loop detector is a prominent example. Ultrasonic and radar devices may function as small area sensors with detection areas as long as 20 to 30 ft (6 to 9 m). When used in this manner, the length of time the moving vehicle is in the detection zone is not needed for the intersection control logic. The small area sensor is also referred to as a point sensor or point detector.

SOUND-SENSITIVE VEHICLE SENSOR: A sensor that responds to sound waves produced when a vehicle passes through the detection area of the sensor.

SPEED ANALYSIS SYSTEM: A type of hardware assembly composed of two loop detectors and auxiliary logic. The two loops are in the same lane, a known distance apart. A vehicle passing over the loops produces two actuations. The time interval between the first and the second is measured, and a speed is computed.

SPLASHOVER: An unwanted actuation caused by a vehicle in a lane adjacent to the lane in which the sensor is located. Often occurs where long loops are used.

SPLICE BOX: See Pull Box.

SPLIT: A division of the cycle length allocated to each of the various phases (normally expressed in percent).

SPVD: See Self-Powered Vehicle Detector.

STRETCH DETECTOR: See Extended Call Detector.

SYSTEM DETECTOR OR SENSOR: Sensors located to provide information to central control computers selecting appropriate control programs to meet the traffic demands.

TAPESWITCH: A temporary sensor consisting of two strips of metal encased in a flat ribbon that is temporarily affixed to the roadway. When a vehicle tire crosses the tapeswitch, the two metal strips make contact, thereby closing an electrical circuit.

TERMINAL: Any fitting used for making a convenient electrical connection.

TF: The Underwriter’s Laboratories, Inc. (UL) designation for fixture wire, solid soft copper conductor, insulated with thermoplastic.

TFF: Same as TF, except has stranded copper conductor.

THHN: Building wire, plastic insulated, 90 °C, 600 volt, nylon jacketed.

THRESHOLD (LOOP): A minimum level of percent change in inductance that occurs to produce an actuation.

THW: Building wire, plastic insulated, heat, flame, and moisture resistant, 75 °C.

THWN: Same as THW with overall nylon jacket.

TIME HEADWAY: The time separation between vehicles approaching an intersection, measured from the front of the lead vehicle to the front of the trailing vehicle.

TIME-BASED COORDINATION: Control systems in which basic coordination is provided by a highly accurate real time clock within the traffic signal controller. This information is used to determine the local signal timing of offset and cycle/split or actuated timing based on the Time-of-Day/Day-of-Week. Also called time-based coordinated control.

TIME-OF-DAY/DAY-OF-WEEK (TOD/DOW): Timing plan selections based on the clock and calendar rather than on system traffic information. TOD/DOW systems operate with no system sensors (or at a sensor density Level of 0 or 0.5). These systems measure no characteristics of traffic flow. They may use clocks or wireline or wireless communications to provide the timing plan selection. Two categories of traffic control systems have predominantly TOD/DOW characteristics. These are the time based coordination and the interconnected control categories. In addition, isolated intersection control may sometimes use TOD/DOW considerations.

TIMER: Obsolete name. See Controller Unit.

TOTALLY ISOLATED SYSTEMS: Control systems without either timebased or communications-based connections between intersections. Thus, these systems operate each intersection without consideration of the operation of adjacent signalized intersections.

TRAFFIC DETECTOR OR SENSOR: A device by which automotive vehicles, light and heavy rail, buses, or pedestrians are enabled to register their presence with a traffic-actuated controller.

TRAFFIC PHASE: Right-of-way and clearance intervals in a cycle assigned to any independent movement(s) of traffic.

TRAFFIC-ACTUATED CONTROLLER ASSEMBLY: A controller assembly for supervising the operation of traffic control signals in accordance with the varying demands of traffic as registered with the controller by traffic flow sensors.


TRAFFIC ADAPTIVE (CONTROL) SYSTEM: Control systems that operate with 2 sensors per lane per link for optimum efficiency (Sensor web density level of 3.0). Adaptive systems do not have cycle, split, and offsets in the classic sense. They forecast traffic into the near future and proactively reoptimize selection, sequence, and duration of phases every several seconds. These systems need to measure the flow of individual vehicles so that they can predict the future flow of the individual vehicles or small packets of vehicles. System response to traffic changes is proactive.

TRAFFIC ADJUSTED CONTROL SYSTEM: Traffic Adjusted systems operate with few sensors (or at sensor web density levels of 1.0 and 1.5) and use fixed timing plans, the selection of which is adjusted by a minimum number of system sensors at periodic intervals. The sensors in these systems measure system-wide variations in traffic demand using either volume plus weighted occupancy or predominant direction of flow. They then use pattern matching to choose from among a set of preselected plans. This definition builds on the original definition by Kell and Fullerton in the Manual of Traffic Signal Design. The first usage of this term was by Automatic Signal in describing one of their control systems in the 1950s. System response to variations in traffic flow is relatively slow.

TRAFFIC RESPONSIVE CONTROL SYSTEM: Control systems that use flexible timing plans in which the offsets, splits, and phase durations can be adjusted in reaction to changes in traffic on a nearly cycle-by-cycle basis. They contain at least one sensor per lane per link and operate at a moderate sensor density (Sensor web density levels of 2.0 and 2.5). Cycle, split, and offset are each optimized in the selection of the flexible timing plan to be implemented. These systems typically use macroscopic measures of traffic flow on individual links such as platoon and other characteristics. System response to traffic changes is prompt.

TRAILING CAR: With a volume density controller, when the time headway between two consecutive cars passing over a sensor exceeds the allowable gap imposed by the controller, an event called Gap-Out occurs. The first of these cars is called the last car and the second is called the trailing car.

TRANSDUCER: That part of a sensor that transmits and receives or simply receives energy from the detection zone and converts the received signal into electrical form.

TRANSMISSION LINE: See lead-in cable.

TW: The UL designation for thermoplastic insulated wire for use in conduit and underground and wet locations. It is a common building wire having a soft copper conductor, which may be either solid or stranded.

TWISTED PAIR: Two insulated conductors twisted together with each end marked for identification.

TWO-COIL MAGNETIC DETECTOR: A magnetic detector with two coils. This sensor is capable of serving as a directional sensor.

TYPE 170 (179) AND 2070 CONTROLLER: One of the two major types of traffic signal controllers, the other being NEMA controllers. The traffic management function is implemented through standardized hardware and specialized software. Inputs from loop detectors, magnetic detectors, magnetometers, and a variety of over-roadway sensors may be used to provide traffic flow information to these controllers.

UL: Underwriter’s Laboratories, Inc., a chartered, nonprofit organization that maintains and operates laboratories for the examination and testing of devices, systems, and materials relative to life, fire and casualty, hazards, and crime prevention.

UL APPROVED: A product that has been tested and approved to Underwriter’s Laboratories standards.

ULTRASONIC SENSOR: An active vehicle sensor that transmits sound energy from a transducer at a high frequency (one that is above the upper range of human hearing) and that senses the reflection of the energy from a vehicle in its detection zone.

UNCOORDINATED (SIGNAL) CONTROL: Form of signal control for a single signalized intersection that does not have control relationships between intersections nor any data flow between intersections. The system of intersections is not responsive to traffic in real time. The individual isolated intersection may have local actuated control or Time of Day/Day of Week changes based on historical traffic flows. Also called isolated signal control. See also totally isolated systems.

VAC: Voltage Alternating Current.

VARIABLE INITIAL INTERVAL: A controller design feature that adjusts the duration of initial interval for the number of vehicles in the queue.

VDC: Voltage Direct Current.

VEHICLE DETECTOR (SENSOR) SYSTEM: A system for indicating the presence or passage of vehicles (NEMA). See Sensor System.

VEHICLE STANDARD: A test unit that produces a change in the loop inductance equivalent to a conventional American sedan.

VEHICULAR PHASE: A traffic phase allocated to vehicular traffic.

VIDEO IMAGE PROCESSOR (VIP): A type of passive sensor that analyzes a video image of a roadway by digitizing it and subjecting the digitized information to algorithms or neural networks that differentiate moving objects from background patterns.

VOLUME DENSITY CONTROLLER: An actuated controller that has a gap-reduction factor based on opposing phase vehicle time waiting.

VOLUME EXTENSION MODE: Operation of a multiple sensor design in which the green is extended by heavy traffic operating at a speed below the Design Speed. The speed is so low that the extension is attributable to the heavy volume. Compare with Volume Density Controller.

WEIGH-IN-MOTION (WIM) SYSTEM: A system that estimates a moving vehicle’s gross weight and the portion of that weight carried by each wheel, axle, or axle group, or combination thereof, by measurement and analysis of dynamic vehicle tire forces (ASTM E 867 and E 1318).

WIM: See Weigh-In Motion.


WATER BLOCKED: Impervious to water entrance and migration as a water blocked lead-in cable or water blocked splices.

XHHW: Cross-linked polyethylene insulated wire, rated at 90 °C in wet locations.

YELLOW CHANGE INTERVAL: The first interval following the green right-of-way interval in which the signal indication for that phase is yellow (NEMA), indicating the imminent change of right-of-way.


ZONE OF DETECTION: See Detection Zone.

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