8 Signal Coordination

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Traffic EngineeringTopic: Signal Coordination

Dr. Henry Liu

CE 4211/5211 Traffic Engineering

University of Minnesota

email: [email protected]


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Signal Coordination

For situations with relatively closely spacedintersections

Vehicles commonly maintain their grouping

for well over 1000 feet. Common practice is

to coordinate signals less than mile apart

on major streets and highways


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L

L

TimeEqually Spaced Intersections

1

2

3

Speed


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L

L


1

2

3 Stop Delay


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L

L


1

2

3Stop Delay


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L

L


1

2

3


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Benefits Prime benefit: reduction in stops and delays

It is common to consider the benefit of a coordination

plan in terms of costs as a weighted combination of stopsand delay:

Cost = A X (total stops) + B X (total delay) + other terms

A and B are decisions of the local jurisdiction

Encourage preferred speed: signals set such that to

incur more stops for speeds faster than the design speed Set up platoons (shorter headways)

Stop fewer vehicles, esp important for short blocks withheavy flows which may overflow the available storage


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Purpose Usually the physical layout of the street systemand the major traffic flows determine the purpose

Consider the type of system: 1-way, 2-way, or

mixed network Consider the movement to be progressed:examine tradeoff

Set an objective: max bandwidth, min delay, minstops, and combinations?

Understand limitations


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Factors lessening benefits

Inadequate roadway capacity

Existence of substantial side frictions, includingparking, loading, double parking, and multiple driveways

Complicated intersections, multiphase control

Very short signal spacing

Heavy turn volumes, either into or out of the street


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Exceptions to the coordinated

scheme

Easy coordination may not always be possible. Forexample, a very busy intersection located in not ascongested area. The engineer may not want to use the cycletime of the busy intersection as the common cycle time.

Another example is the existence of a critical intersectionthat causes queue spillback.


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The time-space diagram

and ideal offsets Offset: the difference between the green initiation

times at two adjacent intersections.

usually expressed as a positive number betweenzero and the cycle length.

Often, the ideal offset: the value such that thefirst vehicle of a platoon just arrives at the downstreamsignal, the downstream signal turns green.

t(ideal) = L/S where L = block length, S= vehiclespeed

Offset = mod C {OFFSET}


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Offset determination on a

two-way street The fact that the offsets are interrelated presentsone of the most fundamental problems of signaloptimization.

Actual offsets and travel times are distinct. While theengineer might desire the ideal offset to be the same as

the travel time

In link i, the actual offsets are related by:

Define the actual offset and ideal offset by:

where j represents the direction, and i the link

nCtt iSBiNB =+ ,,


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Offset Optimization Define the actual offset and ideal offset by:

where j represents the direction, and i the link

In a number of signal optimization programs thatcan be used for two-way arterials, the objective is tominimize some functions of the discrepancies betweenthe actual and ideal offsets.

The simplest form is perhaps the sum of thesquares or of the discrepancies, weighted by the link

volumes:

),(),(),( ijett ijidealijactual +=

= ij ijidealijactual ttijvZ ,2

),(),( ])][,([


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The bandwidth concept and

maximum bandwidth


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Bandwidth and efficiency of a

progression

Efficiency defined as:

Efficiency =(bandwidth/cycle length) X 100%

An efficiency of 40-55% is considered as good.

The bandwidth is limited by the min green in thedirection of interest.

Nonstop volume =3600 (BW) (L)/h/C vph

Where BW = measured or computed bandwidth (s); L= number of through lanes; h = headway (s/veh), C =

cycle length (s)


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Effective progressions on 2-

way Streets

If an appropriate combination of cycle length,block length, and platoon speed, then the task of goodprogression in both directions becomes easy

Therefore, whenever possible, in new town or newdevelopment, these appropriate combinations mustbe considered seriously.

Green Band in


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CC

C

C

L

L


Green Band in

North Direction

(NEMA 2)

Green Band in

South Direction(NEMA6)

Offset of Int. 2

Relative to Int. 3

(Ref. to Start of Green)

1

2

3

Offset of Int. 2Relative to System Time


Yield Point of Int. 2

Relative to System Time


Green Band in


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Green Band in

South

Direction

(NEMA6)

CC

C

L

L


Green Band in

North Direction

(NEMA 2)

1

2

3

C

L S=2L/CC=2L/S

Offset of Int. 2

Relative to Int. 3


C

L

OFFSET=C/2

C

L

Old Green Band in


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C

C

C

TimeUnequally Spaced Intersections

1

3

2

Old Green Band in

South Direction(NEMA6)

Old Green Band in

North Direction

(NEMA 2) New Green Band

in

South Direction

(NEMA6)

New Green Band in

North Direction

(NEMA 2)

2

L

L

L2

L1

Regain Green Band in


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C

C

C

L2

L1

Time

1

3

2


North Direction

(NEMA 2)

Lose Green Band in

South Direction

(NEMA6)

New Green Band

in

South Direction

(NEMA6)

New Green Band in

North Direction

(NEMA 2)

Move Offset

ORIGINAL PHASING DIAGRAM


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ORIGINAL PHASING DIAGRAM

for Intersection 2

(Equally-Spaced Intersections)

REVISED PHASING DIAGRAM

for Intersection 2

(Unequally-Spaced Intersections)



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C

C

C

L2

L1

Time

1

3

2

g

North Direction

(NEMA 2)


South Direction

(NEMA6)

Adjust Phasing


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Actuated Signal CoordinationNew Concepts:

1. Background Cycle Length

2. Yield Point

3. Sync Phase4. Force Off


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1. To provide synchronization and maintain the backgroundcycle length, all coordinated intersection have the same

system clock reference point, which is usually the start point

of signal coordination plan.

2. The sync phase of every coordinated intersection has fixed

series of yield points, and the difference between yield points

is the background cycle length.

3. These yield points are also local clock reference points toother non-sync phases. The sync phase has minimal

bandwidth, i.e. the sync phase has to start at the time of

minimal bandwidth earlier than yield point.4. To do so, all other phases have to be cut at certain points,

which are so-called force-off points. These force-off points

are usually referenced to the local clock reference point.

Yield PointLocal Clock Reference Point = 0 sec


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Background Cycle Length

System Clock Reference Point = 0 sec

Sync Phase (usually NEMA 2)

Initial (Minimum)

Green


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Phase Interval Times

Interval Phase

1 2 3 4 5 6 7 8

Walk

Ped Clear

Initial 6 14 6 8 9 10 6 10

Extension 2.0 3.0 3.0 2.0 2.0 3.0 3.0 2.0

Max Green 20 50 15 35 20 50 15 35

Yellow 3 5 3 4 3 5 3 4

Red 1 1 1 1 1 1 1 1

Permit

Max Recall

Min Recall

Ped Recall

Lag Phase

Yield Point = 70 secLocal Clock Reference Point = 0 sec

Check barrier: F.O. 1 < F.O.6 + 2 = 43 sec


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4

Background Cycle Length = 120 sec


NEMA 2 Bandwidth = 48 sec

6 6 6 4 8 5

9 4 10 6 6 4 10 5

F.O. 1 < 120 - 4 - 6 - 4 - 8 - 5 - 48 = 45 sec

14 sec

F.O. 3 < 120 - 4 - 8 - 5 - 48 = 55 sec

F.O. 4 < 120 - 5 - 48 = 67 sec

F.O. 8 = F.O. 4 < 120 - 5 - 48 = 67 sec

F.O. 7 < F.O. 8 - 10 - 4 = 67 - 14 = 53 sec

F.O. 6 = F.O.7 - 12 = 53 12 = 41 sec


F.O. 5 = F.O. 6 - 40 sec -4 sec

= 41 44 = -3 = 117 sec

Yield Point = 70 secLocal Clock Reference Point = 0 sec


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4 46 6

4

Background Cycle Length = 120 sec


6 6 6 4 8 5

9 10 10 5


14 sec

F.O. 1 (< 43 sec) = 33 - 10 - 4 = 19 sec

F.O. 3 (< 55 sec) = 67 - 30 - 4 = 33 sec

F.O. 4 (< 67 sec) = 67 sec

F.O. 8 = F.O. 4 = 67 sec

F.O. 7 (< F.O. 8 - 14 sec) = 67 - 34 - 4 = 29 sec

F.O. 6 = F.O. 1 - 2 sec = 17 sec


F.O. 5 = F.O. 6 - 40 sec -4 sec

= 17 - 40 - 4 = - 27 sec

= 120 - 27 = 93 sec

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Hypothetical Intersection

8 Signal Coordination

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