Behind the Scenes: Traffic Signal Timing

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Behind the Scenes Traffic Signal Timing
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Traffic Signal Control

• Why put in a traffic signal?
• To improve safety
• To improve traffic flow
• Warrants
• Traffic counts
• Engineering study

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Types of Traffic Signals

• Fixed Time
• Uses historical data
• Less efficient

• Traffic Actuated
• Responds to traffic
• More efficient

• Non-coordinated
• Far away from other signals
• Operates independently

• Coordinated
• Close to other signals
• Operates with other signals

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Traffic Signal Equipment

• Traffic signals
• Vehicle and pedestrian detectors
• Controller cabinet
• Controller

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Traffic Signals
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Vehicle Detectors
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Controller Cabinet
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Traffic Signal Controller
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Traffic Signal Settings

• Objective
• Minimize vehicle stops
• Minimize delay
• Speak the language
• Phases
• Cycle length
• Bandwidth
• etc...

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Phases

• Phase
• A traffic signal indication that controls the
  movement of one or more traffic movements.
• Phase Numbers

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Phase Times

• The phase time is the number of seconds that the
  phase is on.
• While a phase is on, it goes through 3 intervals
• Green (cannot be less than 5 seconds)
• Yellow (4 seconds)
• Red (1 second)
• While a phase is not on, it stays in ?Red.

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Non-Conflicting Phases

• Phases that can run at the same time without
  causing problems in the intersection - e.g., two
  vehicles trying to go to the same place.
• Example phases 1 5 or phases 3 8

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Phase Sequence

• Fixed sequence in which phases follow each other.
• E.g. 15?16?26?38?47?15

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7
6
1
5
2
3
8
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Cycle Length

• Time it takes to cycle through the phase
  sequence.
• All movements have a chance to go through the
  intersection during a cycle.
• Rule of Thumb Minimum 60 seconds
• Rule of Thumb Maximum 180 seconds.

15?16?26?38?47?15 ?16
1 cycle
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Time Budget

• Have to budget cycle time to phases.
• E.g. 15?16?26?38?47

1 Cycle
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Traffic Volumes
Volumes in Vehicles Per hour Assume one lane in
each direction.
150
350
800
400
200
800
450
150
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Easy Timing of a Traffic Signal

• 1. Add up the total number of vehicles that go
  through the intersection in each lane in an hour.
• 2. Select the critical volume for each phase.
  Critical volume is the largest lane volume for a
  phase that must go through the intersection
  during the hour.
• 3. Calculate the total critical volume by adding
  the phase critical volumes.
• 4. Determine the percentage of total critical
  volume for each phase.
• 5. Multiply that percentage by the cycle length
  to get the number of seconds for those movements
  - assuming 2 phases.

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Timing Example

• Assumptions
• volumes in vehicles per hour
• one lane in each direction
• 2-phase cycle

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Timing Example - Step 1
Total Volume per Lane
500
1200
1000
600
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Timing Example - Steps 2 3
Critical Volume for each Phase Total Critical
Volume
500
1200
1800
1000
600
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Timing Example - Step 4
Phase Percent of Total Critical Volume
67
33
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Timing Example - Step 5
Phase 40 53 67 80 93 107
Cycle Length 60 80 100 120 140 160
Phase 20 27 33 40 47 53
66
33
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Timing of a Traffic Signal

• You can divide the allotted time up to handle
  exclusive left turns.
• Phases can overlap.
• You can have exclusive/permissive phases.
• Dont forget the red and yellow.

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Magic Numbers

• Headway In a normal lane, vehicles cross the
  stop line every 2 seconds when discharging from a
  queue.
• Headway increases by 2 for each 1 ft decrease in
  lane width.
• Lost time About 5 seconds of the phase time is
  needed to start and stop the traffic stream.

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Lane Layout

• Normal lanes are 12 ft wide.
• Left turn movements usually have their own
  lane(s).
• Lanes typically not narrower than 10 ft.
• Curb lanes may be wider for bicycles, trucks,
  drainage.

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The Exercise

• Given
• Intersection geometry
• Traffic volumes
• Determine
• Lane layout
• Phase sequence
• Phase times (and cycle length)
• Estimate traffic performance (extra credit)

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Intersection Geometry
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Traffic Volumes
Volumes in vehicles per hour