cete_39

Traffic Engineering And Management 39. Vehicle Actuated Signals

Chapter 39

Vehicle Actuated Signals

39.1 Introduction

Now-a-days, controlling traffic congestion relies on having an efficient and well-managed traffic

signal control policy. Traffic signals operate in either pre-timed or actuated mode or some com-

bination of the two. Pre-timed control consists of a series of intervals that are fixed in duration.

They repeat a preset constant cycle. In contrast to pre-timed signals, actuated signals have

the capability to respond to the presence of vehicles or pedestrians at the intersection. Actu-

ated control consists of intervals that are called and extended in response to vehicle detectors.

The controllers are capable of not only varying the cycle length & green times in response to

detector actuations, but of altering the order and sequence of phases.

Adaptive or area traffic control systems (ATCS) belong to the latest generation of signal-

ized intersection control. ATCS continuously detect vehicular traffic volume, compute optimal

signal timings based on this detected volume and simultaneously implement them. Reacting

to these volume variations generally results in reduced delays, shorter queues and decreased

travel times. Coordinating traffic signals along a single route so that vehicles get progressive

green signal at each junction is another important aspect of ATCS.

In the subsequent pages, the operating principles and features of Vehicle-Actuated Signals

& Area Traffic Control Systems will be briefly discussed.

39.2 Vehicle-Actuated Signals

39.2.1 Basic Principles

As stated earlier, Vehicle-Actuated Signals require actuation by a vehicle on one or more

approaches in order for certain phases or traffic movements to be serviced. They are equipped

Dr. Tom V. Mathew, IIT Bombay 1 April 2, 2012


with detectors and the necessary control logic to respond to the demands placed on them.

Vehicle-actuated control uses information on current demands and operations, obtained from

detectors within the intersection, to alter one or more aspects of the signal timing on a cycle-

by-cycle basis. Timing of the signals is controlled by traffic demand. Actuated controllers may

be programmed to accommodate:

• Variable phase sequences (e.g., optional protected LT phases)

• Variable green times for each phase

• Variable cycle length, caused by variable green times

Such variability allows the signal to allocate green time based on current demands and opera-

tions. A proper clearance interval between the green & the red phases is also ensured.

39.2.2 Advantages of Actuated Signals

The various advantages of actuated signals are stated below:

• They can reduce delay (if properly timed).

• They are adaptable to short-term fluctuations in traffic flow.

• Usually increase capacity (by continually reapportioning green time).

• Provide continuous operation under low volume conditions.

• Especially effective at multiple phase intersections.

39.2.3 Disadvantages of Actuated Signals

The main disadvantages are as following :

• If traffic demand pattern is very regular, the extra benefit of adding local actuation is

minimal, perhaps non-existent.

• Installation cost is two to three times the cost of a pretimed signal installation.

• Actuated controllers are much more complicated than pretimed controllers, increasing

maintenance costs.

• They require careful inspection & maintenance to ensure proper operation.



39.2.4 Types of Actuated Control

There are three basic types of actuated control, each using signal controllers that are somewhat

different in their design:

1. Semi-Actuated Control

2. Full-Actuated Control

3. Volume-Density Control

Semi-Actuated Control

This type of controller is used at intersections where a major street having relatively uniform

flow is crossed by a minor street with low volumes. Detectors are placed only on the minor

street. The green is on the major street at all times unless a call on the side street is noted. The

number and duration of side-street green is limited by the signal timing and can be restricted

to times that do not interfere with progressive signal-timing patterns along the major street.

Full-Actuated Control

This type of controller is used at the intersections of streets or roads with relatively equal

volumes, but where the traffic distribution is varying. In full actuated operation, all lanes of

all approaches are monitored by detectors. The phase sequence, green allocations, and cycle

length are all subjected to variation. This form of control is effective for both two-phase and

multiphase operations and can accommodate optional phases.

Volume-Density Control

Volume-density control is basically the same as full actuated control with additional demand-

responsive features. It is designed for intersections of major traffic flows having considerable

unpredictable fluctuations.

39.2.5 Detection for Actuated Signalization

The various types of detectors used for detection of vehicles are as following:

• Inductive loop detectors

• Magnetometer detectors



• Magnetic detectors

• Pressure-sensitive detectors

• Radar detectors

• Sonic detectors

• Microloop detectors etc.

The vast majority of actuated signal installations use inductive loops for detection purpose.

Now, the type of detection is of greater importance than the specific detection device(s) used.

There are two types of detection that influence the design and timing of actuated controllers:

1. Passage or Point Detection:- In this type of detection, only the fact that the detector

has been disturbed is noted. The detector is installed at a point even though the detector

unit itself may involve a short length. It is the most common form of detection.

2. Presence or Area Detection:- In this type of detection, a significant length (or area)

of an approach lane is included in the detection zone. Entries and exits of vehicles into

and out of the detection zone are remembered. Thus, the number of vehicles stored in

the detection zone is known. It is provided by using a long induction loop, or a series of

point detectors. These are generally used in conjunction with volume-density controllers.

39.2.6 Actuated Control Features

Regardless of the controller type, virtually all actuated controllers offer the same basic functions,

although the methodology for implementing them may vary by type and manufacturer. For

each actuated phase, the following basic features must be set on the controller:

Minimum Green Time

Each actuated phase has a minimum green time, which serves as the smallest amount of green

time that may be allocated to a phase when it is initiated. Minimum green times must be set for

each phase in an actuated signalization, including the non-actuated phase of a semi-actuated

controller. The minimum green timing on an actuated phase is based on the type and location

of detectors.

• In case of Point Detectors,

Gmin = tL + [hInteger(d/x)] (39.1)



Where,

Gmin = minimum green time in second

tL = assumed start-up lost time = 4 sec

h = assumed saturation headway = 2 sec

d = distance between detector & stop line in m

x = assumed distance between stored vehicles = 6 m

• In case of Area Detectors,

Gmin = tL + 2n (39.2)

Where,

tL = start-up lost time (sec)

n = number of vehicles stored in the detection area

Unit Extension

This time actually serves three different purposes:

1. It represents the maximum gap between actuations at a single detector required to retain

the green.

2. It is the amount of time added to the green phase when an additional actuation is received

within the unit extension, U.

3. It must be of sufficient length to allow a vehicle to travel from the detector to the STOP

line.

In terms of signal operation, it serves as both the minimum allowable gap to retain a green

signal and as the amount of green time added when an additional actuation is detected within

the minimum allowable gap.

The unit extension is selected with two criteria in mind:

• The unit extension should be long enough such that a subsequent vehicle operating in

dense traffic at a safe headway will be able to retain a green signal (assuming the maximum

green has not yet been reached).



• The unit extension should not be so long that straggling vehicles may retain the green or

that excessive time is added to the green (beyond what one vehicle reasonably requires

to cross the STOP line on green).

The Traffic Detector Handbook recommends that a unit extension of 3.0 s be used where

approach speeds are equal to or less than 30 mile per hour, and that 3.5 s be used at higher

approach speeds. For all types of controllers, however, the unit extension must be equal to or

more than the passage time.

Passage Time Interval

It allows a vehicle to travel from the detector to the stop line. It is analogous with ’Unit

Extension’.

P = (d/S) (39.3)

Where, P = passage time, sec

d = distance from detector to stop line, metre

S = approach speed of vehicles, m/s

Maximum Green Time

Each phase has a maximum green time that limits the length of a green phase, even if there are

continued actuations that would normally retain the green. The maximum green time begins

when there is a call (or detector actuation) on a competing phase. The estimation can be done

by any of the following methods:

• By Trial signal timing as if the signals were pretimed

Ci =L

[1 − V C/(1615(PHF )(v/c))](39.4)

Where, Ci = Initial cycle length, sec

L = Total lost time, sec

VC = Sum of critical lane volumes, veh/hr

Knowing the initial cycle length, green times are then determined as:

gi = (Ci − L) ∗VCi

VC(39.5)



RedNum

ber

of V

ehic

les

in q

ueue Green time based on

phase time extension

Green time based ontarget v/c ratio

Green

Time (s)

Green extension time

8

6

4

2

0

Figure 39:1: Queue accumulation polygon illustrating two methods of green time computation

Where gi = effective green time for Phase i, sec

VCi = critical lane volume for Phase i, veh/hr

The effective green times thus obtained are then multiplied by 1.25 or 1.50 to determine

the maximum green time.

• By Green-Time Estimation (HCM) Model

Traffic-actuated controllers do not recognize specified cycle lengths. Instead they deter-

mine, by a mechanical analogy, the required green time given the length of the previous

red period and the arrival rate. They accomplish this by holding the right-of-way until

the accumulated queue has been served.

The basic principle underlying all signal timing analysis is the queue accumulation polygon

(QAP), which plots the number of vehicles queued at the stop line over the duration of the

cycle. The QAP for a simple protected movement is illustrated in the Fig. 39:1. From Fig. 39:1,

it’s clear that queue accumulation takes place on the left side of the triangle (i.e., effective red)

and the discharge takes place on the right side of the triangle (i.e., effective green).

There are two methods of determining the required green time given the length of the pre-

vious red time. The first employs a target v/c approach. Under this approach, the green-time

requirement is determined by the slope of the line representing the target v/c of 0.9. If the phase

ends when the queue has dissipated under these conditions, the target v/c will be achieved.

The second method recognizes the way a traffic-actuated controller really works. It does not

deal explicitly with v/c ratios; in fact, it has no way of determining the v/c ratio. Instead it

terminates each phase when a gap of a particular length is encountered at the detector. Good

practice dictates that the gap threshold must be longer than the gap that would be encountered



when the queue is being served. Assuming that gaps large enough to terminate the phase can

only occur after the queue service interval (based on v/c = 1.0), the average green time may

be estimated as the sum of the queue service time and the phase extension time.

Therefore,

Average Green Time = Queue Service Time + Phase Extension Time

Now,

Queue Service Time(gS) =fqqrr

(s − qg)(39.6)

Where,

qr = red arrival rate (veh/s)

qg = green arrival rate (veh/s)

r = effective red time (s)

s = saturation flow rate (veh/s)

fq = calibration factor = 1.08 - 0.1(actual green time/maximum green time)2

Green extension time(ge) = [exp(λ ∗ (u + t − ∆))/Φq] − (1/λ) (39.7)

Where, q = vehicle arrival rate throughout cycle (veh/s)

u = unit extension time setting (s)

t = time during which detector is occupied by a passing vehicle(s)

= [3.6(Ld + Lv)]/SA

Lv = Vehicle length, assumed to be 5.5 m

Ld = Detector length (m)

SA= Vehicle approach speed (kmph)

∆ = minimum arrival (intra-bunch) headway (s)

λ = a parameter (veh/s) = Φq/(1 − ∆q)

Φ = proportion of free (unbunched) vehicles in traffic stream = exp(−b∆q)

b = bunching factor

This green-time estimation model is not difficult to implement, but it does not lead directly

to the determination of an average cycle length or green time because the green time required

for each phase is dependent on the green time required by the other phases. Thus, a circular

dependency is established that requires an iterative process to solve. With each iteration, the

green time required by each phase, given the green times required by the other phases, can be



Table 39:1: Recommended Parameter ValuesCase ∆(s) b

Single Lane 1.5 0.6

Multilane

2 lanes 0.5 0.5

3 lanes 0.5 0.8

determined.

The logical starting point for the iterative process involves the minimum times specified for

each phase. If these times turn out to be adequate for all phases, the cycle length will simply

be the sum of the minimum phase times for the critical phases. If a particular phase demands

more than its minimum time, more time should be given to that phase. Thus, a longer red

time must be imposed on all of the other phases. This, in turn, will increase the green time

required for the subject phase.

Recall Switch

Each actuated phase has a recall switch. The recall switches determine what happens to the

signal when there is no demand. Normally, one recall switch is placed in the on position, while

all others are turned off. In this case, when there is no demand present, the green returns to

the phase with its recall switch on. If no recall switch is in the on position, the green remains

on the phase that had the last ”call.” If all recall switches are on and no demand exists, one

phase continues to move to the next at the expiration of the minimum green.

Change and Clearance Intervals

Yellow and all-red intervals provide for safe transition from green to red. They are fixed times

and are not subject to variation, even in an actuated controller. They are found in the same

manner as for pretimed signals.

y = t + [S85/(2a + 19.6g)] (39.8)

ar = (w + l)/S15 (39.9)

Where,

y = yellow time, sec



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extensible portion

maximum greenminimumgreen

Figure 39:2: Operation of an Actuated Phase

ar = all red interval, sec

S85 = 85th percentile speed, m/s

S15 = 15th percentile speed, m/s

t = reaction time of the driver = 1 sec (standard)

a = deceleration rate = 3 m/s2 (standard)

g = grade of approach in decimal

w = width of street being crossed, m

l = length of a vehicle, m

39.2.7 Operating Principle

The Fig. 39:2 illustrates the operation of an actuated phase based on the three critical settings:

minimum green, maximum green, and unit or vehicle extension. When the green is initiated

for a phase, it will be at least as long as the minimum green period. The controller divides

the minimum green into an initial portion and a portion equal to one unit extension. If an

additional call is received during the initial portion of the minimum green, no time is added to

the phase, as there is sufficient time within the minimum green to cross the STOP line (yellow

and all-red intervals take care of clearing the intersection). If a call is received during the last

U seconds (Unit Extension) of the minimum green, U seconds of green are added to the phase.

Thereafter, every time an additional call is received during a unit extension of U seconds, an

additional period of U seconds is added to the green. Note that the additional periods of U

seconds are added from the time of the actuation or call. They are not added to the end of

the previous unit extension, as this would accumulate unused green times within each unit

extension and include them in the total green period.



Table 39:2: Recommended Detector Locations & Timing Parameters

Approach Detector Set-Back Mimimum Passage

Speed (To front of loop) Green Time

(kmph) (m) (sec) (sec)

24 12 8.0 3.0

32 18 10.0 3.0

40 24 12.0 3.0

48 30 14.0 3.5

56 41 18.0 3.5

64 52 22.0 3.5

72+ Volume density or multiple detectors recommended

The green is terminated in one of two ways:

1. a unit extension of U seconds expires without an additional actuation,

2. the maximum green is reached.

The maximum green begins timing out when a call on a competing phase is noted. During the

most congested periods of flow, however, it may be assumed that demand exists more or less

continuously on all phases. The maximum green, therefore, begins timing out at the beginning

of the green period in such a situation.

Now-a-days, in India, detectors are placed mostly at stop lines. In that case, the green times

for phases are primarily determined by arrival headway. The green time is extended until the

gap between two vehicles becomes equal to or greater than the pre-determined threshold value.

Generally threshold of 4 seconds is considered.

39.2.8 Concept of Semi-Actuated Controller

Principles

• Detectors on minor approaches only.

• Major phase receives a minimum green interval.

• The green remains on the main street until a call for service on the side street is registered.



Side

Detector

Street

Main Street

Stop line

Figure 39:3: Semi-Actuated Control

• If the main street has had enough green, the side street is given the green for just enough

time to guarantee that its vehicles are processed.

• Usually Point Detectors are used.

• Detectors can be placed at either stop line or upstream location.

Advantages

• It can be used effectively in a coordinated signal system.

• Relative to pre-timed control, it reduces the delay incurred by the major-road through

movements during periods of light traffic.

• It does not require detectors for the major-road through movement phases and hence, its

operation is not compromised by the failure of these detectors.

• Generally the main street indeed has the green whenever possible.

Disadvantages

• Continuous demand on the phases associated with one or more minor movements can

cause excessive delay to the major road through movements if the maximum green and

passage time parameters are not appropriately set.



Detector

Figure 39:4: Full-Actuated Control

• Detectors must be used on the minor approaches, thus requiring installation and ongoing

maintenance.

• It also requires more training than that needed for pre-timed control.

39.2.9 Concept of Full-Actuated Controller

Principles

• Detectors on all approaches.

• Each phase has a preset initial interval.

• Phases are sequenced according to ”calls” for service on all approaches.

• Green interval is extended by a preset unit extension for each actuation after the initial

interval provided a gap greater than the unit extension does not occur.

• Green extension is limited by preset maximum limit.

• Generally Point Detectors are used.

• Detectors can be placed at either stop line or upstream location.



Advantages

• Reduces delay relative to pre-timed control by being highly responsive to traffic demand

and to changes in traffic pattern.

• Detection information allows the cycle time to be efficiently allocated on a cycle-by-cycle

basis.

• Allows phases to be skipped if there is no call for service, thereby allowing the controller

to reallocate the unused time to a subsequent phase.

Disadvantages

• Initial and maintenance cost is higher than that of other control types due to the amount

of detection required.

• It may also result in higher percentage of vehicles stopping because green time is not held

for upstream platoons.

39.2.10 Concept of Volume-Density Controller

Volume-Density Controllers are designed for intersections of major traffic flows having consid-

erable unpredictable fluctuations. They are generally used at intersections with high approach

speeds (≥ 45 mi/hr). Here, detectors are placed on all approaches. Generally this type of

controller is used with Area Detectors. To operate efficiently, this type of control needs to

receive traffic information early enough to react to existing conditions. So, it is essential that

detectors be placed far in advance of the intersection.

39.2.11 Numerical Example : Full-Actuated Control

An isolated suburban intersection of two major arterials is to be signalized using a full actuated

controller. Area detection is to be used, and there are no driveways or other potential entry

points for vehicles within 90 m of the STOP line on all approaches. The intersection is shown

in the figure and all volumes have already been converted to tvus for convenience. Left-turn

slots of 75 m in length are provided for each approach. The tvu conversions assume that a

protected left-turn phase will be provided for all approaches.

Solution:



Figure 39:5: Intersection for the Example

Step 1: Phasing

The problem statement indicates that protected left-turn phasing will be implemented on all

approaches. Note that Kennedy Avenue has double left-turn lanes in each direction and that

Monroe Street has a single left-turn lane in each direction. At a heavily utilized intersection

such as this, quad-eight phasing would be desirable. Each street would have an exclusive LT

phase followed by a leading green in the direction of heavier LT flow and a TH/RT phase. Such

phasing provides much flexibility in that LT phasing is always optional and can be skipped in

any cycle in which no LT demand is noted. The resulting signalization has a maximum of four

phases in any given cycle and a minimum of two. It is treated as a four-phase signal, as this

option leads to the maximum lost times. Quad-eight phasing involves overlaps that would be

taken into account if this were a pretimed signal. As an actuated signal, the worst-case cycle,

however, would occur when there are no overlap periods. This would occur when the LT flow

in opposing directions are equal. Thus, the signal timing will be considered as if this were a

simple four-phase operation without overlaps. The controller, however, will allow one protected

LT to be terminated before the opposing protected LT, creating a leading green phase. The

four phases are:

• Phase I-Protected LT for Kennedy Avenue

• Phase 2-TH/RT for Kennedy Avenue

• Phase 3-Protected LT for Monroe Street

• Phase 4-TH/RT for Monroe Street



Step 2: Unit Extension

For approach speeds of 64 kmph, the recommended unit extension (from Table 2) is 3.5 s.

Step 3: Minimum Green Times and Detector Placement

The problem specifies that area detection shall be employed. For area detection, the far end

of the detection zone is placed such that the passage time is equal to unit extension. Since all

approaches (including LT approaches) have a 64 kmph approach speed, the far end of detectors

should be located as follows:

U = 3.5 = P = d/(64/3.6)

d = 3.5 ∗ (64/3.6) = 62.22 ≈ 62m

The near end of the detection zone would be placed within 0.3 m of the STOP line.

The minimum green time for area detection is variable, based on the number of vehicles sensed

within the detection area when the green is initiated. The value can vary from the time needed

to service one waiting vehicle to the time needed to service Int(62/6) = 11 vehicles. The range

of minimum green times can be established for each approach. In this case, all values will be

equal, as the approach speeds are the same for all approaches and the detector location is com-

mon to every approach, including the LT lanes, all of which are long enough to accommodate

a 62 m setback.

Gmin/low = 2.0 + (2 ∗ 1) = 4.0 sec

Gmin/high = 2.0 + (2 ∗ 11) = 24.0 sec

Step 4: Critical-Lane Volumes

As the volumes given have already been converted to tvus, critical-lane volumes for each phase

are easily identified:

• Phase 1 (Kennedy Ave, LT) - 400/2 = 200 tvu/h

• Phase 2 (Kennedy Ave, TH/RT) - 1,600/4 = 400 tvu/h

• Phase 3 (Monroe St, LT) - 110/1 = 110 tvu/h

• Phase 4 (Monroe St, TH/RT) - 700/2 = 350 tvu/h

Therefore, VC = (200+400+110+700) = 1,060 tvu/h



Step 5: Yellow & All-Red times With a 64 kmph average approach speed for all movements,

the S85 may be estimated as (64 + 8) = 72 kmph, and the S15 may be estimated as (64 - 8) =

56 kmph. Then:

yall = 1.0 + (72/3.6)/(2 ∗ 3) + 19.6(0.01 ∗ 0) = 4.3sec

ar1,2 = (16 + 6)/(56/3.6) = 1.5sec

ar3,4 = (36 + 6)/(56/3.6) = 2.7sec

Y1,2 = (4.3 + 1.5) = 5.8sec

Y3,4 = (4.3 + 2.7) = 7.0sec

There are four phases in the worst-case cycle. The total lost time is equal to the sum of the

yellow and all-red intervals in the cycle:

L = 2*5.8 + 2*7.0 = 25.6 sec

Step 6: Maximum Green Times and the Critical Cycle

The initial cycle length for determining maximum green time is: Ci = 25.6/[1-1060/(1615*0.96*0.98)]

= 84.8 sec

Green times are found as

G1 = (84.8 − 25.6)(200/1060) = 11.2sec

G2 = (84.8 − 25.6)(400/1060) = 22.3sec

G3 = (84.8 − 25.6)(110/1060) = 6.1sec

G4 = (84.8 − 25.6)(350/1060) = 19.5sec

Gmax1 = (1.5 ∗ 11.2) = 16.8sec

Gmax2 = (1.5 ∗ 22.3) = 33.5sec

Gmax3 = (1.5 ∗ 6.1) = 9.2sec

Gmax4 = (1.5 ∗ 19.5) = 29.3sec

With area detection, the minimum green for all lane groups, including LT lanes, can be as

high as 24.0 s. This is inconsistent with Gmax values for the LT Phases 1 and 3. Increasing

the maximum greens beyond the computed values, however, will lead to an excessively long

critical cycle length. Thus, it is recommended that the LT lanes use point detectors, placed so

that the Gmin for Phases 1 and 3 is a constant 4.0 s. The above Gmax results will work in this

scenario. The Gmax results for Phases 2 and 4 (through phases) are close to the high value of



Gmin for these phases, but would provide some flexibility even in peak periods. It is, therefore,

not recommended that any of these times be arbitrarily increased. The critical cycle length

becomes:

CC = 16.8 + 5.8 + 33.5 + 5.8 + 9.2 + 7.0 + 29.3 + 7.0 = 114.4 sec

39.3 Area Traffic Control

39.3.1 Definition and Principle

Area Traffic Control (ATC) can be defined as a technique which provides for a centralised

control of numerous signal installations distributed throughout an urban area, such that there

is a planned co-ordination between signals at different junctions. ATC systems are intelligent

real-time dynamic traffic control systems which are designed to effectively respond to rapid

variations in dynamic traffic conditions. The technique invariably employs digital computers

for achieving the desired objective. So, ATC can be called as a combination of Traffic Respon-

sive Signals & Co-ordinated Signals.

In ATC systems, traffic signals in a particular area are co-ordinated in such a way that an

objective function is optimized. The objective function can be delay, stops, queue length etc.

or a weighted combination of these. Mostly, optimization of a weighted combination of delay

and number of stops is taken as the objective function.

39.3.2 Objectives

The various objectives of an area traffic control system are:

• Minimizing journey time for vehicles.

• Minimizing vehicular stops, resulting in less noise, less pollution and less consumption of

fuel.

• Reducing accidents.

• Discouraging use of certain areas.

• Minimizing person-time.

• Ensuring a smooth & safe traffic flow.



39.3.3 Major Building Blocks of ATC

Major Building blocks of the Area Traffic Control Systems are:

• Vehicle Detectors

• Intersection Controller

• Communication Network

• Application Software

• Central (Regional) Control System

Vehicle Detectors

Detectors or Sensors are the devices used to detect the presence of vehicle. They act as a nodal

point between vehicle and intersection controller. Function of detector is to sense data for

average speed, vehicle flow, vehicle density, queue length measurement, tracking etc. Various

kinds of vehicle detectors such as ultrasonic, microwave radar, infrared laser radar, non-imaging

passive infrared, video imaging, acoustic array, magnetic loop etc. are used but the most

common among these is the inductive loop vehicle detector.

Intersection Controller

It is the micro-macro computer placed at intersection for temporary storage of data. However

in some of the systems liberty is given to the intersection controller to handle split optimization.

In most of the systems it transfers the data obtained from detectors to central control. Central

control processes the data and sends back the information to intersection controller which then

implements the signal timings as instructed at the intersection.

Communication Network

The communication network transfers data from the signal controllers, sensed by the detectors

to the central control station where optimized signal timings and phases are determined and

it again transfers information to the signal controller as per the data processed. It transfers

the data obtained from detectors to central control. Central control processes the data and

sends back the information to intersection controller which then implements the signal timings

as instructed at the intersection.



Signalhardware

Vehicle

DataDecision

ControlCentral

ControllerIntersection

Detector

Decision Data

Figure 39:6: Flow of Data & Decisions in ATC System

Application Software

Application software is the software used behind the whole ATC system which performs the

entire task. It defines the user services that planners want to deliver and presents an overall

structure that can deliver these services effectively. It is a basic design scheme. It is an

enormous program involving multiple systems, multiple stakeholders, multiple procedures for

implementation, and wide-ranging effects which builds the architecture.

Central Control System

It is the heart of Area Traffic Control System. It enjoys the absolute authority to control

every intersection. It consist of master computer which takes various complex decisions such

as selecting cycle time, remove or add an intersection from the coordination area, isolate a

junction, selecting a plan from the database etc.

39.3.4 Methodology of Detection

Most of the traffic control systems are designed with three distinct detection location strategies:

1. Upstream detection for arrival rate

2. Downstream detection for departure rate or occupancy detection

3. Both upstream and downstream detection

Generally these traffic control systems works in order to respond the traffic demand which is

estimated or predicted based on arrival rate (or headway). With this demand and departure

rate (if known) one can find expected queued vehicles waiting for green signal. To implement



real time control systems for heterogeneous type of complex traffic flow and associated vehicu-

lar interactions is challenge for traffic engineers. So, departure headway variable is considered

here as a reliable input for our control system, both 1 and 3 detection systems are not suitable.

Using 2nd system of detection, number of departures and departure headway can be measured.

Headways are more important in the traffic flow theory as the flow rate is reciprocal to the

mean headway. The study of headways is closely connected to the study of traffic intersection

capacity. The mathematical analysis and simulation of traffic operations are based on theo-

retical models, which must be evaluated against the properties of real-time data. Accordingly,

the vehicle headway studies have been concentrated on the statistical analysis of headway data.

Green times and cycle times are adapted according to the actual traffic states in previous

cycles in order to improve the efficiency of the signal. Control is determined by continuous

monitoring the departure headways of vehicles in active phase. Green times are given to a

traffic stream in order to serve all vehicles waiting at the signal during the previous red phase.

The signal switches happens when the detector records a headway gap, which is longer than a

pre-defined threshold value. This green time is usually constrained to be within minimum and

maximum values, which are mainly determined by predicted state of system in future. The

assigned green times are thus variable according to the variability of queues forming during the

red phase and to the variability of vehicle headways. For vehicle actuated logic with only Stop-

line Detectors there is no accurate methodology to estimate the residual queue and red-time

arrival rates using the Stop-Line detectors alone, as the detector placing and signal grouping

becomes extremely critical in the context of highly heterogeneous traffic. The expected queued

vehicles at in-active phase are determined by the number of vehicles arriving during the red

phase.

The framework of an adaptive signal control with stop-line detection is shown in Fig. 39:7.

The signal controller block is getting initial policy as an input at the beginning of control.

The signal controller block which gives signal parameters input to the control the traffic light.

Traffic lights are responsible for phase termination decisions and vehicle flow profiles will be

captured through stop-line detection system. Controller uses the termination and departure

information to determine the state of phases and reset the control parameters if required.

39.3.5 Traffic Control Methods

The following are the main types of methods in general use:

• Fixed time plans based on historical data and calculated off-line by a computerized op-



Initial Policy

Signal Controller

(State of phase)Utilization

Phase

for next phaseControl policy

departureReal time

andterminationinformation

Monitoring(Detection)andTermination lights

controlIntersection

Figure 39:7: Framework of Adaptive Signal Control with Downstream Detection

timizing technique. The information about vehicular movement is obtained manually or

through detectors and fed to the computer, which then determine the signal settings, and

transmit the settings to the signals. Examples of this type are the Combination Method

and TRANSYT (Traffic Analysis Study Tool).

• Co-ordinated systems with local response at each signal. Example of this type are the

FLEXIPROG (Flexible Progressive) and EQUISAT (Equally Saturated).

• Fully responsive systems such as S.P.G. (Signal Plan Generation) and PLIDENT (Platoon

Identification).

Two of the most important commercial implementations of ATC system are SCOOT and SCAT.

39.3.6 SCOOT

Introduction

The Split Cycle Offset Optimization Technique (SCOOT) is an urban traffic control system

developed by the Transport Research Laboratory (TRL) in collaboration with the UK traf-

fic systems industry. SCOOT is an adaptive system, which responds automatically to traffic

fluctuations. It continuously measures traffic volumes on all approaches of intersections in

the network and changes the signal timings to minimize a Performance Index (PI), which is

a composite measure of delay, queue length and stops in the network. It is a cyclic, para-

metric, centralized, traffic-responsive signal control system, which does not take advantage of

vehicle-actuated control tactics at local intersections. SCOOT performs a real-time incremen-

tal optimization of signal settings utilizing a traffic simulator. SCOOT has proved to be an

effective and efficient tool for managing traffic on signalized road networks.



FIGURE TO BE DRAWNFigure 39:8: Operating Principle of SCOOT

Working Principle

The system consists of a number of SCOOT cells or computers, each cell being able to control

up to 60 junctions, handling input data from up to 256 vehicle counting detectors on street.

Unlike the SCAT system, the SCOOT detectors are placed as far upstream (usually 14 m from

the stop-line) from the approach to the junction as possible and are then calibrated to strike a

balance between flow and occupancy. Fig. 39:8 clearly depicts the working principle of SCOOT

where the detectors placed upstream sense the occupancy and the information is transmitted

to the central computer. The SCOOT traffic model and optimizers use this information to

calculate signal timings to achieve the best overall compromise for coordination along all links

in the SCOOT area.

The main philosophy of the SCOOT traffic signal control system is to react to changes in

observed average traffic demands by making frequent, but small, adjustments to the signal

cycle time, green allocation, and offset of every controlled intersection. For each coordinated

area, the system evaluates every 5 minutes, or 2.5 minutes if appropriate, whether the com-

mon cycle time in operation at all intersections within the area should be changed to keep

the degree of saturation of the most heavily loaded intersection at or below 90%. In normal

operation SCOOT estimates whether any advantage is to be gained by altering the timings. If

an advantage is predicted then one or more of the timings are changed by small amounts. By

this means frequent, but small, changes allow the signal timings to match fluctuations in traffic

demand. There are no large and abrupt changes in signal timings, although overtime, major

changes in splits, cycle time and offset can occur.

Optimization Procedures

SCOOT has three optimization procedures by which it adjusts signal timings

• Split Optimizer

• Cycle Time Optimizer

• Offset Optimizer

These gives SCOOT its name i.e. Split Cycle and Offset Optimization Technique. Each op-

timizer estimates the effect of a small incremental change in signal timings on the overall



performance of the region’s traffic signal network. A performance index is calculated based on

predictions of vehicle delays and stops on each link.

• Split Optimizer : The Split Optimizer works at every change of stage by analyzing the

current red and green timings to determine whether the stage change time should be

advanced, retarded or remain the same. The Split Optimizer works in increments of 1 to

4 seconds.

• Cycle Time Optimizer : The Cycle Time Optimizer operates on a region basis once every

five minutes, or every two and a half minutes when cycle times are rising rapidly. It

identifies the critical node within the region and will attempt to adjust the cycle time

to maintain this node with 90% link saturation on each stage. If change in cycle time is

required, it can increase or decrease the cycle time in 4, 8 or 16 second increments.

• Offset Optimizer: The Offset Optimizer works once per cycle for each node. It operates

by analyzing the current situation at each node using the cyclic flow profiles predicted

for each of the links with upstream or downstream nodes. It then assesses whether

the existing action time should be advanced, retarded or remains the same in 4 second

increments.

Other Features

SCOOT systems can be linked to other traffic management and control systems such as:

• Variable Message Signs

• Emergency Green Wave Routes

• Fault Identification & Management

• Diversions

• Fixed Time Plan

• Green Confirmation

• Manual Control



Limitations

The main limitations of SCOOT are:

• Inability to handle closely spaced signals due to its particular detection configuration

requirements.

• Interface is difficult to handle.

• Traffic terminologies are different from those used in India.

• Primarily designed to react to long-term, slow variations in traffic demand, and not to

short-term random fluctuations.

39.3.7 SCAT

Introduction

The Sydney Co-ordinated Adaptive Traffic Control (SCAT) System was developed by the Roads

and Traffic Authority (RTA) of New South Wales, Australia in the late 1970’s. SCATS is a

hierarchical, cyclic, traffic responsive signal control strategy. The system operators may choose

between combinations of different goals including, minimize stops, minimize delay and maxi-

mize throughput. Similar to SCOOT, SCATS adjusts cycle time, splits and offsets in response

to real-time traffic demand to minimize overall stops and delay. However, unlike SCOOT, it is

not model based but has a library of plans that it selects from and therefore relies extensively

on available traffic data. It can loosely be described as a feedback control system.

SCATS is an automated, real time, traffic responsive signal control strategy. Under SCATS, the

timing of the signals is governed by computer-based control logic. The system has the ability

to modify signal timings on a cycle-by-cycle basis using traffic flow information collected at the

intersection approach stop lines.

Working Principle

The system is very flexible, powerful, expandable, and yields unprecedented monitoring and

management possibilities. For control purposes, the total system is divided into intersection,

regional and a central system management control. The distribution of the regional computers

is determined by the economics of communication. Each regional computer maintains au-

tonomous control of its region. The interrelationship of the computers is shown in Fig.39:9.



functionsStrategicTrafficcontrol

CentralManagement

System

Management

1−32

tactical traffic controlTraffic controllers regional computer

upto 250 per

ComputerRegional Regional

ComputerRegionalComputer

RegionalComputer

RegionalComputer

Figure 39:9: SCAT Computer Hierarchy

Input data for the SCATS system is collected via a system of traffic sensors. The sensors

may be inductive loop detectors imbedded in the pavement or video image devices mounted

overhead on the signal strain poles or attached on mast arms. The traffic information collected

in the field involves the discharge characteristics (i.e., flow and occupancy during the green

phase) on each intersection approach. This data is processed by the local controller or trans-

mitted to a regional control center. SCATS have a hierarchical control architecture consisting

of two levels, strategic and tactical. At the strategic level, a subsystem or a network of up to

10 intersections is controlled by a regional computer to coordinate signal timings. These sub-

systems can link together to form a larger system operating on a common cycle time. At the

tactical level, optimization occurs at the intersection level within the constraints imposed by the

regional computer’s strategic control. Tactical control allows early termination of green phases

when the demand is less than average and for phases to be omitted entirely when there is no de-

mand. All the extra green time is added to the main phase or can be used by subsequent phases.

SCATS supports four modes of operations. The first or Normal Mode provides integrated

traffic responsive operation. In the second or Fall-Back Mode time-of-day plans are imple-

mented when computer or communication failure occurs. In the third mode or Isolated Control

Mode local vehicle actuation with isolated control works, while in the fourth mode,the normal

signal display shows flashing yellow or flashing red on all approaches. SCATS gathers data

on traffic flows in real-time at each intersection. These data are transmitted via the traffic

control signal box to a central computer. The computer makes incremental adjustments to

traffic signal timings based on minute-by-minute changes in traffic flow at each intersection.

SCATS performs a vehicle count at each stop line, and also measure the gap between vehicles

as they pass through each intersection. As the gap between vehicles increases the traffic signal

approach is wasting green time, and SCATS seeks to reallocate green time to where demand is

greatest.



The expected benefit from SCATS comes from its ability to constantly modify signal tim-

ing patterns to most effectively accommodate changing traffic conditions. While the potential

benefits from this control structure may be significant, few research studies have documented

the effect of implementing this method of signal control.

Limitations

The main criticisms of SCAT are:

• It lacks user-friendly interface features to support day-to-day operations & programming

tasks.

• The error messages (flags & alarms) are not easy to decipher & do not provide the

opportunity for corrective actions by system operators.

• Very much costly.

• Does not have the predictive capability of SCOOT.

39.3.8 Comparison of SCOOT & SCAT

SCOOT SCAT

Upstream detection Downstream detection

Centralized system Distributed system

Fixed traffic region Adjustable region

Fallback - fixed Fallback - VA

Model based Algorithmic

39.4 Conclusion

Modern actuated controllers give the traffic engineers a great deal of flexibility in dealing with

variations in demand. Area traffic control system along with Vehicle actuated signals can reduce

traffic delays substantially. These are highly complex subject. Timing of VA signals is almost

as much an art as a science, and more then one solution is possible. Regarding ATC systems,

SCOOT and SCAT are popular in advance countries but such systems cannot cope up with

Indian situations without adaptation to Indian traffic scenario. Presently, an advance ATC

system known as CoSiCoSt has been developed considering the Indian Traffic scenario.



39.5 Acknowledgments

I wish to thank my student Mr. Kaniska Ghosh for his assistance in developing the lecture

note, and my staff Mr. Rayan in typesetting the materials. I also wish to thank several of my

students and staff of NPTEL for their contribution in this lecture.


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