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Smart Traffic Signals
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Motivation
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  • The Idea:
    • Use "demand for service" in addition to signal timings to improve the performance of traffic signals
  • Measures of Performance:
    • Delay (e.g., shorten phases late at night)
    • Throughput
    • Safety (e.g., provide a safe phase termination for high-speed movements by reducing the chance of being in the indecision zone)
Variants:
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  • Semi-Actuated Control:
    • Uses detection only for the minor movements
  • Fully-Actuated Control:
    • All movements are detected and all phases are actuated
Detecting the "Demand for Service"
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  • Pedestrians:
    • Principally user-activated push buttons
  • Vehicles:
    • Inductive loops
    • Digital cameras equipped with image analysis algorithms
    • Acoustic detectors
Operating Modes
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  • Detectors:
    • Pulse Mode - point detection that results in a short "on" pulse
    • Presence Mode - time-interval detection that starts with the arrival of the vehicle and ends with its departure
  • Controller Memory:
    • Locking - the first actuation on a channel during the red interval triggers a call for service and is retained until the assigned phase is serviced (regardless of whether there are demanders waiting to be served)
    • Non-Locking - the actuation is recognized only during the time the demander is present (hence can't be used with pulse detection)
Detection for High-Speed (> 35mph) Approaches
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  • Visualization of the Indecision Zone:
    • images/intersection_indecision-zone.png
  • Defining the Indecision Zone:
    • Distance from stop line
    • Travel time to stop line (e.g., 85% of drivers stop if they are more than 3 secs from the stop line)
    • Stopping sight distance (SSD)
Detection for High-Speed Approaches (cont.)
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  • Position of Upstream Detector:
    • Typically located at the start of the indecision zone
  • Number of Detectors:
    • Typically 3 or 4
Common Designs - Basic
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  • Characteristics:
    • Detection zone at the stop line
    • Detectors in presence mode
    • Non-locking memory
  • Guidelines:
    • Detection zone should not be shorter than 20 feet
    • A queued vehicle should not be able to stop between the trailing edge of the detection zone and the crossroad
Common Designs - Volume-Density
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  • Characteristics:
    • 6-foot detector upstream of the stop line (determined by the desired maximum allowable headway)
    • Detectors in presence mode
    • Locking memory
  • Guidelines:
    • Not suitable for large turning movements
Common Designs - Multiple Detector
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  • Characteristics:
    • One 6-foot detector at 60 feet (this is the last detector)
    • One 6-foot detector at the upstream safe stopping distance (this is the first detector)
    • Detectors in presence mode
    • Non-locking memory
  • Guidelines:
    • Suitable for high percentage of turning movements because these vehicles "gap out" as they slow to make the turn
There's Always More to Learn
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