Lecture -Signalised Intersection.pdf

7/21/2019 Lecture -Signalised Intersection.pdf

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Signalized intersection

Controlling traffic movement (RT, TH,LT) through sequentialdisplay of lights

Green, Yellow, Red

One complete sequence - Signal Cycle



Traffic signals -

a) Pre timed

b) Demand actuated

- Fully actuated

- Semi actuated

# Pre-timed have fixed Signal Cycle duration; this can change during peak/offpeak

# Demand actuated changes signal cycle time based on traffic flow - detector

based

[In India, Traffic police controlled signal time is demand actuated in principle]

# Semi actuated - only one leg is demand actuated; other is priority

movement

[Good for minor road traffic junction along major road, mid-block pedestrian

crossing]



Signal Control

- isolated intersection

- arterial system- network system

# Isolated intersection

- spacing such as not to interfere with others

- demand actuated good option

# Arterial system

- a series of signals along a major arterial

- co-ordination between signal time

- pre-timed good option



# Network system- takes into consideration a grid of intersections

- busy urban core street network

- pre-timed good option

# Semi actuated - best suited for midblock pedestrian crossingor major-minor street crossing



Signal Components

- Phase- Ring

Phase - single/combination of movement which are given

right of way (i.e. permission to move) simultaneously (by

showing green & yellow)

Ex. 2/3/4 phase signal

Ring - sequence of phases

- single ring

- multiple ring [Barrier (concurrent termination)]



[A1→B1] & [A2→B2] –Two Ring working simultaneously

[when A1 ends B1 can start & when A2 ends B2 can start]

Barrier is determined by: Max.[(A1+B1),( A2+B2)]



Signal phasing

- it is the process of identifying the sequence of phases

[see previous examples]

Each phase does have its Green, Yellow/Amber, All Red & Red

timesGreen - go

Yellow - Red imminent, so stopAll Red - Red at all phases, safety feature (if someone chooses to

crossing yellow but could not clear)



Basic objective of phasing is to minimize conflicts

More phases - more elimination of conflict

- more delays in each phase (Y+AR)

- inefficiency rises, LOS drops

Delay:

i) start up time lost at the onset of Green

ii) Yellow & AR time

iii) Adherence to minimum Green/Yellow

(to allow pedestrians adequate time)



Two phase Signal:

- where right turning volumes are low

- where pedestrian volume & left turning volumes are low

Three phase Signal:

- where an exclusive phase is given for high right turning

volume on one street

- where an exclusive phase is given for pedestrians (Ex.

CBD/SCHOOL)

Four phase Signal:

- exclusive right turning phases for both streets due to

high volumes



Start up delayUnutilized Yellow +

All Red



Cycle length & delay

- Delay is minimum at Co

- Sensitivity of delay to cycle time is much less when cycle length is increased

Co



Allocation of Green time

Time between phases are allocated according toRatio of critical flow between phases

Ratio applied to available cycle time

i.e. optimal cycle time (Co) - yellow time (y)(all phase)

- All red time (AR) (All phase)

Yellow time (Y) is calculated based on Dilemma Zone Analysis



Max. Green time is also checked

- It means Red time in some leg

- Check whether queue length reaches next intersection

- Not applicable for isolated signal, but crucial for

arterial system or network system



Signal Co-ordination

The cycle time of the signals can be coordinated in such a way that a band of

vehicle in one direction meets consecutive Green →[choosing offset time]



Width of band in opposite directions can be made equal or

close - balanced design

May not be optimal as peak hour volume may demand

preferential treatment.

Change on cycle time in consecutive intersection may lead to

fluctuating band width.



Capacity & Performance Analysis

Degree of saturation(X)

Ratio of existing traffic volume to its capacity

- for each approach lane- for the whole intersection

Performance Analysis through LOS

- based on average delay for each approach lane



Step – 2

Lane utilization factor from [Signal Design & phasing scheme]

- how each lane is expected to move traffic

Saturation flow/service rate (S) will depend heavily on this

In Previous example:

S(TH) = 1800 veh/hr S(TH+RT) = 1600 veh/hr S(TH+LT) = 1700 veh/hr



Step -3

Estimation of saturation flow



LOS according to delay [HCM 2000]

LOS A - ˃10s/vehicle B - 10-20

C - 20-35

D - 35-55

E - 55-80

F - 80+

So, EB lane approach delay of 19.1 S - LOS B

Similarly LOS can be calculated for all approaches.

Delay for the entire Intersection can be calculated by weighing approach

delays by the approach volumes

LOS for Intersection can be calculated from that

Lecture -Signalised Intersection.pdf

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