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Traffic Signals
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Terminology
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  • Movement:
    • A pair of directed street segments (e.g., straight, right turn, left turn)
  • Phase:
    • The timing unit associated with one or more movements with the same initial directed street segment (e.g., northbound straight and northbound right)
Terminology (cont.)
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  • Interval:
    • A duration of time during which the signal indications do not change (e.g., a pedestrian phase contains three intervals: Walk, Flashing Don't Walk, Solid Don't Walk)
  • Cycle:
    • The sequence of signalizations for all movements at an intersection
Users and Movements
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  • Users:
    • Vehicles
    • Pedestrians
    • Bicyclists
  • Types of Movements:
    • Protected - no conflict is possible
    • Permissive - the movement can be made only after yielding
An Example 4-Leg Intersection
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Vehicular Movements are Shown with Purple Arrows
Phases are Numbered in Gold

images/four-leg-intersection_typical.png
Ring-and-Barrier Diagrams
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  • Purpose:
    • Organizing phases
  • Components:
    • Rings illustrates sequential movements
    • Barriers avoid conflicting movements (e.g., separate N-S movements from E-W movements)
Ring-and-Barrier Diagrams (cont.)
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An Example for a 4-Leg Intersection

images/ring-and-barrier_typical.png
Left-Turn Options
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  • Permissive:
    • Reduces overall delay but also reduces safety
  • Protected:
    • Reduces delay for turning vehicles but increases overall delay
  • Prohibited:
    • Reduces conflicts but increases delay (overall and for turning vehicles)
Left-Turn Options (cont.)
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Ring-and-Barrier Diagram for Protected Left-Turn Phasing

images/ring-and-barrier_protected-left.png
Nerd Humor
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http://imgs.xkcd.com/comics/long_light.png
(Courtesy of xkcd)
Vehicular Green Interval
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  • Minimum Green:
    • The least amount of time that a green signal indication will be displayed for a movement (which can be based on driver expectancy, pedestrian crossing times, and/or queue clearance)
  • Maximum Green:
    • The maximum amount of time that a green signal indication can be displayed in the presence of conflicting demand
Vehicular Change and Clearance Intervals
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  • Yellow Change:
    • Based on driver perception+reaction time and the distance needed to safely stop or to travel safely through the intersection
    • Permissive - A driver can enter the intersection during the entire yellow interval and be in the intersection during the red indication as long as the vehicle entered the intersection during the yellow interval
    • Restrictive A - A vehicle may not enter an intersection when the indication is yellow unless the vehicle can clear the intersection by the end of yellow
    • Restrictive B - A vehicle may not enter an intersection unless it is impossible or unsafe to stop
  • Red Clearance:
    • An interval at the end of the yellow change interval during which the phase has a red-signal display before the display of green for the following phase
Systems of Traffic Signals
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  • Isolated:
    • Each traffic signal operates in isolation
  • Coordinated:
    • Signals at multiple intersections are synchronized to enhance the operation of one or more directional movements in a system
Systems of Traffic Signals (cont.)
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An Example Time-Space Diagram for Coordinated Signals

images/traffic-signal_coordinated_time-space.png
Systems of Traffic Signals (cont.)
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  • Some Existing Optimization Packages:
    • Synchro
    • PASSERTM II
    • TRANSYT-7F
  • Complicating Factors:
    • Heavy side street volumes
    • Turn bay interactions
    • Oversaturated conditions
There's Always More to Learn
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