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					                    DETECTORS



CE 7670: Advanced Traffic Signal Systems


              Tapan K. Datta, Ph.D., P.E.


                            Winter 2003
Detectors

   Basic function
       to detect the presence of vehicles or
        pedestrians
   Two types
     Presence detector
     Passage detectors
   Presence detector
     generate output signals as long as the
      detected object is in the influence zone
     long loops (6 feet X 20-70 feet) are
      used, OR
     series of (6 feet X 6 feet) loops
   Passage detectors
     generates a short duration output
      signal based on arrival or motion of the
      detected vehicle
     small loops are used (6 feet X 6 feet)
        • may be square, diamond, octagon, or
          circles
Set back farther to
allow lag time to
calculate signal
timing




         140’                                 Group of 6’ X 6’
                                              small loops-presence
   80’                                        Better for
                                              maintenance/repair




                      Driveway Traffic
                      Detection (6’ X 8’)
                      • To pick up driveway
                      activity
One long loop
(70’ to 80’ long)
     Detector/Controller
     Configurations
   Locking Detection Memory (LDM)
      • a vehicle calls for a green by its presence,
        which is remembered and held by the
        controller.
      • The controller holds the ‘call’ after the
        vehicle leaves the detection area, until it
        has been satisfied by the green phase
      • Once the need is registered,no other phase
        will be given a green until the phase which
        was called receives a green
      • associated with point or small detection
        areas
                  Disadvantages


Suppose this vehicle
                            If this vehicle turns right on
was able to turn left
                            red, the detector still
during the permissive
                            remembers that a call was
phase, the detector still
                            made and will give a green
remembers that a call
                            phase, even if the vehicle is
was made and will give
                            no longer present.
an exclusive left turn
                            The green time is essentially
phase, even if the
                            wasted
vehicle is no longer
present.
    Example of Actuated
    Controller Settings
Approach Detector Initial      Unit      Min
 Speed   Setback Interval   Extension   Green
 (mph)    (feet)  (sec)       (sec)     (sec)
   15      77       8.5       3.5        12


  20       103     10.5       3.5        14


  25       120     10.5       3.5        14


  30       120     10.5       3.5        14

  * one unit extension per phase
Non-Locking Detection Memory

    Waiting call is dropped or forgotten
     by the controller as soon as the
     vehicle leaves the detection area
        detectors are 60’ X 50’
    Is desirable for left turn lanes with
     permitted/protected left turn phases
  Non-Locking Detection Memory

                           Use longer detector and
                           place detector close to
With non-locking
                           the stop bar in order
detection memory, if
                           NOT to trap vehicles
the vehicle is able to
                           waiting to turn left on
turn left during the
                           the permissive phase
permissive phase, a
green for the exclusive
                           Also use amber an all-
left turn phase will NOT
                           red intervals to protect
be given and will be
                           vehicles that have not
SKIPPED.
                           completed their left
                           turn during the
                           permitted phase, but
                           are outside of the
                           detection area.
        Loop Length Design

 Suppose allowable gap between vehicles
  = 3 seconds
 Length of vehicle = 18 feet


   L = length of detection
   L = 1.47V*(3 - V1) - 18
    
where: V1 is the selected vehicle interval=1 sec
       V is the approach speed
        Loop Length Design

 Assume V = 30 mph
 L =1.47(30)*(3 - 1) - 18 = 70.2 feet


   If you assume a longer vehicle
       the loop length will be smaller
High Speed Approach (V 35 mph)
   Approach       Distance from
    Speed     intersection in feet for
    (mph)     probability of stopping
                10%           90%

     35       102 feet      254 feet

     40       122 feet      284 feet

     45       152 feet      327 feet

     50       172 feet      353 feet

     55       234 feet      386 feet
 The distance from the intersection
  for 90% probability of stopping will
  give the distance at which the loop
  should be placed
 The distance from the intersection
  for 10% probability of stopping
  means that 10% of the vehicles will
  be able to stop if they see a yellow
  light, given the distances above
         Controllers with Locking
           Detection Memory
   Fully Actuated Controllers
       Multiple small loops
   This design assumes an emergency stop on
    a dry road
       The first detector is not placed far enough
        upstream to give adequate option zone
        protection
       If the first detector is pushed back, then the
        allowable gap is so long that it will frequently
        max out
   Semi-actuated
     No detector on high speed and for
      major road
     generally uses green extension system

     Volume Density Controllers

        Small detector at the
         end of option zone




                  Option Zone
    Controllers with non-locking
        detection memory
       Cannot detect properly with only
        one detector
 This detectors
 registers cars
passing through




                  384’         70’


                                  Long Detector
                         This detector will detect all cars
                         have passed through the queue
             Verify call
First call




                                  25’
                           254’

                     384’




   Area detection is used to identify the number of vehicles
                                     TYPICAL DETECTOR DESIGN
                                                                      PEDESTRIAN PUSHBUTTON
 TWO SETS OF LOOPS                                                    FOR EAST – WEST CROSSING
 FOR 30 – 35 MPH SPEEDS


                                                                       NOTE:
                                                                       FOR THIS APPROACH, DETECTION FOR THE RIGHT




                            140 FT
                                                                       TURN ONLY LANE WAS NOT REQUIRED
 DIAMOND LOOPS FOR
 PRESENCE DETECTION




                                      80 FT
        MAJOR DRIVEWAY                                                           TWO 6FT X 20FT LOOPS
                                                                                 FOR PRESENCE OPERATION



TWO 6 FT X 8 FT SYOPLINE LOOPS
TO PICK UP DRIVEWAY ACTIVITY

                                                                                    10 FT




 FOUR 6FTX20FT LOOPS FOR
 PRESENCE OPERATION SPACED
 10FT APART FOR 54 FT                                                 LONG LOOP FOR PRESENCE DETECTION
 DETECTION ZONE
                                                  140 FT

                                                           220 FT
PEDESTRIAN PUSH BUTTON (AT CURB)
FOR NORTH – SOUTH CROSSING




                                                                    TWO SETS OF LOOPS FOR 40 MPH SPEEDS
         Detection in Left-Turn Lanes

                                         LENGTH VARIES



                                         50 FT              26 FT       13 FT




                                                 80 FT



                                                    STOP LINE


NOTES:

1. LOCATE STOPLINE IN RELATION TO CROSS STREET TURNING RADIUS.

2. MEASURE BACK 80 FT FROM STOPLINE TO ESTABLISH BACK LOOP.

3. MEASURE 50 FT TOWARD STOPLINE TO ESTABLISH FRONT OF BACK LOOP.

4. ALLOW 2 FT GAP BETWEEN LOOPS AND MEASURE 28 FT TO STOPLINE.

5. FRONT LOOP SHOULD BE WITHIN 13 FT OF CROSS STREET TRAFFIC LANE.
   FRONT LOOP SHOULD BE QUADRUPOLE DESIGNED TO DETECT SMALL VEHICLES.
 Multiple Point Detection System
 (Speed <= 55mph)
45 MPH
                 85 FT
                                                                      LOOPS ASSUMED TO BE 6 FT WIDE
                                  145 FT            225 FT




50 MPH
         60 FT
                         115 FT
                                           185 FT
                                                             267 FT




55 MPH
           85 FT

                                  145 FT
                                                    225 FT
                                                                        325 FT



           CONTROLLER DATA: DETECTOR MEMORY: LOCKING

           MINIMUM GREEN : 8 ½ TO 14 SEC (DEPENDENT ON TRAFFIC VOLUMES, SPEED AND NUMBER OF TRUCKS)
Multiple Point Detection System
(Speed <= 55mph)

                     LOGIC CARD TO                                   TO CONTROLLER ASSEMBLY
 55 MPH              RECOGNIZE SPEED
                                                            REGULAR DETECTOR UNIT NO. 2
                  SPECIAL DETECTOR UNIT NO 1



          54 FT      49 FT       64 FT          76 FT                76 FT



          60 FT
                      115 FT
                                  185 FT
                                                 267 FT

                                                                     349 FT




  CONTROLLER DATA: DETECTOR MEMORY: LOCKING

 MINIMUM GREEN : 8 TO 14 SEC (DEPENDENT ON TRAFFIC VOLUMES, SPEED AND NUMBER OF TRUCKS)
Vehicle Detection and
Surveillance

   Intrusive technologies
     Inductive Loop Detectors
     Fiber Optic Sensors

     Magnetic Sensors

     Piezoelectric Sensors

     Pneumatic Road Tube

     Weigh-in-Motion (WIM)
Principal Components of
Inductive Loop Detector
Vehicle Detection and
Surveillance

   Non-Intrusive technologies
     Infrared Sensors
     Microwave Radar

     Passive Acoustic Array Sensors

     Ultrasonic Sensors

     Video Image Sensors
Infrared Sensors
Strengths and Weaknesses of Various
Sensor Technologies
Strengths and Weaknesses of Various
Sensor Technologies
Strengths and Weaknesses of Various
Sensor Technologies
Invasive Detector
   Small coil of wire embedded in
    protective housing and installed
    under the surface of the roadway
     Electronic amplifiers required
     Does not work as a presence detector

     Minimum speed 3 to 5 mph
Non-invasive Detection

    You do not have to alter the
     pavement to use this system
    Types
    1.   Radar detectors
    2.   Sonic detectors
    3.   Video Image Processing (VIPs)
    4.   Special purpose detectors
Radar Detection

  Operates on the Doppler effect , as a
   microwave signal is emitted by the
   detector and reflected back at the
   detector by approaching vehicles
  Transmits microwave energy toward
   the roadway from the detector’s
   antenna
  Senses the frequency of the microwave
   change in the reflected energy and
   obtains vehicle speed from the signal
Radar Detection

   Two types of radar units
       Antenna and detection electronics is
        fabricated as a single unit and located
        over the roadway




       Separate antenna and detection
        electronics
    Radar Detectors

 Detectors require FCC approval
 Newly developed detectors
     Detect moving vehicles as well as
      stopped vehicles
     Covers single or multiple lanes

     Provides digital and instantaneous
      speed information
Sonic Detectors
  Transmit pulses of ultrasonic energy
   through transducers toward the
   roadway
  Located over the roadway

  Presence of vehicle causes these
   ultrasonic beams to reflect back to the
   transducers and it:
     • Senses the reflected wave
     • Converts to electrical energy
     • Relays the energy to a transceiver which
       provides vehicle presence information
        Video Image Processing
        System(VIPS)
   Research was conducted in the mid
    1970’s by the University of Minnesota
     Camera, digitizer, formatter, interface,
      electronics, microprocessor and power
      supply was used
     Theory: one camera to replace numerous
      detectors
   1970’s and 80’s
       Japan, UK, Germany, Sweden and France
        used VIPs successfully
Video Image Detection
System (VIDS)
 Algorithm for generating both
  presence and passage detection and
  speed
 Problems:
     Shadows
     Lighting (illumination) change

     Reflection (camera difficulties)
California Tests

   Camera positioning
     Traffic volumes
     Different weather and lighting
      conditions
   Summary of VIDS and VIPS
                         Entire intersection
                         can be surveyed
                         using one camera

                         Can have remote
                         or automatic
                         control
Special Purpose Detectors

 Used in ITS
 Most technology came out of defense
  research in the 1950’s, 60’s and 70’s
     Automatic Vehicle
      Identification/Electronic Toll
     Bus and emergency vehicle pre-emption

     Over-height vehicles for underpass

     Over-weight vehicles

     Buses
Signal Preemption
                                                Fire
                                               Station




 *                                    *
                                          Fire truck sends
                                            out signal to
                                              controller
     Controller will either cut red
     phase short or will lengthen
     the green phase to allow
     the firetruck through the
     intersection
Bus Detectors

 Radio frequency transmitters
 Loop Detectors
 Curbside unit
    Video Image Processor

   Used in vehicle presence
    detection and other
    purposes
   Video signal from the
    monitoring camera is the
    input to this unit
   Video detectors may be
    used to replace loop
    detectors
Vehicle Presence Detection
at an Intersection
Automatic Vehicle
Identification System (AVI)
 Used for toll collection
 Vehicle mounted transponder or tag
  where vehicles are identified
  electronically and tag amount is
  reduced at every station where its
  used
 Roadside reader unit
 Computer and data storage unit
   Pedestrian Signals
       Most are demand actuated
   Radio detectors
COMMUNICATION
     CONCEPTS
 Communication Concepts

Communication transfers information
from one location to another
     Distributed Traffic Control System
      requires communication between
      • Local Controller to Master Controller or to a
        computer
      • Detector to Controller to Signal Equipment
   Freeway Control System requires
    communication between field
    devices, such as:
      • Vehicle Detectors
      • Ramp Meters
      • Changeable Message Sign (CMS)
      • Highway Advisory Radio
   Video Monitoring requires
    communication between video cameras
    and control center command data to the
    camera for Pan, Tilt and Zoom (PTZ)
    uses a separate communication link
Communication Link and
Signaling Techniques
 Path over which information travels
  is called a link or channel
 Media used in traffic control systems
  include:
       Cable
        • twisted pair
        • coaxial
        • fiber optic
   Wireless
    • radio networks at various frequencies
    • point to point directional microwave radio
    • air path optics
   Lease Local Telephone Services
    • voice grade lines
    • digital communication services
 Community Antenna Television (CATV) cable
  channels
 Switched Telecommunication Services
    • dial up wire line service
    • cellular phone service
    Bandwidth
   The information carrying capacity of
    a link depends on:
     the time the link is available
     the bandwidth of the link (channel)
      describes the range in sinusoidal
      frequencies that can transmit through
      the link without significant weakening
      or distortion
            Magnitude
Amplitude




            A                     a

                        (2f*t)       phase angle
                                                         Time (t)
                                      relative to
                                      the origin
                                                        Frequency   f= 1
                                           Period (T)                  T
                          a = A sin
                         (2f*t)
 A Wider Channel Bandwidth allows
  more information to pass along the
  link in a given period of time
 Telephone channel can send 300 to
  330 Hz. It has a bandwidth of 3000
  Hz or 3 KiloHZ
     1 Hertz (Hz) = 1 cycle/second
 A video camera requires a
  bandwidth of 4 megahertz
Attenuation or Weakening of
          Signals
 Attenuation or Weakening of Signals
  represents a decrease in magnitude
  or power of a signal in transmission
  between points
 Decibal (dB) expresses signal strength

      dB = 10 log 10 Pr
                      Ptr
 Pr = receiver power
 Ptr = transmitter power
     Interconnection
   A communication network must
    address the interconnection
    architecture among data receivers
    and data sources

   Most direct is point to point
Point to Point
              1
                      2


 Central
                          3
Facility or
 Master
   Unit
                      4

                  5
Multi-drop Scheme
              1
                      2


 Central
                          3
Facility or
 Master
   Unit
                      4

                  5
    Communication Channel
      Capacity Concepts
   Time concept
    • Time during which the channel is available
   Bandwidth concept
    • Bandwidth of the channel with a range of
      sinusoidal frequencies that can be
      transmitted through the channel without
      significant weakening or distortion
    • Frequency is expressed in Hertz or Hz,
      which is equal to one cycle per second
    • Wider the bandwidth, greater the capacity
      of a channel
             Typical Telephone
             Channel Bandwidth



 Received Signal
Transmitted Signal




                     0   1,000                  3,000
                                        2,000
                             Frequency, Hz
                Modulation and
                Demodulation
   Modulation
       Transforms the signal into a form suitable for
        the transmission system
       Transmitting binary data requires at least 2
        types of modulated signals
   Demodulation
       Reconstructs the original signal at the
        receiving end
   MODEM (Modulator- DEModulator)
       Devices at each of the channel
              Modulation and
              Demodulation

                                                       OUTPUT
INPUT                                       RECEIVER
        MODULATOR



                       MODULATION


                                 FILTER 1

             CHANNEL

                                 FILTER 2


                       DEMODULATION
Frequency Modulation

Signal
    Frequency Modulation

 One of several frequency modulation
  (FM) systems is commonly utilized
 Varying frequencies represent different
  signal values
 Can use either 2 or 3 discrete signaling
  frequencies
    Amplitude Modulation

Signal




                           Time
     Amplitude Modulation

   Amplitude Modulation (AM) changes the
    strength of the carrier using signals of
    large amplitude and smaller amplitude
   Tone keyeing – type of modulation,
    where the sinusoidal carrier is cut on and
    off to represent two binary values
   Various keyed tone signals are mixed
    before transmission, and the composite
    signal is sent over the channel
            Transmission of
            Mixed Keyed Tone
INPUT
NO. 1   KEYED TONE
        MODULATOR
                                   MIXER            CHANGER
INPUT
NO. 2   KEYED TONE
        MODULATOR


          OUTPUT                           FILTER
          NO. 1
                     DEMODULATOR
                                           NO. 1


          OUTPUT                           FILTER
          NO. 2      DEMODULATOR
                                           NO. 2
Phase Modulation


                   Time
    Phase Modulation

 A close relative of Frequency Modulation
 Phase of the carrier signal w.r.t. some
  reference is varied to represent differing
  signal values
 More difficult to demodulate than
  Frequency Modulation, since the
  reference by which phase changes are
  measured, must be generated at the
  receiver
      Typical Traffic Control
      Communication System

SIGNAL UNIT                             MODEM     MODEM



CONTROLLER    MODEM                             DATA
                               MODEM
                                            CONCENTRATOR



                                             MODEM


  COMMUNICATIONS      MODEM
   CONTROL UNIT

                                 OTHER
                              INPUT LINES
  CENTRAL COMPUTER
    Typical Intersection
    Controller Communications
                          D
                          4
                                  PB2




                                          D
                                          3


                     D
                     1




                    PB1                  LOCAL
                              D
                                        CONTROL
Di = Loop Detector            2           UNIT
PBj = Ped. push button

				
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