Signal Timing Fundamentals

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WELCOME

• Signal Timing Fundamentals
• The Web seminar will begin at 12 noon EDT.

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HOUSEKEEPING

• Synchronize your audio and web connections.
• All participant phone lines are muted.
• Questions may be asked via the Chat Room.
• If you have technical difficulties dial
  1-800-305-5208 to contact the Genesys help desk.

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EARNING CEU AND/OR PDH

• Successful completion of this Web seminar
  includes
• Verification of attendance
• Completion of course evaluation
• Verification of learning objectives (online quiz)
• These requirements must be met to earn 1.5 PDH or
• .15 IACET CEU.
• At the conclusion of the course you will receive
  an email
• with directions to the online quiz. An
  additional fee may
• apply.

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INSTRUCTOR

• Philip J. Tarnoff
• Director
• Center for Advanced Transportation Technology
• University of Maryland-College Park
• tarnoff_at_eng.umd.edu

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Traffic Signal Fundamentals
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Course Objectives

• Define the basic traffic signal timing variables
  of cycle, split and offset, understand the manner
  in which they are calculated based on traffic
  characteristics.
• Identify three types of signal controllers,
  including their functional capabilities,
  applications, and limitations.
• Understand the relationship between actuated
  controller timing and effectiveness.
• Define the performance measures used for
  assessing signal system effectiveness.

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Agenda

• Why good signal timing is important
• Signal timing concepts
• Types of signal controllers
• Performance measures

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Signal Timing Benefits

• 15 to 40 reduction in delay
• 10 to 40 less stops
• 10 reduction in fuel consumption
• 22 reduction in emissions
• National savings in fuel consumption of 400
  million barrels of oil per year
• Source National Transportation Operations
  Coalition (NTOC). Traffic Signal Report Card.
  April 2005.

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Agenda

• Why good signal timing is important
• Signal timing concepts
• Types of signal controllers
• Performance measures

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Signal Timing Concepts

• Cycle Time required for signal to display a
  complete sequence of colors
• Split Time allocated to a given movement
  relative to cycle length
• Offset Start of cycle at one intersection
  relative to start of cycle at adjacent
  intersection

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Signal Timing Concepts
Cycle
Split (A phase)
Offset
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Time-Space Diagram
Space
Bandwidth
Speed
Time
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Cycle Length Fact and Fiction

• Fact
• Increased cycle length increases intersection
  capacity
• Shorter cycle lengths reduce delay
• Fiction
• Cycle length has a significant impact on capacity
• Offset and cycle length are independent
• Longer cycle lengths always reduce congestion

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Cycle Length and Capacity
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Websters Formula for Optimum Cycle Length

• C (1.5L5)/(1-Y)
• Where
• C Cycle length in seconds
• L Lost time or (no. of phases) (lost
  time per phase)(all red time)
• Y Sum of the critical lane flows/1900

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Sample Calculation of Cycle Length

• Using Websters equation, calculate the cycle
  length for an intersection with the following
  characteristics
• Two phases (E-W and N-S)
• Assume lost time on each phase 2.5 sec.
• 1 lane eastbound flow 75 vph
• 1 lane westbound flow 100 vph
• 2 lane northbound flow 2600 vph
• 2 lane southbound flow 1500 vph
• 2 lane flows equally divided in each lane

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Cycle Length and Progression (Offset)
Space
Ideal multiple of cycle length
Time
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Longer Cycle Lengths CanIncrease Congestion

• Upstream throughput may exceed downstream link
  capacity
• Turning bay storage can be exceeded
• Increased vehicle headways with long cycle
  lengths (longer green times)

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Cycle Length and Headway
Space
Headway
Time
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The Bottom Line On Cycle Length
Results for an Isolated Intersection
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Basic Facts About Split

• Allocates intersection capacity to conflicting
  movements
• Directly entered on pretimed controllers
• Implicitly selected for actuated controllers
  through
• Maximum green times
• Minimum green times

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A Correct Split Is Important
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Conclusions About Split

• Split i Cycle (CLFi)/(Total CLF)
• Bad splits produce increased delays particularly
  at high volumes
• CLF Critical Lane Flow

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Offsets Can Produce Smooth Flow

• Offset is the time relationship between
  intersections
• Offset effectiveness is limited for saturated
  conditions
• Offset is useful for greater distances between
  intersections.
• Value of offset depends on mid-block friction

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Platoons Determine Offset Effectiveness
Space
Platoon Dispersion
Time
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Conclusions About Offset

• Offset Spacing - Queue Length
• Speed Discharge Rate
• Offset must take queue length into account
• If midblock friction is low, coordination (the
  use of offsets) is useful for greater distances
  between intersections.

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Sample Calculation of Offset

• Calculate the relative offset between two
  intersections for the following conditions
• Spacing 1000 ft.
• Avg. speed 50 fps.
• Avg. Queue 16 veh.
• Queue Discharge 0.5 veh./sec.

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Phase Sequences

• Leading/lagging turns present four options for
  NB-SB and EB-WB movements
• If time-space diagrams are used, best procedure
  is to determine offsets for favored direction
• Try different phase sequences to achieve best
  greenband in reverse direction

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Dual-Ring Controllers
Barrier
Barrier
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Why Do I Need to Know All of This?

• Signal timing optimization programs do NOT always
  provide the best signal timing
• Cannot optimize for oversaturation
• Do not model mid-block sources well
• Do not model platoon dispersion
• Do not permit evaluation of alternative phase
  sequences
• Independent analysis is mandatory

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Agenda

• Why good signal timing is important
• Signal timing concepts
• Types of signal controllers
• Performance measures

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Controller Fundamentals

• Pretimed Operates using a fixed cycle and split
  for a predefined time period
• Full-Actuated Timing for all approaches
  determined by traffic actuation
• Semi-Actuated Timing on one or more, but not
  all approaches determined by traffic actuation
• Volume-Density Variable initial timing with gap
  reduction on actuated approaches

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Actuated Controller Intervals
Variable Green
Yellow Clearance
Green Clearance (Ped FDW)
Initial Interval
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Conventional Actuated Controller Green Phase
Veh. Int. Exceeded
Max. Vehicle Interval
Green Clearance
Act.
Act.
Act.
Act.
Start of Phase
Min. Green
Maximum Green
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More Definitions

• Time for controller to gap out A B
• A Vehicle interval (also called passage time)
• B Time duration of vehicle detection
  (Detection Zone Vehicle Length) / Speed

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Example of Time Duration of Detection

• Example Vehicle speed 60 ft./sec.
• Detector length 50 ft.
• Avg. vehicle length 18 ft.
• Time duration (5018)/60
• or
• Time duration 1.13 sec.

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Relationship Between Gap Out Time and Delay
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Gap-Out Time
2
1
6
4
7
20
Delay (sec/veh)
15
10
5
1500
1200
900
600
300
Total Approach Volume (vph)
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Vehicle Extension

• Ideally should be as short as possible
• Eliminates problem of increasing headways
• Maximizes intersection throughput
• Includes passage time (ideally, most vehicle
  intervals should be set to zero)
• Short passage times are good but should be used
  with discretion, WHY?

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Volume Density Controller Green Phase
Veh. Int. Exceeded
Gap Reduction
Green Clearance
Act.
Act.
Act.
Start of Phase
Min. Green
Maximum Green
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Volume Density vs.Conventional Control

• Simulation studies will always show volume
  density inferior to conventional control
• Advantages
• Avoids premature termination of green
• Permits use of short extension
• Settings should reduce gap as rapidly as possible

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Coordinating Actuated Controllers

• Requires semi-actuated control
• Main-street phase typically not actuated
• Unused time from side-street phases returned to
  main street
• Coordination provided at end of main-street phase

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Pretimed Controller Coordination
Offset
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Coordination of Actuated Controller
Yellow Change
Offset
Phase A
Phase B
Green Clearance (Ped FDW)
Extra Green From Phase B
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Time-Space Diagram
Space
Extra Green
Time
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Coordinating Actuated Controllers

• Uncertain start of green disrupts progression
• During saturated conditions, maximum greens
  should be timed as if controllers are pretimed
• Heavy side street traffic reduces time available
  to main street

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Maximum Greens

• Undersaturated calculation
• (Max Green) (Avg. green time) 1.7
• Oversaturated calculation
• (Max Green) (Cycle) Vc / (Sum of Vc)
• Max green for actuated controller split for
  pretimed controller when oversaturated

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Agenda

• Why good signal timing is important
• Signal timing concepts
• Types of signal controllers
• Performance measures

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Measures of Effectiveness Fundamentals

• Stops Number of vehicles stopping per unit time
  (stops/hr.)
• Delay Stopped time delay summed over all
  vehicles (veh.-sec./hr.)
• Travel time Time required to traverse a
  specific route (sec.)
• Capacity Number of vehicles traversing a
  specific location per unit time (veh./hr.) Can be
  intersection capacity or link capacity

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Closing Thoughts

• Signal timing is the most cost-effective action
  we can take to reduce delays due to recurring
  congestion
• There is a difference between the actions taken
  with saturated vs. under-saturated conditions
• The type of control required depends on roadway
  and traffic conditions

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BEFORE YOU GO

• Remember to submit sign-in sheets and evaluation
  forms
• Online quiz information will follow in an email
  to course registrants. The quiz must be taken
  within two weeks of the course.
• Questions/Comments
• Aliyah N. Horton
• Senior Director
• Professional Development and Outreach
• ITE
• 1099 14th St., NW, Suite 300 West
• Washington, DC 20005
• 202-289-0222 ext. 137 ahorton_at_ite.org