Traffic Signal Control

8/7/2019 Traffic Signal Control

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BY

Dr. Mahdi Damghani


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Ohio in the US

Wolverhampton in the UK


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Control traffic movement

Reduce congestion

Improve safety

Regulate flows within a road network

They use less space than a roundabout with a

similar capacity


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Minimises space requirements compared with

other types of junctions

Caters for unequal approach flows

Links with other junctions

Relatively low cost


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Delays (particularly at off-peak times)

Risk of certain type of accidents (nose to tail

collisions)

Maintenance costs (equipment, setting &

monitoring)

No U-turns


7/51

Signal heads

Vehicle detectors

Signal controller (microprocessor)

Traffic computer


8/51

The objective is to design both the intersection and

the signal to create a safe & fair common shared

road surface to the road users (cars, trucks,

pedestrians, cyclists, etc)


9/51

Visibility of signal head

70 m for maximum speed of 50 km/hr

125 m for maximum speed of 70 km/hr


10/51

The lane should be 3-5 m wide

Where the vehicle should stop is at stop line

In the UK a secondary signal is diagonally located

opposite to the primary one

pedest

primary

secondaryStop line

Pedestrian crossing (3-12 m wide)


11/51

Traffic islands;

Provide safe refuge to pedestrians

Separate opposing traffic streams

Provides place for the secondsignal column

If lanes are carrying different

streams then two heads are

provided


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The central island for pedestrians should be

considered in a manner that each pedestrian would

occupy 0.5 m2

Waiting pedestrian should be protected by

guardrails

Bus lanes should terminate before the stop line

The stop-line for cyclists is usually ahead of that

for other vehicles


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User will comply to safety (obeying the rules) if

they perceive it as being fair

Compliance by pedestrians may not be required by

lawA degree of fairness is achieved by

Maximum red time

For drivers is around 120 s

For pedestrians is around 60 s

Minimum green time

Often between 10-15 s, however it can be reduced to 5 s for

vehicle actuated traffic signals


18/51

The end of right of way for a stream is anticipated by ayellow indication

The duration of yellow depends on the maximum allowablespeed of the approach

3 s for speed limit of 50 km/hr 4 s for speed limit of 60 km/hr

5 s for speed limit of 70 km/hr

For the comfort of passengers on buses and tramslonger period of yellow is required

3 s for speed limit of 30 km/hr 8 s for speed limit of 70 km/hr

Cyclists require 2 s yellow period

Pedestrians do not require yellow period


19/51

The start of a right of way for a stream is denoted

by a short combined yellow & red indication for 1s

(not more than 2s) so that the drivers can get into

gearThe above situation does not exist for trams, buses,

cyclists & pedestrians (generally)


20/51

An example of UK 4 state traffic light:

1&2 stop

3 go if clear

4 stop if safe to do so


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It is critical to the safety of intersections

Defined as the time needed to elapse between the

end of right of way for one stream (start of red) &

the start of right of way another incompatible

stream (start of green)


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eruztttt !

Clearance

time

C an e er

timeClearintime

Entry time

The clearin distance S0 is the distance fr m the st p-line t the

c nflict p int f r the streamin l sin ri ht f way plus 1 ehicle

len th (0 f r cyclist, 6m f r ehicles 15m f r trams)

The entry distance Se

is the distance fr m the st p-line t the

c nflict p int f r the streamin ainin ri ht f way

Chan e er time is an all wance made f r those ehicles that are

unable to stop durin the yellow period (0-3s)


23/51

Consider two streams. The one losing right of way

has S0=13m , whereas the stream gaining right of

way has Se=11m. Calculate the clearance time

ss

s

s

s

e

r

u

49.319.13

111/11

9.110/)136(

3

6mlengthvehicleaverage

3stimechangeover

m/s11speedentryreasonable

m/s10speedclearingreasonable

$!!

!!

!!

!

!

!

!

!


24/51

Usual sequence of signals

Fixed

period

Fixed

period

Variable

period

Variable

period

Variable

period


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Some streams can simultaneously opt for green

light without conflict so they are grouped into

s ages

Streams with similar green times should begrouped together

Keep the number of stages to minimum

For a 4-arm intersection 2 stages are preferred

More than 2 stages may result from the need of

public transport to have its own priority and so

adds another stage, etc


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Some terms;

Intergreen period The time between the end of one stage to the start of the next

stage, i.e. the duration of the stage transition normally equal

to maximum clearance time

Signal plan


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INTERGREEN INTERGREEN INTERGREEN

GREEN A RED RA

GREEN A RED RA

RED RA GREEN A RED

RED RA GREEN A RED

RED RA GREEN A RED


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A junction carrying the following traffic flows is to

be controlled by two stage traffic signals

The composition of the traffic is 81%

cars, 15%

heavy goods vehicles & 4% public service

vehicles with PCU factors of 1.0, 2.3 & 2.0

respectively.

Left turning

traffic

(Veh/hr)

Straight ahead

traffic

(Veh/hr)

Right turning

traffic

(Veh/hr)

Saturation flow

(PCU/hr)

North arm 15 167 40 1575

South arm 44 171 45 1825

East arm 25 357 140 1975

West arm 58 459 75 2195


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Calculate the optimum cycle time and effective green

times for the junction assuming:

Intergreens: 5 seconds

Amber: 3 seconds Red/amber time: 2 seconds

Sum of Start and End lost times per green stage: 2 seconds

It has been suggested that a fixed all-red period of an

extra 12 seconds should be provided between the

North-South and the East-West stages to assistpedestrian movements. Estimate the queue lengths and

delays with and without the pedestrian facility for the

West arm of the junction.


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West

arm

East

arm

North

arm

South

arm

15

167

40

45

171

44

357

25

140

58

45975


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1.2352.0)(0.042.3)51.0()0.181.0(Factor !vvv!

0.33375)/2195459(58523.1y0.326140)/1975357(25523.1y

0.17645)/1825171(44523.1y

0.17440)/1575167(15523.1y

flosaturation

floactual

!v!

!v!

!v!

!v!

!y


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Saturation flow

Capacity of the stream

The number of vehicles passing through a point whilst

there is a constant supply of vehicles (vehicles inqueue)

Can be measured from counting the number of vehicle

at 6s intervals passing through the stop-line when green

period of the traffic signal starts

It is mostly between 1800-2000 veh/hr for a single lane

of average width and a straight path


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Y is the sum of the worst case y values for each

stage within the signal cycle

!!

!

YY WS

The biggest ratio in phase 1 phase 2


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West

ar

East

ar

North

ar

South

ar Phase diagra

This proble suggests the

intersection should be controlledusing 2 phases. The first phase

allows for North & South with

their turnings on per itted basis.

The second phase allows for

passage of East & West with

their turnings on per itted basis(dotted line)


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l1

l2

laIL

lll

lagg

nstage!!

!

!!

_

21

changestageperLost time

'periodgreenEffective

intergreen


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!s es s es

l)(


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L is the lost time per cycle

seco s!v!v! laIL

Amber duration

(fixed)

Intergreen

duration (fixed)


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C0

is optimum cycle time and is calculated as

1551

y1551

stages

ma

!!

L.L.C


40/51

Calculate C0

as

seconds530.509-1

58)5.1(0 !

v!C


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Calculate effective green period for each phase

(stage)

seconds188)-53(095.0333.0)('

seconds98)-53(095.0

176.0)('

0ma

2

0

ma

1

2

1

!!!

!!!

LY

yg

LY

yg

stage

stage


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Calculate actual green period for each phase

sec'

sec'

22

11

!!!

!!!

algg

algg


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Calculate new L which is new lost time due to

extra added red time for pedestrians

sec20128 !!L


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Re-calculate all parameters with new L

sec322333

sec33)2071(509.0333.0'

sec172318

sec18)2071(509.0

176.0'

sec71509.01

520)5.1(

2

2

1

1

0

!!

!!

!!

!!

!

v!

g

g

g

g

C


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Delays on an approach can be obtained using


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For no pedestrian provision delay becomes;

sec3.8

5sec3

647.00.610)519.0(

203.0

PCU/Second61.03600

5219

PCU/Second203.03600

731

519.053

18

!

!

!v!

!!

!!

!!

d

c

x

s

q

P


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Queue length can be obtained as

'

2ma

0 gCr

qr

qr

qd

!

The lengthof effective red


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Queue length for no provision for pedestrians is

5PCU.3

3.5PCU18)-53(203.0

PCU4.32

18)-53(203.0)3.8203.0(ma !

!v

!vv


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For pedestrian provision

sec9.17

5sec3

716.00.610)465.0(

203.0

PCU/Second61.03600

5219

PCU/Second203.03600

731

465.071

33

!

!

!v!

!!

!!

!!

d

c

x

s

q

P


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Queue length considering provision for pedestrians

is

7.7PCU

7.7PCU33)-71(203.0

5PCU.72

33)-71(203.0)84.17203.0(ma !

!v

!vv


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GREEN A RED A/R

0 357 14 21 28

RED GREENA/R A

Signal

group

g(s)

N-S 8

E-W 17

Documents

Traffic Signal Control