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    Traffic Signals

    Uploaded by

    lakshmidileep
    Traffic signals regulate traffic flow at intersections through stop and go phases. There are advantages like increased traffic handling, reduced collisions, and safe pedestrian crossings. Disadvantages include possible rear-end collisions and confusion from failures or improper design. Signal timing is determined using methods like Webster's, which calculates green times based on traffic flows and saturation levels for each phase. Sample problems demonstrate designing two-phase and four-phase signals using this method.

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    © All Rights Reserved

    Traffic Signals

    Uploaded by

    lakshmidileep
    35 views6 pages
    Traffic signals regulate traffic flow at intersections through stop and go phases. There are advantages like increased traffic handling, reduced collisions, and safe pedestrian crossings. Disadvantages include possible rear-end collisions and confusion from failures or improper design. Signal timing is determined using methods like Webster's, which calculates green times based on traffic flows and saturation levels for each phase. Sample problems demonstrate designing two-phase and four-phase signals using this method.

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    Traffic signals regulate traffic flow at intersections through stop and go phases. There are advantages like increased traffic handling, reduced collisions, and safe pedestrian crossings. Disadvantages include possible rear-end collisions and confusion from failures or improper design. Signal timing is determined using methods like Webster's, which calculates green times based on traffic flows and saturation levels for each phase. Sample problems demonstrate designing two-phase and four-phase signals using this method.

    Copyright:

    © All Rights Reserved
    35 views6 pages

    Traffic Signals

    Uploaded by

    lakshmidileep
    Traffic signals regulate traffic flow at intersections through stop and go phases. There are advantages like increased traffic handling, reduced collisions, and safe pedestrian crossings. Disadvantages include possible rear-end collisions and confusion from failures or improper design. Signal timing is determined using methods like Webster's, which calculates green times based on traffic flows and saturation levels for each phase. Sample problems demonstrate designing two-phase and four-phase signals using this method.

    Copyright:

    © All Rights Reserved
    of 6

    TRAFFIC SIGNALS

    Traffic signal is a control device to regulate traffic flow at an intersection by stop and
    proceed.

    Advantages

    1. Increases traffic handling capacity


    2. Reduce right angled collisions
    3. Safe pedestrian crossing
    4. Safe crossing of traffic
    5. Reasonable speed is maintained
    6. Economical than manual control
    7. Quality of traffic stream is better.

    Disadvantages

    1. Possibility of rear end collision is increased.


    2. Improper design will result in violation of signal.
    3. Failure of signals may causes confusion

    Terms used

    Cycle : The time required for one complete sequence of traffic movement for all meeting
    roads.

    Phase : A part of cycle time allotted for traffic movement in one direction or a
    combination of movement at same time

    Phase 1-NS and SN

    Phase 2 EW and WE E

    Two phase system

    S
    Phase 1-NS

    W Phase 2-EW

    Phase 4 WE E

    Phase 3-SN

    Four Phase system

    Types of signals

    (1)Traffic signals

    (a) Fixed time signal –Signal timing is constant


    (b) Manually operated signal – Signal time is manually controlled based on traffic
    (c) Traffic actuated signal – Signal timing is varied depending upon traffic on
    different arms.

    (2) Pedestrian signal

    (3) Special signal

    Signal coordination : Setting of different signals along a road .There are four
    systems.

    (a) Simultaneous system –All signals along a road shows same phase
    (b) Alternate system – Adjacent signals show red and green
    (c) Simple progressive system – Time schedule to allow continuous movement
    (d) Flexile progressive system – All signals are traffic actuated and controlled by a
    central computer.
    DESIGN OF TRAFFIC SIGNALBY WEBSTERS METHOD

    Data required

    - Number of phases (n)


    - Saturation flow of different phases (S1,S2,…Sn)
    - Normal flow of each phase(q1,q2…qn)
    - All red time for pedestrian crossing (R)
    - The amber time for one phase =2sec

    y1=q1/S1

    y2=q2/S2

    y3=q3/S3

    …yn=qn/Sn

    Y=y1+y2+y3+….yn

    Lost time L=2n+R

    Optimum cycle time Co=(1.5L +5)/(1-Y)

    Green time for phase 1 G1 = (y1/Y) (Co-L)

    Green time for phase 2 G2 =( y2/Y) (Co-L)

    ………Green time for phase n….Gn= (yn/Y)/(Co-L)

    Problem 1

    Two cross roads A and B have normal flows 800PCU/hr and 500PCU/hr respectively. The
    saturation flows on A and B are 2500PCU/hr and 2000PCU/hr respectively. The all red time for
    pedestrian crossing is 12sec. Design a two phase traffic signal by Webster method.

    Solution

    SA = 2500PCU/hr, SB =2000PCU/hr

    qA= 800PCU/hr , qB=500PCU/hr

    n=2

    R=12sec

    yA = qA//SA =800/2500= 0.32

    YB =500/2000= 0.25
    Y= yA + yB =0.57

    L=2n+R = 2x2+12 =16sec

    Co=(1.5L +5)/(1-Y) =(1.5x16+5)/(1-0.57) =29/0.43=67.4sec

    G1 = (yA/Y) (Co-L)=(0.32/0.57)x(67.4-16) = 28.9sec

    G2=(0.25/0.57) x(67.4-16) =22.5 sec

    Cycle time=28.9+2+ 22.5+2+12= 67.4 sec

    Phase diagram

    28.9 sec 2 36.5 sec


    G A RR R RRRRR PhaseA

     R   G  A  R Phase B
    30.9 22.5 2 12

    Problem 2

    Two cross roads 1 and 2 meet at an intersection. The normal flows and saturation flows are
    given below.

    Direction NS SN EW WE

    Normal flow 700 400 500 560

    Saturation flow 2100 1600 1700 1800

    The all red time for pedestrian crossing is 10sec. Design a two phase traffic signal by Webster
    method.

    Solution

    SNS = 2100PCU/hr, SSN=1600PCU/hr SEW 1700 PCU/hr SWE=1800 PCU/hr

    qNS = 700PCU/hr, qSN=400PCU/hr qEW 500 PCU/hr qWE=560 PCU/hr

    n=2

    R=10sec

    YNS = qA/SA =700/2100= 0.33 YSN =400/1600 =0.25

    YEW =500/17000= 0.29, YWE =560/1800= 0. 31


    Y1= max (YNS , YSN) = max ( 0.33, 0.25) =0.33

    Y2= max (YEw , YWE) = max ( 0.29, 0.31) =0.31

    Y=Y1+Y2=0.33+0.31=0.64

    L=2n+R = 2x2+10 =14sec

    Co=(1.5L +5)/(1-Y) =(1.5x14+5)/(1-0.64) =72.2sec

    G1 = (y1/Y) (Co-L)=(0.33/0.64)x(72.2-14) = 30sec

    G2=(0.31/0.64) x(72.2-14) =28.2 sec

    Cycle time=30+2+ 28.2+2+10= 72.2 sec

    Phase diagram

    30 2 40.2
              Phse 1

              Phase 2
    32 28.2 2 10

    Design of four phase signal

    For four phase signal determine y1, y2, y3and y4.

    Y=y1+y2+y3+y4

    Then determine optimum cycle time Co= (1.5L +5)/(1-Y)

    Find G1, G2, G3 and G4

    Phase diagram contains four phases.

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