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Signal Timing Fundamentals

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... Start of cycle at one intersection relative to start of cycle at adjacent intersection Time-Space Diagram Cycle Length Fact and Fiction Fact: ... – PowerPoint PPT presentation

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Title: Signal Timing Fundamentals


1
WELCOME
  • Signal Timing Fundamentals
  • The Web seminar will begin at 12 noon EDT.

2
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.

3
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.

4
INSTRUCTOR
  • Philip J. Tarnoff
  • Director
  • Center for Advanced Transportation Technology
  • University of Maryland-College Park
  • tarnoff_at_eng.umd.edu

5
Traffic Signal Fundamentals
6
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.

7
Agenda
  • Why good signal timing is important
  • Signal timing concepts
  • Types of signal controllers
  • Performance measures

8
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.

9
Agenda
  • Why good signal timing is important
  • Signal timing concepts
  • Types of signal controllers
  • Performance measures

10
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

11
Signal Timing Concepts
Cycle
Split (A phase)
Offset
12
Time-Space Diagram
Space
Bandwidth
Speed
Time
13
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

14
Cycle Length and Capacity
15
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

16
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

17
Cycle Length and Progression (Offset)
Space
Ideal multiple of cycle length
Time
18
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)

19
Cycle Length and Headway
Space
Headway
Time
20
The Bottom Line On Cycle Length
Results for an Isolated Intersection
21
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

22
A Correct Split Is Important
23
Conclusions About Split
  • Split i Cycle (CLFi)/(Total CLF)
  • Bad splits produce increased delays particularly
    at high volumes
  • CLF Critical Lane Flow

24
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

25
Platoons Determine Offset Effectiveness
Space
Platoon Dispersion
Time
26
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.

27
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.

28
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

29
Dual-Ring Controllers
Barrier
Barrier
30
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

31
Agenda
  • Why good signal timing is important
  • Signal timing concepts
  • Types of signal controllers
  • Performance measures

32
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

33
Actuated Controller Intervals
Variable Green
Yellow Clearance
Green Clearance (Ped FDW)
Initial Interval
34
Conventional Actuated Controller Green Phase
Veh. Int. Exceeded
Max. Vehicle Interval
Green Clearance
Act.
Act.
Act.
Act.
Start of Phase
Min. Green
Maximum Green
35
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

36
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.

37
Relationship Between Gap Out Time and Delay
25
Gap-Out Time
2
1
6
4
7
20
Delay (sec/veh)
15
10
5
1500
1200
900
600
300
Total Approach Volume (vph)
38
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?

39
Volume Density Controller Green Phase
Veh. Int. Exceeded
Gap Reduction
Green Clearance
Act.
Act.
Act.
Start of Phase
Min. Green
Maximum Green
40
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

41
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

42
Pretimed Controller Coordination
Offset
43
Coordination of Actuated Controller
Yellow Change
Offset
Phase A
Phase B
Green Clearance (Ped FDW)
Extra Green From Phase B
44
Time-Space Diagram
Space
Extra Green
Time
45
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

46
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

47
Agenda
  • Why good signal timing is important
  • Signal timing concepts
  • Types of signal controllers
  • Performance measures

48
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

49
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

50
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
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