Vehicle Detectors

7/31/2019 Vehicle Detectors

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PDHengineer.comCourse T-5008

Vehicle Detectors

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VEHICLE DETECTORS

COURSE #C-5010

Robert T. Hintersteiner, P.E.

Transportation/Forensic Engineer

This course will discuss alternative types of vehicle detectors that can beplaced at an intersection to detect vehicular traffic. States and local governmentaljurisdictions have their own standards for the type, placement, size, of vehicledetector used. The guidelines for the placement of vehicle detectors are set forth in

the Traffic Detector Handbook, Second Edition1, published by the Institute of

Transportation Engineers, and by various standards promulgated by theDepartments of Transportation in different States.

A vehicle detector is comprised of two separate devices. The first devicesenses the presence of vehicles (detector sensor unit), and the second device(detector amplifier unit) transmits a signal to the traffic controller. Each lane orapproach may have a detector, which informs the controller that there is a demandfor that particular approach. The detector units are the primary source ofinformation for an actuated controller in determining when the right-of-way should

be changed, and for what duration.

This course will discuss different types of vehicle detectors, along with theirbasic design criteria. The design and operation of these detector units can determinethe operations for an isolated intersection and/or for an intersection within a system.The detectors and detection systems will be discussed as follows:

1. Definitions2. Detection Types

3. Loop Detector Installation4. Detection Design and Operation5. Curb and Shoulder Installation6. Time Delay Detectors

Course #5010 Page 1 of 47

1ITE Bookstore, 1099 14th St., NW Suite 300 West, Washington, D. C. 20005-3438,

(202) 289-0222, www.ite.org.


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Manufacturers have developed different types of sensing units to detectvehicles, which are classified as follows:

Pressure sensitive; Inductance; Radar (Sonar, Ultrasonic); Magnetic; Light emitting (Optical); and, Video imaging.

DEFINITIONS:

There are many different types of detector sensing units, with varyingapplications, in use today:

General Definitions:

Actuation: The sensing of a vehicle or pedestrian.

Advisory Detection: The ability to sense a vehicle on one or more

intersection approaches solely for the purpose of modifying the phase

sequence and/or length of time for the other approaches to theintersection.

Cross talk: Any electrical current generates a magnetic flux or electrical field

around a wire, and this electrical field can induce a current in the adjacentwire (transformers operates on this principle).

Current flow: The right-hand rule is where the thumb points in the direction

of the current flow and the fingers point in the direction of the magneticflux field, or in a counter clockwise direction. When a vehicle goes overthe lane or roadway loop detector (a wire placed in the pavement), achange in the magnetic flux field is registered in the detector amplifier unit.

Detector or Detector Unit:2 A device for indicating the presence or passage

of a vehicle or a pedestrian.

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2Traffic Detector Handbook, Second Edition, Institute of Transportation Engineers.


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Detector Amplifier:3 A device that is capable of sensing and intensifying the

electrical energy produced by a detector sensor, which then transmits anelectrical signal to the controller.

Detection Area (Zone Of Detection): An area that is being detected. It mayconsist of one or more detector sensing units to cover the entire detectionzone.

Detector Sensor: The actual unit that senses the presence of a vehicle.

Lead-In-Cable: A shielded pair of wires leading from the splice box

servicing the detector sensing unit to the terminal board located in thecontroller cabinet. Each pair of wires is shielded to prevent cross talk or

inducted current from an adjacent pair of signal cables and/orLead-In-Cables.

Passage Detection: The ability to sense a vehicle passing or moving through

the detection field or zone, ignoring the presence of a stopped vehiclewithin the detection zone.

Presence Detection: The ability to sense the presence of a vehicle, whether

moving or stopped, as it enters the detection area or zone.

Spill Over Actuation: An unwanted vehicle actuation caused by a vehicle in

a lane adjacent to the lane that is being detected.

Types of Detectors:4

Advisory Detectors: A point or calling detector used in a system to determine

which signal plan shall be used or to extend the signal timing for thatphase.

Bi-directional Detector: A detector sensing unit that is capable of being

actuated by vehicles proceeding in either direction. It also indicates inwhich direction the vehicles are going. The direction is determined by

VEHICLE DETECTORS


4Ibid.

3National Electrical Manufactures Associations - NEMA Standard 11-6-1975, TS-1 1983 page 5.


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which detector sensor unit is actuated first in a series of detector sensingunits.

Calling or Point Detector: A detector sensing unit that is installed in a

selected location to detect vehicles as they approach an intersection. Thecalling detector amplifier transmits a signal to the controller indicating thata vehicle has passed a certain point in the street (Figure 1).

FIGURE 1

POINT OR CALLING DETECTORS

OR

Classification Detector: A classification detector sensing unit consists of

one or two calling or point detector sensing units placed at apredetermined distance to determine the volume of traffic, the speed oftraffic, and the length of each vehicle. By measuring the time that avehicle takes to travel over the detector sensing unit(s), analog formulasare used to determine the speed of traffic and/or the length of the vehicle.Classification detectors can be permanentlyinstalled in the pavement ormounted overhead on a highway. Classification detectors can be installed

temporarily in a roadway lane, which are self containing traffic counters 5

(Figures 2 & 3).

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5Nu-Metrics - Hi-State or Count card Magnetic Sensing units.


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Directional Detector: A detector

sensing unit that is capable of being actuated only by vehicles proceeding

in one specified direction (i.e. two-way traffic on a one-lane roadway ordriveway; or regulating the closing of a parking gate after a vehicle has leftthe gate area in a driveway or at an automatic toll booth).

Extension Detector: The detector sensing unit is placed in advance of an

intersection to sense vehicles that are standing still for a predeterminedtime. The controller will only act upon the signal from the amplifier duringthe green interval for that approach. The extension detectors will increasethe green time for that particular phase. Extension detectors are used toimprove transit operations at intersections, and they are also used toreduce extremely long queues on exit ramps from highways (Figure 4).

FIGURE 4

EXTENSION DETECTOR

LoopDetectorsPermanent

SENSING UNITS

EXIT

RAMP

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FIGURE 2CLASSIFICATION DETECTOR

Loop DetectorsPermanent

TemporaryRoad Tubes

SENSING UNITS


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Light-sensitive Detector: A detector sensing unit that utilizes a light-sensitive

device for sensing the passage of an object, which then transmits a specialtype of light beam directed at the sensor. For example, a high intensitylight beam transmitter is mounted on an emergency vehicle, and the optical

detector sensing unit is mounted over the intersection. Upon sensing thehigh intensity light beam, the sensor sends a signal to the controllerpreemption unit to change the existing signal phasing into a preselectedphasing program for the passage of the emergency vehicle (Figures 5 &

6)6.

Infrared Detector: A detector sensing unit that senses radiation in the

infrared spectrum, which is used as a calling detector. The amplifier unit iscall a Transducer.

Loop Detector: A detector sensing unit that senses a change in inductance

from wires embedded in the pavement, caused by the passage orpresence of a vehicle within the inductance field. Currently, inductanceloops are the most widely used type of detection.

Magnetic Detector: A detector sensing unit that senses a change in the

earth's magnetic field by the movement of a vehicle near its sensor, whichis used as a calling detector. A magnetic detector sensing unit can beplaced in the center of two lanes to detect traffic in both lanes (Figure 7).

VEHICLE DETECTORS


6Opticom by 3M.


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Magnetometer Detector: A detector sensing unit that measures the difference

in the level of the earth's magnetic forces caused by the passage orpresence of a vehicle near it sensor. Single lane point detection orpresence detection with a number of units placed in series and/or parallelare used on bridges where loop detector sensing units could be affected

by the steel in the bridge deck (Figure 8).

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Microwave Detector: A detector sensing unit that detects the presence of

moving pedestrians and/or vehicles. Microwave detectors are found inautomatic moving doors, and are used for traffic sensing where loops andother types of detectors cannot be installed (Figure 9).

Non-directional Detector: A detector sensing unit that is capable of being

actuated by vehicles proceeding in any direction.

Pedestrian Detector: A detector sensing unit that is responsive to the

operation by, or the presence of, a pedestrian. Push buttons are presentlyused, but there are new products that can sense the presence of apedestrian by radar, microwave, or video camera.

Pneumatic Detector: A pressure-sensitive detector sensing unit that uses a

pneumatic tube as a sensor. Existing pressure-sensitive detector sensingunits are still being used today as calling detectors, but new ones are notbeing installed (Figure 10) due to the high construction installation andmaintenance costs of each unit.

Pressure-sensitive Detector: A detector sensing unit that reacts to the

pressure of a vehicle passing over the surface of its sensor. This type ofdetectorsensing unit, which consist of road tubes placed across a lane tocounttraffic, is also knownas an Automatic Traffic Counter (ATC).

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Radar (Sonic) Detector: A detector sensing unit that is capable of sensing

the passage of a vehicle through its field in the form of emitted microwaveenergy. These were widely used during the 1940's and 1950's, but were

replaced by loop detectors because the transducers used in them werevery expensive to replace. They are making a comeback in today's marketsince the transducers have been replaced by circuit boards andmicrochips. Radar detectors are used for point detection per approach,and do not provide for separate lane controls (Figure 11).

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Sampling Detector: Any type of vehicle detector sensing unit used to obtain

representative traffic flow information.

Side-fire Detector: A vehicle detector sensing unit with its sensor located to

one side of the roadway on top of a 3 meter (10 foot) pole.

Sound-sensitive Vehicle Detector: A detector sensing unit that responds tosound waves generated by the passage of a vehicle over the detectionarea.

Ultrasonic Detector: A detector sensing unit that is capable of sensing the

passage or presence of vehicles through its field in the form of emittedultrasonic energy (Figure 11).

Video Enhanced Detector: A detector sensing unit that is capable of

sensing every vehicle per lane per approach per intersection using videoimages, by selecting certain pixels in the image to detect the location ofvehicles on the screen by using analog equations (Figure 12). This is a

spin-off of space age technology.

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Amplifier Modes:

Carryover Output: The ability of the detector amplifier unit to continue its

output for a predetermined length of time following an actuation.

Controlled Output: The ability of the detector amplifier unit to produce apulse that has a predetermined duration regardless of the length of time avehicle is in the field of influence or zone of detection.

Detector Mode: A term used to describe the operation of a detector channel

output when a detection occurs.

Presence Mode: Detector output continues if any vehicle remains within the

field of influence or zone of detection. Presence mode is used in long

detection areas. The signal to the controller discontinues when a vehicleleaves the detection zone.

Pulse Mode: Detector amplifier unit produces a short output pulse when

detection occurs.

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Limited Presence Mode: Detector output continues for a limited period of

time if a vehicle remains in the field of influence or zone of detection. Forexample, when a vehicle is parked in the field of influence, the detectorwill stop registering its presence and will reset itself.

Time Delay Output: The ability of a detector amplifier unit to delay its

output for a predetermined length of time. Time delay outputs are used forpresence detection in the left and right turn lanes, and also for extensiondetectors.

DETECTION TYPES:

The design and placement of vehicle detection systems within an intersectioncan determine the efficiency of that intersection, and also the efficiency of thearterial. Detectors are used to inform the controller that a vehicle is approaching theintersection and should be given the right-of-way during the next occurrence of thisphase, within the predetermined cycle length. These detectors are use to extend thegreen time for that phase, and when the actuation ceases, the green time will alsocease. Detection areas are designed to provide actuation while the vehicles arewithin or passing over the detection zone, and this detection lasts until the last

vehicle has left the area of detection or when the maximum time for that phase hasbeen reached. There are basically two methods used to detect vehicles: Calling orPoint Detectors, and Presence Detectors.

Calling or Point Detectors:

Calling detectors are set back from the intersection and can be either aboveor below ground. The below ground detector units can be either loop,magnetic, or magnetometer. The above ground detector units can be either

radar, microwave, or video imaging types, and they are fixed to an overheadmast arm extending over the lanes or are side mounted on a pole located onthe side of the roadway. The area of detection is perpendicular to the travellane and can be set back from the Stop Line or at the Stop Line. Calling orpoint detection holds the call in memory until that particular green phase hasbegun and an initial green interval has completed its timing. Any additionalactuation will extend the Green interval by the predetermined passage time.

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The time is set high (3 to 10 seconds) depending upon the distance thedetector unit is located from the Stop Line. If an actuation is cut short by themaximum green time, then the controller will automatically place a call intomemory so that this signal phase will occur during the next cycle.

Presence Detectors:

Presence detectors are installed in the pavement using detector loops orabove ground using video cameras, and are located parallel to traffic. Thearea of detection extends from the Stop Line for a distance back in the laneto detect traffic as it approaches the intersection. This distance can vary from6 m (20 feet) to 36.6 m (120 feet), as required by the local governmentalagency. For left turn lanes, the detection area can be extended into theintersection with the intention to extend the green time to give motoristsenough time to clear the intersection before the clearance interval starts.Normally the area of detection will extend back from the Stop Line about 18m (60 feet) or three car lengths. On a side street the detection area can beextended to within 1.5 m (5 feet) of the projected curb line or travel way. Thedetection area can be controlled by a single loop, multi-loops, or multi-pointdetectors placed close together. Above ground presence detector units are ofthe microwave or video imaging types. Both these types of detectors must beplaced overhead, and be programed to detect traffic on an approach or alane of travel. They will sense presence and motion within the area of

detection.

Types of Detectors:

Detector units are described as follows:

Loop Detection:

The standard loop detector sensing unit is usually rectangular in shape.

The rectangular box is 1.8 m (6 feet) wide, and the length variesaccording to the length of the zone of detection, size of the lane, and/orthe approach width. The standard Loop Inductance Wire is a #14

AWG7, with a plastic jacket to prevent nicking or cuts; however, eachState has its own standard wire size. The Loop Inductance Wire is

VEHICLE DETECTORS


7American Wire Gage.


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placed in a saw slot which is 10 mm (3/8 inch) wide and 57 mm (2 1/4inch) to 8.3 mm (3 1/4 inch) deep. The Loop Inductance Wire is laid in acontinuous run without any splices, starting from the pull box, out to the

detector sensing unit, around the detector sensing unit many times (1 to 6

layers), and then back to the pull box. The wire is tamped down with awooden stick (paint stirrer) and then liquid epoxy is used to fill the sawcut. The number of turns per loop can be determined by the formulas set

forth in the Traffic Detector Handbook,8 or by referring to the local Statestandard loop design. Figure 13 shows three types of loop installations.

1st Layerof Sealant

2nd Layer

of Sealant

DepthofSawCut

Plastic Filler

Loop Wire Sealant

FIGURE 13

LOOP WIRE INSTALLATION

The three typical types of loop installation methods are: place the loops inthe with the sealant above and below the wires; above the wires; and witha Plastic Filler (polyethylene rope) which permits the wires to flex with thepavement to reduce stress on the wires.

Magnetic Detector:

Magnetic detector sensing units are calling or point detectors. Magnetic

detector sensing units are placed between 15 m (50 feet) to 30 m (100feet) back from the Stop Line. Since their magnetic fields will not registera change in their flux for vehicles traveling less then 8 KPH (5 MPH), theyhave to be placed back from the intersection in order to be able to detectmoving vehicles. The magnetic probe(see Figures 1 & 7) is placed in a

VEHICLE DETECTORS


8Traffic Detector Handbook, Second Edition, published by ITE.


16/48

100 mm (4 inch) diameter plastic conduit (600 mm) 2 feet under thepavement, perpendicular to the travel lane and in the center of the travellane. For an approach roadway with two lanes, a probe is placed in thecenter of the two lanes. The magnetic probe is placed in the plasticconduit extending from the pull box about three quarters of the width ofthe roadway under the lane(s) to be detected. A flexible wooden or plasticstick of known length is used to push the probe to its desired location.The probes connecting wire is then spliced into the Lead-In-Cable.

Magnetometer Detectors:

Magnetometer detector sensing units can be used for point detection, andmulti-magnetometer detectors can be used for presence detection. Thesensitivity radius varies between 450 mm (1.5 feet) and 900 mm (3 feet),

depending upon the number of probes connected in series. Two or threemagnetometer probes can be connected together in series to detect anytype of vehicle (autos, trucks, buses, motorcycles, and bicycles), andshould be placed between 900 mm (3 feet) and 1.5 m (5 feet) apart in thecenter of the lane of travel. Each amplifier channel can have up to 6

probes connected to the input device9. Each magnetometer probe requiresa hole drilled about 3 mm (1/8 inch) diameter larger than the probe'sdiameter, to a depth of 480 mm (19 inches) to 610 mm (24 inches), whichplaces the probe into the subbase and below the pavement. A pavement

saw cut trench is then installed between each probe and to the curb oredge of pavement. The probes wire is placed in a conduit leading to apull box to be connected to a Lead-In-Cable. For point detection, one tothree probes are placed at a distance back from the Stop Line. Forpressure detection, at least 4 sets of magnetometers with spacing of 3.4 m(11 feet) are required to cover the entire area of detection with a length of

12 m (40 feet)10 (see Figure 14). Many governmental agencies use themagnetometers in bridge decks because of the depth of the loop saw cutswould cut into the steel reinforcing bars, which would affect the electric

field around the wires. The Magnetometers are rarely used for lanedetection due to the high installation cost of each probe, which includesthe cost of drilling and saw cutting the pavement.

VEHICLE DETECTORS


10ITE Traffic Detector Handbook Second Edition, Page 101, Figure 99.

9Ibid. page 98.


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3.4 m 3,4 m 3.4 m 0.9.m0.9m

12 m(40 feet)

1.8

m

(6

ft)

FIGURE 14

TYPICAL MAGNETOMETER

MagnetometerMagnetic Field Detection Area

PRESSENCE DETECTOR INSTALLATION

Light Emitting Preemption:

Light emitting preemption detector sensing units consist of three basicparts: the infrared light emitting unit which is placed on an emergencyvehicle; the light sensing unit which is placed over the approach lanes atthe intersection; and the amplifier unit which transmits the signal to thetraffic controller. The light beam which is emitted is sensed by the opticalsensing unit mounted over the center of the intersection from a distance of150 m (500 feet) to 550 m (1800 feet). The controller then changes thetraffic signal program into a preemption program, providing vehicleclearances to all phases, followed by a pre-designed Green indication for

the direction of the emergency vehicles. There is one sensor unit facingeach approach to the intersection (see Figures 5 & 6). It should beunderstood that if the distance is less than 150 m (500 feet) the time topre-empt the traffic signal phase could be less than the vehicle clearanceinterval, and the pedestrian clearance time could be short, leaving thepedestrian in the middle of the intersection and/or in the path of theemergency vehicle. The preemption signal could also provide false calls atother intersections along the route if the emergency vehicle turns or stopsprior to the end of the route [anywhere between 550 m (1800 feet) and

150 m (500 feet)]. Another problem can occur when a short Greenindication appears on an opposing phase with very little notice to themotorist. All preemption systems have to be planned and tested out toprotect both pedestrians and the motoring public. Other preemptionsystems use a radio frequency instead of a light frequency, with similarsetups and drawbacks.

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Hardwire Preemption:

Some preemption systems use hardwire from a central computer systemto control emergency vehicles routes. This is accomplished by the central

fire command choosing a preselected route plan. The fire control centerpresses the desired route button which in turns activates the pre-designedemergency route in the central computer. These pre-designed plans canchange the signal at each intersection as the emergency vehicle travelsalong its route. A high degree of planning and time testing is required sothat the signal is Green when the first emergency vehicle reaches eachintersection. This system can reduce the time that traffic is interrupted.The advantage of this method is that the system is hardwired, and it willnot be affected by a short route or blockage of the signal by overhangingtrees. This system can also separate emergency vehicles coming from

different directions.

Microwave detectors:

The microwave detector sensing units are similar to that of a radardetector, since the emitted energy is reflected back to a sensing unit orantenna, which denotes the passage of a vehicle. The operating frequency

is normally in the K-band (24 gHz) or the X-band (10 gHz)11. A Dopplereffect is achieved when the units antenna is place at a slight angle to

traffic (see Figure 9). For vehicles or pedestrians to be detected, theymust exceed 5 kph or 4.4 ft/sec (3 mph).

Video Imaging Detectors:

Video Imaging Detector System (VIDS) cameras can detect vehicles in anumber of different locations within the field of vision. These locationsare specified by the user with interactive graphics, which can be changedas many times as desired. To achieved detection, the detection area or

points are programmed into the interactive graph program, and when avehicle passes these pixel dots or points a signal is sent to the controller.These points can be arranged in lines, squares, or rectangles to detectvehicles in each lane. The VIDS can be used to determine turning

VEHICLE DETECTORS


11ITE Traffic Detector Handbook Second Edition, page 164.


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movements, volumes, speed, classification, queuing lengths, etc., (seeFigure 12).

The installation of overhead detector sensing units shall conform to themanufacturers specifications, and also to the best position that can beobtained for the desired results.

LOOP DETECTOR INSTALLATION

The installation of the surface pavement type loop detector sensing units shallconform to the following general guide lines, except when modified by the localgovernmental agency having jurisdiction. Pavement designs and installations are

critical in the service life of the loop detector unit. The three major types ofpavement used to construct todays roadways are gravel, asphalt, and/or concrete.The life of the loop detector unit depends upon the thickness of the pavement, thetype of pavement used, and the depth that the loops are installed into the pavement.

Pavement Types:

There are four different types of surfaces: gravel pavement (commonly knowas Item 4 base course), asphalt concrete pavement, concrete pavement, and

structural bridge slab pavement.

Gravel Pavement:

Standard surface detection sensor units cannot be installed in a dirt orgravel pavement. Overhead microwave type detectors are most suitablefor point detection on such roadways. Dirt or gravel roads are usuallyfound on rural roadways and driveways.

Asphalt Pavement:

Asphalt Pavement (a/k/a Bituminous Concrete) is the pavement surfacetreatment most commonly used in the world today due to its reasonablecost. Typical asphalt pavement thickness varies from 165 mm to 230 mm(6.5 inches to 9 inches). Loop detector sensor units are saw cut into the

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pavement to a depth of 50 mm to 90 mm (2 to 3.5 inches), dependingupon the number of turns per loop detector in the sensing unit. The top ofthe loop wires should be at least 40 to 50 mm (1.5 to 2 inches) below thesurface of the pavement, thus placing the entire loop detector sensing unitinto the binder course of the asphalt pavement. The life of the loopdetector sensing units can be related to the minimum depth below thesurface. If the loop wire is placed close to the surface of the pavement,the vehicular traffic wears down the pavement surface, causing and theloop wire to be exposed to the elements. At intersections where there isextremely heavy truck traffic, the braking of the vehicles causes thepavement to ripple (washboard effect), shifting the loop detector sensingunits vertically and horizontally. In summary, the loop detector sensingunit fails when the pavement fails.

There are three methods used today in installing loop detector sensingunits:

1. Loop detector sensing unit are installed in the pavement by saw cuttingafter the top or wearing course (surface) has been installed. Theadvantage of using this method is that it can be used in existingpavement and the saw cut can be seen by roadway contractors whoare responsible for replacement.

2. Installing the loop detector sensing units by saw cutting the bindercourse, and then installing the asphalt top course. To indicate thelocation of the buried or paved over loops, many jurisdictions place anotch in the adjacent curb at the start and end of each loop detectorsensing unit; however, this cannot be done at all locations. Theadvantage of this method of installation is that it reduces the effects ofpavement failures, and it reduces the depth of saw cutting required.The major disadvantage of this method of saw cutting the bindercourse is that in future years the exact location of the loop detector

sensing units may be difficult. Therefore, the governmental agency cannot hold a contractor responsible if the detector sensing unit is cut ordestroyed during future work. The local governmental agency havingjurisdiction must mark off the location of the loop detector units priorto construction.

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3. Prefabricated loop detector units are placed on top of the bindercourse, and the turns are placed side by side in a horizontal format,and the top course is placed on top of the wires. The advantage is thesame as stated in section 2 above, in addition to saving the cost of sawcutting. The disadvantage is also the same as stated in section 2 above,with the added problem that if the asphalt is too hot it can melt theplastic protective coating on the loop wire. Since the thickness of thetop course is the only protection for the loop wire, as the pavementwears under heavy traffic usage, this protective thickness is reduced,and the wires can become damaged.

Concrete Pavement:

Loop detector sensor units are saw cut into the pavement to a depth of50 mm to 90 mm (2 to 3.5 inches), depending upon the number ofturns per loop detector sensing unit. The loop wires should be placedat least 40 to 50 mm (1.5 to 2 inches) below the surface of thepavement. At concrete joints and cracks the loop wire shall be placedin a plastic sleeve with slack in the wire (see Figure 15).

If plastic sleeves are not used, the loop wire will snap (break) duringcold weather as the concrete shrinks, and in hot weather the joints will

VEHICLE DETECTORS



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compress and the sharp edges could cut the loop wire. Remember thatloop wire must be a continuous run without splices from the pull boxto the loop detector sensing unit and back to the pull box. Once theloop wire is cut or broken, the entire loop detector sensing unit mustbe replaced in its entirety. The cost of installing a loop detectorsensing unit ranges from $2,000 to $6,000 per unit.

Concrete with Asphaltic Pavement Overlay: Loop detector sensing units

are the same as mentioned above.

Metal objects within the detection zone: Metal objects within an approach

lane shall be avoided because they will have a direct influence upon theinductance field. The loop detectors are split in two to avoid a manholecover. If the manhole cover is along the edge of the loop detector, it shouldbe directed around the manhole cover by at least 300 mm (1 ft.) (see Figure16). Small water or gas boxes will not have an influence upon the detectionarea as long as they are located near the center of the loop.

Thickness of Pavement:

The thickness of the pavement also has an affect on the life of loop detectorsensing units. The minimum thickness of the pavement should be at least 200mm (8 inches) to provide proper clearance between the wearing surface and

the subbase. Water seeping into the saw cut, from either the surface orsubbase, can cause the pavement to develop potholes along the loop sawcuts. Placing the loop wire in the middle of the pavement increases the life ofthe pavement and the loop sensor units. The majority of loop failures occurin private driveways where the combined pavement thickness ranges from 50mm to 90 mm (2 to 3.5 inches). For driveways exiting at a traffic signal, thetotal pavement thickness should be the same as the intersecting roadway, or200 mm (8 inches) for a distance of at least 30 m (100 feet) from theextended curb or edge of shoulder.

Saw Cutting Detector Loops:

Loop detector saw cut slot shall be installed without any sharp edges or

corners so that the inductance wire plastic coating will not be nicked or cut

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causing the wire to be grounded. The corners shall be cut at an angle or a hole

drilled at each corner (see Figure 16).

SAW CUT SLOTS

CORNER CUTS

FIGURE 16

CORNER SLOT TREATMENT

ManholeCover

DETECTION DESIGN AND OPERATIONS:

The operation of a traffic signal depends upon the placement of the loopdetector (detector sensing units). Each type of loop detector is designed to fulfill aspecific function, and their installation can determine the efficiency of the trafficsignal operation. The placement of the loop detector within the lane of travel and thecontrol setting of the amplifier can determine the response time of the traffic signaltiming module. The loop detector only senses the presence or passing of a vehicle,and this information is transmitted to the amplifier to determine the Green intervaltiming for that particular phase. Amplifier and timing modular settings are used todetermine the length of Green time for each phase, which will be discussed in detailin a future course. The present course will discuss the type, size, and location of aloop detector, which can be used to obtain the desired results using traffic patterns,calling, and presence detection.

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Traffic Patterns:

The size and shape of each loop detection and the configuration of thedetection area depends upon the traffic patterns that exist or are proposed for the

intersection. The detection required at an isolated intersection differs from that of anintersection located within an arterial system. To design the type of detectionsystem desired, the following traffic patterns should be determined:

{ Uniform Traffic Patterns - Vehicle traffic is uniform throughout the

day (i.e. CBD traffic patterns).

{ Directional Traffic Patterns - Vehicle traffic is highly directional

during peak highway hours (i.e. commuter traffic, inbound andoutbound flow).

{ Off Peak Traffic Patterns - Vehicle traffic is balanced on all

approaches during non peak highway hours (i.e. shopping traffic,noontime traffic, night time traffic).

{ Event Traffic Patterns - Vehicle traffic is highly directional to and

from special events (i.e. sporting complex, holiday events).

{ Acombinationof all of the above traffic patterns.

The most common type of detection systems currently in use are the callingand presence detection sensor units.

CALLING DETECTION:

Calling detectors (Calling Detectors Sensing Units) [overhead or within the

pavement] are placed back from an intersection to detect vehicles as theyapproach the intersection. The distance from the Stop Line varies accordingto the posted speed limit of the roadway and the physical features of theadjacent properties. Calling loop detectors are used in the Speed ExtensionCalling Detection System, the Vehicle Calling Detection System, and theVolume Density Detection System.

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Speed Extension Calling Detection System:

The location of the Extension Calling Detector is determined by the

Dilemma Zone.

Dilemma Zone Method:

The Dilemma Zone Method is used to determine the distance requiredfor a vehicle to stop safely before reaching the Stop Line, withoutcoming to a skid stop, when a vehicle passes over the calling detector.To determine the minimum distance to place the calling detector fromthe Stop Line, use the equation for the Safe Stopping Distance on wet

pavement12. The Safe Stopping Distance is based on the Reaction

Time of the driver (usually about 2.5 seconds for design)13, plus theactual Skid Stopping Distance to a stop.

D = Vt+ V2/(30(f! g))

D = Distance (feet or meters).V = Velocity (feet per sec. or meters per sec.)T = Time (seconds).Vt = Vehicle speed x time [includes perception to reaction time].

f = coefficient of friction for wet pavement14.g = percent of grade divide by 100 [added for upgrade and

subtracted for down grade].

If using the English system with MPH the formula is:

D = 1.47Vt+ V2/(30(f! g))

If using the Metric system with KPH the formula is:

D = 0.278Vt+ V2/(255(f! g))

VEHICLE DETECTORS


14AASHTO 1965 & 1984, f = friction factor of the roadway on wet pavement (0.4 to 0.29);

g = grade of roadway to Stop Line.

13AASHTO 1965 & 1984, Reaction Time from varies from 0.64 to 1.7 seconds.

12ITE Transportation And Traffic Engineering Handbook 1976, page 611.


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The computed minimum distance that calling detector sensing unitsshould be placed back from the Stop Line at various speeds when f=0.29, g = 0, and t= 2.5 seconds are:

At 55 mph (88 kph) the distance would be 550 feet, use 550 feet.At 40 mph (64 kph) the distance would be 331 feet, use 330 feet.At 30 mph (48 kph) the distance would be 214 feet, use 210 feet.

At 90 kph (56 mph) the distance would be 172 m, use 170 m.At 80 kph (50 mph) the distance would be 142 m, use 140 m.At 50 kph (31 mph) the distance would be 69 m, use 70 m.

Therefore, the calling detector should be placed between 70 m (210feet) and 170 m (550 feet) behind the Stop Line, depending upon theposted speed of the roadway.

Vehicle Calling Detection System:

Vehicle calling detectors are used in urban areas or where there areon-street parking and driveways near the intersecting roadway. Vehicleextension calling detection sensor units are placed between 6 and 15 m(20 and 50 feet) behind the Stop Line just to call that particular phase.The controller has preset timings that determine the length of time thatthe Green indication shall remain lit, which is based upon the actuation

of each vehicle passing over the calling detector. Usually there is aGreen indication of 10 seconds, with the time being extended from 2to 5 seconds depending upon the distance of the vehicle callingdetector from the Stop Line and the average spacing between vehicles.The vehicle calling detectors should always be installed to include thedriveway and on-street parking closest to the intersection.

Common Types of Calling Detectors:

Common types of calling detector units are inductive loops, magnetic,radar, microwave, and ultrasonic, and their installations are explained inthe definition section of this course. Inductive loop detector sensorunits (loop detectors) are custom designed for each location, and areplaced perpendicular to the lane of travel. The length of a loopdetector is 1.8 m (6 feet) and the width varies according to theapproach width. The width of the calling loop detector extends from 1

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m (3 feet) from the curb or shoulder to 1 m (3 feet) from the center lineof the roadway. On a multi-lane approach 1.8 x 1.8 m (6 x 6 feet)calling loop detectors can be installed in the center of each travel lane(see Figure 17).

CALLING DETECTION VOLUMUE DENSITY DETECTION

FIGURE 17

CALLING DETECTION

Calling Detectors

Distance from

Stop Line

Exit Detector

Driveway Extra Detection

Calling Detectors

or

Presense Detector

Volume Density Detection System:

The Volume Density Detection System uses calling loop detectors,and their locations are determined by the Dilemma Zone Method on theapproaches to the intersection. Exit loop detectors are placed ahead ofthe Stop Line to terminate the vehicle call when the last vehicle haspassed the Stop Line. This extra loop detector will terminate the Greenindication after a vehicle passes over it if it does not receive a call from

the first loop detector. If only the extra loop detector receives a call, itwill send a signal to the controller that this phase has detected avehicle. The Volume-Density Controller will adjust the Green indicationfor the Initial Minimum Green, and the Passage Time for each vehicleactuating the calling loop detector. The timing operation will bediscussed in detail in a future course. In areas where there aredriveways and/or parkingis permitted between the StopLine and the

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detectorsensing units, an extra calling detector or loop detectorsshould be installed between 1.5 to 6.0 m (5 to 20 feet) before the StopLine (see Figure 17).

PRESENCE DETECTION:

Presence detection is custom designed for each approach to an intersection,and consists of determining the Detection Area and the different types ofinductance loop detectors to be installed within the detection area.

Detection Area:

Presence detection areas are installed to detect the presence of vehiclesusing the approach lanes and/or the approach roadway. The detection

area usually extends from about 3 m (10 feet) from the adjacent travel wayto about 18.3 to 36.6 m (60 to 120 feet) past the Stop Line. The detectionarea is centered in the travel lane or in the approach roadway, with a cleararea of 0.91 m (3 feet) from adjacent travel lanes and opposing trafficlanes. Once the Detector Area has been determined, presence detectionareas are designed to provide for continuous vehicle detection from thebeginning to the end of the detection area. The type, size, and number ofinductance loop detectors to be used in the Detection Area depends uponthe requirements and design standards of the local governmental agency

having jurisdiction over that particular section of roadway (see Figure 18).

FIGURE 18

DETECTION AREAS

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Left Turning Lanes:

For left turn lanes, the detection area begins about 6 m (20 feet) infront of the Stop Line. It can extend to 24 m (80 feet) behind the Stop

Line. The detection area can cover each individual left turn lane orinclude all the left turning lanes on that particular approach.

Thru Traffic on Main Roadway:

If a detection area is required for thru traffic on the main roadway, thethru lanes are covered from at least 3 m (10 feet) behind the Stop Lineto approximately 24 m (80 feet) behind the Stop Line.

Side Street:

For side street approaches, the entire approach width of the roadwayshall be within the Detection Area, starting at least 3 m (10 feet) fromthe intersecting travel lane to approximately 24 m (80 feet) behind theStop Line. Right and Left turning movements must be included in theDetection Area.

Right Turn Lanes:

Right turn lanes may be included in a detection area or they may havetheir own detection area. If the right turning movement can proceed ona Right Turn On Red signal, the detection area is extended to theintersecting travel lane. The detector amplifier would then have a timedelay feature to permit the vehicle to make the right turn before sendinga call to the controller unit, reducing false calls. The design for theseparate right turn lane detection area would be the same as for theseparate left turn lane.

Left Turn and Thru Lanes:

Many approach roadways have the left turn lane and thru lane sharingthe same lane of traffic. The left turn detection area should be thesame as that for a separate left turn. To accommodate the left turningmovement, the left turn detection area would detect a waiting vehicle

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and place a call to the controller servicing the vehicle(s) waiting tomake a left turn.

INDUCTANCE LOOPS:

The actual inductance loop (presence) detector design varies from State toState. The following section will describe the different types of designs asthey are used by various States and governmental agencies. These loopdetectors vary from very large loops extending 37 m (120 feet) in length to aseries of small loops of 1.8 x 1.8 m (6 x 6 feet). Small area loop detectorsoccur in the following configurations: square; parallelograms; round; anddiamond. Large area loop detectors are rectangular in shape, but the designcan vary from a simple loop to a Quadrupole Loop, with or without a busterloop at the head of the detection area to pick up bicycles and other smallvehicles. The following section will discuss the pros and cons of each typeof loop detector sensor unit design configuration:

Extra Long Loop Detectors:

The extra long loop detectors can vary between 24 and 37 m (80 feet and120 feet) in length, and are designed to detect large spaces or gapsbetween vehicles approaching an intersection. The signal timing is

extended by each succeeding vehicle's actuation, ranging from 0.25 to 1seconds. Under vehicle presence mode, the phase Green Interval willterminate as soon as the last vehicle leaves the detection area, and aftercompleting the initial Green Interval timing. The extra long loop detectorswill hold the actuation for each vehicle while a vehicle is in the detectionarea, providing that the vehicle gaps do not exceed the length of thedetection area.

Loop detectors placed in concrete pavement, across expansion joints,

construction joints, or through asphalt concrete shoulders, can break dueto expansion and contraction of the pavement. If an extra long detectorsensor unit is cut or broken, the entire detection area is lost, and thecontroller shall be placed on recall until the loop detector is replaced.

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The cost of installing a new extra long loop detector ranges between $50

and $60 per linear meter15. The loop detector requires saw cutting thepavement plus placing the inductance wire into the slot. The inductancewire has to start at the junction box, be placed in the saw cut, and go back

to the junction box without a splice. Approximately 91 m (300 feet) ofsaw cutting and wire is required, and the current approximate cost is in the

range of $4,500 to $5,500 per loop16((see Figure 19).

Saw Cut

Inductance Loop

FIGURE 19

EXTRA LONG DETECTOR

LOOP DETECTORS

1.8 M

3 7M

Saw Cut

Inductance Wire

1.8 M

6Mt

o18M

LONG DETECTOR LOOPS

LOOP

2.4 to 3 M

Inductance wirein Saw Cut

Long Loop Detectors:

Long loop detectors can range between 6 m (20 feet) to 18 m (60 feet) inlength. They are placed one behind the other to cover the detection area,spaced at 2.4 to 3 m (8 to 10 feet) apart (see Figure 19). These long loop

detectors are designed for an average queue length of five (5) or morevehicles per cycle, or over 180 vehicles per hour per lane (vphpl) 17. Longloop detectors are used for both high and low traffic volume roadways

VEHICLE DETECTORS


17Based on 36 cycles in a 100 second cycle length.

162006 prices.

15Based on 2005 cost of installation which includes: saw cutting, loop wire, connecting to the

splice box.


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that have large gaps or spaces between vehicles. If the gaps in traffic aregreater than the length of the detection area, then the gap timing would begreater than 1 or 2 seconds, depending on the speed of the vehicles.

Since long loop detectors perform the same function as extra long loopdetectors in detecting vehicle movements with large gaps, the majoradvantage of the shorter loop detectors are that if one is cut or brokenthen the others are still in working condition. Extension timing may have tobe increased by 0.5 second or more to cover the loss of a detector sensorunit.

A common application for long loop detectors are to use them in shorterdetection areas, extending back from the Stop Line between 18 and 24 m(60 and 80 feet). Shorter detection areas are used for left turn lanes and

side streets. For left turn lanes the detection area would start about 6 m(20 feet) before the Stop Line and extend to 21 m (70 feet) behind theStop Line, with a total detection area of 27 m (90 feet). Using two 12 m(40 foot) long loop detectors, with 3 m (10 foot) spacing, they wouldprovide a total detection area of 27 m (90 feet) per lane (see Figure 19).

The current cost of installing one 12 m (40 foot) long loop detector with alinear distance of 38 m (125 feet) would range between $1,900 and $2,300

per loop detector18. In extra long detection areas, three long loop

detectors can be used, with all three being connected in parallel to onedetector amplifier channel. Note that all loop detectors should beconnected in parallel and not in series in case one unit is cut or broken.

Short Loop Detectors:

Short loop detectors can range in length from between 1.8 to 6 m (6 feetto 20 feet) and are placed 0 to 3 m (0 to 10 feet) behind the Stop Line(seeFigure 20). These shorter loop detectors are designed to detect vehicles

during light traffic conditions with short queues.

The current cost of installing one 6 m (20 foot) long loop detection, 24 m

(80 feet), is about $2,000 per detector loop19. The cost of a short loopdetector is higher because of the increased number of linear feet of

VEHICLE DETECTORS


192006 prices

182006 prices.


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inductance wire required, the number of turns required to sense a vehicle,the labor involved in installing the wire, and the additional depth of thesaw cut trench for that wire. Shorter loop detectors should be located todetect at least two vehicles in a queue. The cost of installation variesaccording to the number of loop detectors being installed at one location.

FIGURE 20

SHORT LOOP DETECTORS

DETECTION AREA (1.8 M X 16.5 M)1.8 M X 1.8 M

3 M

DETECTION AREA (1.8 M X 15.3 M)1.8 M X 3.0 M

3 M

High Flux Detectors:

All the above loop detectors are designed to detect normal vehicles, but inmany locations the amplifier's sensitivity is set too low to detect bicycles,motorcycles, or high body trucks. If the amplifiers sensitivity is set tohigh, it could create cross talk or the detecting of vehicles in theadjacent lane. To resolve this type of problem, a High flux inductanceloop is installed. The Quadrupole and the Buster Detector type of loopdetectors are used to increase the flux of the detector loop.

Quadrupole Loop Detectors:

The Quadrupole Loop was designed as a small vehicle detectionsensor unit, and also to reduce false calls from adjacent lanes of travel.Each Quadrupole Loop has an extra saw cut down the middle of theloop detector sensing unit to intensify the inductance or flux in thecenter of the lane (see Figure 21). Quadrupole Loops of 6 to 12 m (20

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to 40 feet) in length can be used independently in short or longdetection areas, or at the Stop Line in a larger detection area.

The cost is one of its drawbacks, i.e., in a 12 m (40 foot) long loop thetotal saw cut would be 49 m (160 feet) [3 x 12 m + 12 m (3 x 40+40)+ lead to splice box]. The cost would range between $3,000 and

$4,000 per detector loop20.

FIGURE 21

QUADRUPOLE LOOP DETECTORS

Quadrupole Loop

SAW CUT

INDUCTANCE WIRE

Buster Detectors:

Buster detectors are used in front of a Large Detection Area, and theyare connected in series to the larger loop detector. The buster detector

is shaped in a 1.8 x 1.8 m (6 x 6 foot) square. The loop wire is placedin one continuous run around the larger loop and then in the smallerloop, with up to 6 additional turns. The small loop detector has ahigher inductance and larger flux lines (see Figure 22).

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202006 prices.


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FIGURE 22

BUSTER LOOPS

Small Loop Detectors:

Small loop detectors are used by many jurisdictions in Large DetectionAreas, with varying designs and configurations. Small loop detectorsuse the following configuration to reduce dead spots within the loopdetector area: a 1.8 x 1.8 m (6 x 6 foot) square; a 1.8 m (6 foot)

diameter loop; a 1000 mm (40 inch) diameter loop in a precastconcrete slab21 [used in Puerto Rico]; and a diamond shape 1.8 m (6feet) in width and 1.8 m (6 feet) in length. A number of these types ofshort loop detectors are placed at specific intervals within theDetection Area. The square, parallelogram, round, and diamond shapeloop detector sensing units are placed in the travel lane's DetectionArea at a spacing which varies from States to State (see Figure 23).

VEHICLE DETECTORS


21Traffic Detector Handbook, Second Edition, Institute of Transportation Engineers, page 135.


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Square Rectangle Diamonds

Hexagon Octagon Circle ChevronModifiedChevron

Quadrupole Triangle Parallelogram or Skewed

FIGURE 23

SMALL LOOP SHAPES

Source: ITE Detector Handbook

Rectangular and diamond shape detector loops are also used

separately and together where the Detection Area covers more thanone lane of travel, as shown in the following two New JerseyDepartment of Transportation Standard Detail Sheets (see Figures 24& 25).

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Detector Loop Wires:

To obtain the desired inductance or flux in each loop detector, the wire sizeand number of turns have to be determined. Different jurisdictions have

pre-designed the number of turns for each loop detector determined by thesize and use of the loop detector. Some jurisdictions require that eachDetection Area be computed and the calculations accompany the signal plansduring the review process. The following example shows the effects ofvariations by State on the design of the loop detector sensor units:

New York State DOT:

Long loop detectors are normally used in New York State, and they arerequired only on State highways. Local jurisdictions have the choice ofusing the NYS DOT standard design or their own design. Table 1 showsa standard 1.8 m (6 foot) wide loop with varying lengths, number of turnsper loop size, and depth of saw cut required. The Detection Area can becomprised of a number of long detector loops in one lane, connected to asingle amplifier channel.

TABLE 1

NYSDOT LOOP WIRING TABLENUMBER DEPTH OF

LOOP LENGTH HOW WIRED OF TURNS SAW CUTOne 1.8 m to 1.8 m (6' to 6') ONE LOOP - ONE AMPLIFIER CHANNEL 5 76 mm (3")

1.8 m to 3.0 m (6' to 10') Wired to a Single Amplifier Channel 4 70 mm (2-3/4")

3.6 m to 6.1 m (12' to 20') 3 64 mm (2-1/2")

6.7 m to 24.4 m (22' to 80') 2 57 mm (2-1/4")

27.4 m to 36.6 m (90' to 120') 1 57 mm (2-1/4")

Two 1.8 m to 1.8 m (6' to 6') TWO LOOPS - ONE AMPLIFIER CHANNEL 5 76 mm (3")

3.6 m to 6.1 m (12' to 20') Parallel Wired to a Single Amplifier Channel 4 70 mm (2-3/4")

6.7 m to 15.4 m (22' to 50') 3 64 mm (2-1/2")

16.8 m to 36.6 m (55' to 120') 2 57 mm (2-1/4")

Three 1.8 m to 1.8 m (6' to 6') THREE LOOPS - ONE AMPLIFIER CHANNEL 6 83 mm (3-1/4")

3.6 m to 6.1 m (12' to 20') Parallel Wired to a Single Amplifier Channel 5 70 mm (2-3/4")6.7 m to 10.7 m (22' to 35') 4 64 mm (2-1/2")

12.2 m to 24.4 m (40' to 80') 3 57 mm (2-1/4")

27.4 m to 36.6 m (90' to 120') 2 57 mm (2-1/4")

ALL LOOPS SHALL BE CENTERED IN LANE, 1.8 m (6 feet) WIDE, BY THE LENGTH SHOWN

Source: NYS DOT

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NYS DOT uses three types of methods to bring the inductance loop wireto the junction box located behind the curb line or on the grass shoulder.

The first is to bring a liquid tight conduit from the pavement to thejunction box (see Figure 26).

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The second type is to use aluminum splice boxes to connect the loopinductance wire to the Lead-In-Cable. An aluminum splice box is placedin the center of the lane between the two detector sensing units. TheLead-In-Cable is then pulled through a conduit to a splice box located onthe shoulder of the road, and thence to the controller cabinet (Figure 27).

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New Jersey State DOT:

NJ DOT requires two drawings for each traffic signal intersection. Oneshows signal configuration, signing, and detection areas. The second

drawing requires detailed electrical wiring diagrams, signal hardware forboth above and below ground, phasing, signal layout, and signalconfiguration. The second plan must contain the actual design and layoutof each loop detector, which must conform to the detection area on thefirst drawing. Table 2 shows the standard design of the loop detectorsand Figures 24 & 25 show the loop detector placement within thedetection area. NJDOTmay have changed the way they compute thedetector units.

TABLE 2

NJDOT APPROXIMATE NUMBER OF TURNS FORDIAMOND LOOPS RECTANGULAR LOOPS

1.8 m x 1.8 m (6' x 6') - 7 Turns 1.8 m x 1.8 m (6' x 6') - 6 Turns1.8 m x 2.4 m (6' x 8') - 6 Turns 1.8 m x 2.4 m (6' x 8') - 5 Turns

1.8 m x 3.0 m (6' x 10') - 6 Turns 1.8 m x 3.0 m (6' x 10') - 5 Turns






Notes:

Depth of saw cut trench shall be a minimum of 38 mm (1.5 inches) below top course of pavement or of sufficientsize to accommodate the number of conductors.

"Diamond" Loops are based on Rectangle measurements given in the loop detection schedule on plan sheets

for each location.

Source: NJ DOT Bureau of Electrical Engineering, Standard Detail Sheet #T-2090.

Commonwealth of Massachusetts DOT:

MA DOT requires the use of only 1.8 m x 1.8 m (6 ft x 6 ft) loop

detectors, 3 turns, with 2 m (10 foot) spacing between each loop detectorwithin the presence detection area. The signal plan requires a detectorschedule list, which contains the following items: Detector Number; Size;Number of Turns; Phase Number; Type of Operations (Presence orCalling); and Time Delay setting, as shown in Table 3.

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N/ACalling636 x 66A, 6B

7 sec.Presence336 x 63A, 3B, 3C

7 sec.Presence536 x 65B, 5D

7 sec.Presence536 x 65A, 5C

N/APresence436 x 64B, 4D

N/APresence436 x 64A, 4C

N/ACalling236 x 62A, 2B

N/APresence136 x 61B, 1D

N/APresence136 x 61A, 1C

DELAY TIMEOPERATIONSPHASE# OF TURNSSIZEDETECTOR NO

TABLE 3

MA DOT DETECTOR SCHEDULE

MA DOT also requires an attached work sheet that determines the total

inductance for each detector amplifier channel using the following formulas 22:

Loop Lead-In-Cable: LLIC = ILoop + Dist.LIC *LI /100

LLIC = Induction in Lead-In-CableIloop = Inductance of Loop [i.e. 1.8 m x 1.8 m (6' x 6') = 74uh]Dist.LIC = Distance of Lead-In-Cable from Loop to controllerLI/100 = Inductance of Lead-In-Cable = 22uh/100 feet (30 m)

Single Loop Connection: For a single loop connection with a 1.8 m

x 1.8 m (6' x 6') loop and a Lead-In-Cable of 60 m (200 feet), the

inductance would be:LLIC = 74uh + 200 x (22uh/100) = 74uh + 44uh = 118uh

Three Loops in a Series Connection: LL = L1 + L2 +Ln + LLIC

For 3 loops in a series LL = 74uh + 74uh + 74uh = 222uhTotal Inductance = LT = LL + LLIC = 44uh + 222uh = 266uh

Three Loops in a Parallel Connection: LT = LL + LLIC1

LL=

1L1

+1

L2+

1LN

=1

74+

174

+1

74=

374

3LL = 74; LL = 25uhLT = 25uh + 44uh = 69uh

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22ITE, Traffic Detector Handbook, Second Edition, pages 15 & 16.


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NEMA STANDARDS:

The National Electrical Manufactures Association (NEMA) specifies an

inductance range of 50 to 700uh for each detector loop with eachLead-In-Cable. MA DOT requires a separate 500 volts DC resistance test foreach detector loop and for each Lead-In-Cable, with a resistance of at least100 mega-ohms. In addition, MA DOT requires a maximum of 3 ohms per305 m (1000 feet) for the entire connection at the terminal board in thecontroller cabinet.

In designing a Detection Area and its loop detectors, one should consult thelocation jurisdiction to determine its design requirements.

CURB AND SHOULDER INSTALLATION:

Each loop detector must be connected to the Lead-In-Cable within the splicebox adjacent to the travel lane. This is accomplished by providing a saw cuttrench from the loop sensing unit to a liquid-tight flexible plastic conduitwhich leads into the nearest splice box. The path of the loop wire is from thesplice box to the connecting saw cut trench, which leads to the loop detectorsaw cut trench, around the loop detector saw cut a number of times, and then

back into the connecting saw cut leading to the splice box, continuously

without a splice.

The splice box entrance is located between 460 and 610 mm (18 and 24inches) below the ground. A 25.4 mm (1 inch) liquid-tight flexible conduit isplaced between the splice box and the connecting saw cut trench. A square issaw cut near the edge of pavement to provide an entrance way for the loopwire to be pulled into the conduit, which is then sealed with epoxy. Theflexible liquid-tight conduit is placed in a hole about 38 mm (1.5 inches) in

diameter drilled through the pavement. If a curb is adjacent to the travel lane,the conduit is placed under the curb section (see Figure 26). If there is nocurb, then a hole is drilled through the pavement into the subbase to thesplice box.

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Each loop wire lead must be twisted around itself to reduce cross talkbetween adjacent loop wires. The inductance loop has to be twisted 16 turnsper meter (5 turns per foot) (see Figures 28).

When using a number of small loop detectors in a single lane detection area,the loop wires can be connected in parallel or they can be connected in series(see Figure 29).

The inductance loops can be connected to the Lead-In Cable in a number ofdifferent locations:

1. at the edge of the travel way,2. in the splice box, or3. in the controller cabinet.

At the Edge of the Travel Way:

All loop wires are spliced together at the beginning of the liquid-tightconduit to a single Shielded-Lead-In-Cable (see Figure 28), and thensealed with exopy. This presents problems when one detector loop failsor is cut. Because all the wire connections are sealed together with exopyinto one block, every loop detector sensing unit then has to be replaced.

In the Splice Box:

Each set of inductance loop wires are drawn through the liquid-tightconduit and then connected to the Shielded-Loop-In-Cable within theJunction Box. Starting at the liquid-tight conduit, each pair of inductanceloop wires is twisted around itself, at a rate of 16 turns per meter (5 turnsper foot), to reduce cross talk from other wires in the same conduit. Aseparate liquid-tight conduit can be used and placed at a distance fromone another, to provide for easier future maintenance in case one

inductance loop wire fails or is cut.

In the Controller Cabinet:

Each pair of twisted inductance loop wires are connected directly to thecontroller cabinet. The preferred method is to provide a separate

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Shielded-Loop-In-Cable from each loop detector to the controller cabinet.This presents problems because of the limited capacity of each conduit,thus requiring additional conduit, and increasing the total cost of theproject. Replacement of the Shielded-Loop-In-Cable in a conduit reducesfuture maintenance costs.

TIMINGS FOR DELAY DETECTORS:

Many lane detector amplifiers are equipped with a time delay feature. A timedelay circuit is used to delay the signal from being transmitted to the controller. Thetime delay feature requires that a vehicle must remain within the detection area orover a particular detector loop for a preset period of time upon actuation, afterwhich a signal is sent from the amplifier to the controller showing that a vehicle isbeing detected. Time Delay Amplifiers are used in three types of detection areas:Queuing Detection Areas, Right Turn On Red Movement Detection Areas, and LeftTurn Movement Detection Areas.

Queuing Detection Areas:

The queuing detection areas are located back from the intersection todetect the length of queue from the Stop Line. The time delay feature isused to detect a vehicle standing within the detection area for a preset

period of time, usually 2 to 3 cycle lengths or 1 to 3 minutes. This type ofqueuing detector is used on exit ramps which have a very heavy trafficvolume during certain shopping or peak highway hours. Its main objectiveis to reduce traffic backup, and/or to prevent blocking a lane of traffic onan expressway or parkway. Another place that this type of queuingdetector is used is at the exit ramps from stadiums or convention centers.

Right Turn On Red Movement Detection Areas:

On side streets where Right Turns On Red are permitted, a time delaydetector amplifier would be useful, since it can delay the detection callfrom 5 to 30 seconds. This permits vehicles to make the Right Turn OnRed without forcing the traffic signal to change phases. A time delayadded to the presence detection for a right lane control will reduce thenumber of side street calls, especially during off peak hours.

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Left Turn Movement Detection Areas:

Time delay detector units are used in left turn lanes when the protected left

turn movement follows the through movement phases, which is known asa Lagging Green Phase. The left turn detection area usually extends intothe intersection, and the time delay is used to permit vehicles to make aleft turn under the permissive Green phase (Green Ball Indication) beforeregistering the call with the controller.

It should be noted that the time delay can be either in the detector amplifier orit can be part of the microcomputer controller, depending on manufacturer.

The use of vehicle detection units at a intersection are essential for the safeand efficient operation of a traffic signal. There are a variety of such units, and theirtype, placement, and size is based upon the standards promulgated by the Statesand local agencies having jurisdiction.

END

VEHICLE DETECTORS

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