1. CE 7670: Advanced Traffic Signal Systems
DETECTORS
CE 7670: Advanced Traffic Signal Systems
Tapan K. Datta, Ph.D., P.E.
Winter 2003

2. Detectors Basic function Two types
to detect the presence of vehicles or pedestrians
Two types
Presence detector
Passage detectors

3. Presence detector
generate output signals as long as the detected object is in the influence zone
long loops (6 feet X feet) are used, OR
series of (6 feet X 6 feet) loops

4. 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

5. Group of 6’ X 6’ small loops-presence Better for maintenance/repair
Set back farther to allow lag time to calculate signal timing
Group of 6’ X 6’ small loops-presence
Better for maintenance/repair
140’
80’
Driveway Traffic Detection (6’ X 8’)
To pick up driveway activity

6. One long loop
(70’ to 80’ long)

7. 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

8. Disadvantages
Suppose this vehicle was able to turn left during the permissive phase, the detector still remembers that a call was made and will give an exclusive left turn phase, even if the vehicle is no longer present.
If this vehicle turns right on red, the detector still remembers that a call was made and will give a green phase, even if the vehicle is no longer present.
The green time is essentially wasted

9. Example of Actuated Controller Settings
* one unit extension per phase

10. 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

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

12. 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

13. 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

14. High Speed Approach (V 35 mph)

15. 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

16. 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

17. Small detector at the end of option zone
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

18. Controllers with non-locking detection memory
Cannot detect properly with only one detector
This detectors registers cars passing through
70’
384’
Long Detector
This detector will detect all cars have passed through the queue

19. Area detection is used to identify the number of vehicles
Verify call
First call
25’
254’
384’
Area detection is used to identify the number of vehicles

20. TYPICAL DETECTOR DESIGN
PEDESTRIAN PUSHBUTTON
FOR EAST – WEST CROSSING
TWO SETS OF LOOPS
FOR 30 – 35 MPH SPEEDS
NOTE:
FOR THIS APPROACH, DETECTION FOR THE RIGHT
TURN ONLY LANE WAS NOT REQUIRED
140 FT
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
DETECTION ZONE
LONG LOOP FOR PRESENCE DETECTION
140 FT
PEDESTRIAN PUSH BUTTON (AT CURB)
FOR NORTH – SOUTH CROSSING
220 FT
TWO SETS OF LOOPS FOR 40 MPH SPEEDS

21. 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.

22. Multiple Point Detection System (Speed <= 55mph)
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)

23. Multiple Point Detection System (Speed <= 55mph)
LOGIC CARD TO
RECOGNIZE SPEED
TO CONTROLLER ASSEMBLY
55 MPH
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)

24. Vehicle Detection and Surveillance
Intrusive technologies
Inductive Loop Detectors
Fiber Optic Sensors
Magnetic Sensors
Piezoelectric Sensors
Pneumatic Road Tube
Weigh-in-Motion (WIM)

25. Principal Components of Inductive Loop Detector

26. Vehicle Detection and Surveillance
Non-Intrusive technologies
Infrared Sensors
Microwave Radar
Passive Acoustic Array Sensors
Ultrasonic Sensors
Video Image Sensors

27. Infrared Sensors

28. Strengths and Weaknesses of Various Sensor Technologies

29. Strengths and Weaknesses of Various Sensor Technologies

30. Strengths and Weaknesses of Various Sensor Technologies

31. 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

32. Non-invasive Detection
You do not have to alter the pavement to use this system
Types
Radar detectors
Sonic detectors
Video Image Processing (VIPs)
Special purpose detectors

33. 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

34. 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

35. 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

36. 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

37. 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

38. Video Image Detection System (VIDS)
Algorithm for generating both presence and passage detection and speed
Problems:
Shadows
Lighting (illumination) change
Reflection (camera difficulties)

39. California Tests Camera positioning Traffic volumes
Different weather and lighting conditions

40. Summary of VIDS and VIPS
Entire intersection can be surveyed using one camera
Can have remote or automatic control

41. 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

42. Fire truck sends out signal to controller
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

43. Bus Detectors Radio frequency transmitters Loop Detectors
Curbside unit

44. 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

45. Vehicle Presence Detection at an Intersection

46. 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

47. Pedestrian Signals
Most are demand actuated
Radio detectors

48. COMMUNICATION CONCEPTS

49. 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

50. Freeway Control System requires communication between field devices, such as:
Vehicle Detectors
Ramp Meters
Changeable Message Sign (CMS)
Highway Advisory Radio

51. 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

52. 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

53. Lease Local Telephone Services
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

54. 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

55. Magnitude A a Time (t) 1 T Period (T) a = A sin (2f*t) Amplitude
phase angle
relative to
the origin 1 T
Frequency f=
Period (T)
a = A sin (2f*t)

56. 1 Hertz (Hz) = 1 cycle/second
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

57. 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 = receiver power
Ptr = transmitter power Pr
Ptr

58. Interconnection
A communication network must address the interconnection architecture among data receivers and data sources
Most direct is point to point

59. Central Facility or Master Unit
Point to Point 1 2
Central Facility or Master Unit 3 4 5

60. Central Facility or Master Unit
Multi-drop Scheme 1 2
Central Facility or Master Unit 3 4 5

61. 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

62. Typical Telephone Channel Bandwidth
Received Signal
Transmitted Signal
1,000
2,000
3,000
Frequency, Hz

63. Modulation and Demodulation
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

64. Modulation and Demodulation
OUTPUT
RECEIVER
INPUT
MODULATOR
MODULATION
FILTER 1
CHANNEL
NOISY
OUTPUT
SIGNAL
FILTER 2
DEMODULATION

65. Frequency Modulation
Signal

66. 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

67. Amplitude Modulation
Signal
Time

68. 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

69. Transmission of Mixed Keyed Tone
INPUT
NO. 1
KEYED TONE
MODULATOR
MIXER
CHANGER
INPUT
NO. 2
KEYED TONE
MODULATOR
OUTPUT
NO. 1
FILTER
NO. 1
DEMODULATOR
OUTPUT
NO. 2
FILTER
NO. 2
DEMODULATOR

70. Phase Modulation
Time

71. 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

72. Typical Traffic Control Communication System
SIGNAL UNIT
MODEM
MODEM
DIRECT CONNECTION OR
LOW-SPEED LINE
DATA
CONCENTRATOR
CONTROLLER
MODEM
MODEM
CONDITIONED VOICE
GRADE LINE
MODEM
COMMUNICATIONS
CONTROL UNIT
MODEM
OTHER
INPUT LINES
CENTRAL COMPUTER

73. Typical Intersection Controller CommunicationsD4
PB2 D3 D1
PB1
LOCAL
CONTROL
UNIT D2
Di = Loop Detector
PBj = Ped. push button