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SCATS is a computer based area traffic management system. It is a complete system that
includes hardware, software, and a unique control philosophy. The system operates in realtime, adjusting signal timings in response to variations in traffic demand and system capacity.
The purpose of SCATS, as with any area traffic control system, is to control traffic on an area
basis rather than on an individual, uncoordinated intersection basis.
New Generation SCATS
SCATS has been constantly improved and enhanced on a regular basis as new technologies
become available. The SCATS team has responded to the needs of the end-user and has now
released SCATS 6 which provides far more flexibility for the decision maker, the Traffic
Engineer and most importantly the accountant!.
This new generation has moved to a PC platform, has increased the number of intersections
that can be connected to one PC, has improved data collection resources and reporting
facilities along with improved management and monitoring methods.
SCATS 6 can be made available in the following formats with pricing to suit the end-user
based on their needs and budget.
Full Real Time Traffic Adaptive
Fixed Time Plans
Dial In Dial Out
Comparisons are available on request between the old and the new with the improvements too
large to itemise in this brochure. Details are available from the contacts listed on page 12, see
the hyperlink e-mail connection.
SCATS has always been a real time adaptive traffic management system. Nothing has
changed except the recognition of the variety of systems required by traffic engineers in
diverse traffic conditions around the world.
In this brochure SCATS is described in its most functional role, that of a real time, responsive,
adaptive traffic management system. Details of the method of fixed time plan operation are
available on request as well as the Dial In Dial Out system which offers unique remote access
to sites in outlying cities that need to be monitored on a daily basis reducing the need for
constant visits to check operation.
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Adaptive System Copes with Unusual Demand
Adaptive SCATS, unlike “fixed-time” or “semi-responsive” systems, requires no precalculation of composite signal timing plans. Logic and algorithms in the system’s controllers
and traffic control computer analyse real-time traffic data from vehicle detectors to produce
signal timings which are suitable for the prevailing traffic conditions.
Many other area traffic control systems control the signals on a “fixed-time” basis in which a
series of signal timing plans are brought into operation at certain times of the day. Each plan
determines the timing of individual signals, and the time relationship between signals is precalculated based on previously surveyed traffic conditions.
A “fixed-time” system is generally unable to cope with unpredicted traffic conditions.
SCATS has addressed this problem when releasing SCATS 6 and has improved decision
making capabilities built into the FTP system to compensate for this deficiency in fixed time
No Need To Update Timing Plans
Furthermore, as traffic conditions change with the passage of time, fixed time plans become
outdated. This requires the area to be resurveyed and new signal timing plans calculated every
few years. Experience has shown this procedure to be expensive and to require resources
which are not always readily available.
As a result, the development of new plans is either deferred beyond the useful life of the old
plans or “ad hoc” changes are made to the plans and timetables, usually resulting in suboptimum performance.
More Responsive Control Method Required
The problems of “fixed-time” systems suggest that a technique more responsive to changing
traffic conditions would be more appropriate and more acceptable to the motoring public.
SCATS Offers Real Time Responsiveness
The implementation of a fully responsive system does not, however, mean that the careful
design of each intersection can be avoided. The present state of technology only allows for
the real-time variation of signal timings at the intersections which have been designed to suit
known or anticipated traffic requirements.
A degree of adaptability of the local design to varying traffic requirements can be
accommodated by a system such as SCATS. This is done by providing a variable sequence of
phases and the ability to omit phases or movements from the sequence on a cycle-by-cycle
basis, when there is no demand.
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Four Modes of Operation
SCATS provides for operation of signals in the system under four modes:
The adaptive mode, known as Masterlink, provides the integrated traffic responsive
In the event of failure of a regional computer or loss of communications, the local controllers
can revert to a form of time based coordination known as Flexilink. In this mode, adjacent
signals are synchronised by reference to the power mains frequency or an accurate crystal
controlled clock and signal timing plans are selected by time of day. Local vehicle actuation
facilities are operational in this mode.
Signals may also operate in an Isolated mode with local vehicle actuation being the sole
The fourth mode is Flashing Yellow in which the normal signal display is replaced by
flashing yellow displays on all approaches or flashing yellow and flashing red to different
Any Mode Can be Used
Provided communications are functional, signal operation can still be centrally monitored in
Flexilink, Isolated and Flashing modes. Any signal may be set to any of the four modes by an
operator using a SCATS workstation or by time of day.
Two Levels of Control
SCATS control of traffic is effected at two levels which determine the three principle signal
timing parameters of traffic signal coordination; phase split, cycle length and offset. These
two levels are referred to as “strategic” and “tactical”.
SCATS “strategic” control refers to the top level of control which is impressed on a network
of coordinated signals by the regional computer. Using flow and occupancy data collected
from loop detectors in the road by the local controllers, the strategic algorithms determine, on
an area basis, the optimum cycle length, phase splits and offsets to suit the prevailing average
traffic conditions. This is carried out for adjacent groups of signals (usually one to ten in size)
which are known as subsystems.
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The subsystem in SCATS is the basic unit of strategic control. Each subsystem consists of
one or more intersections and contains only one critical intersection which requires accurate
and variable phase splits. The intersections in a subsystem form a discrete group which are
always coordinated together and share a common cycle length and inter-related split and offset
selection. Phase splits for minor intersections in the subsystem are, by definition, non critical
and are therefore either non-variable or selected by a matching process which selects splits
which are compatible with the splits in operation at the critical intersection.
Subgroup Linking – building large coordinated systems automatically
To give coordination over larger groups of signals, subsystems can link together to form larger
systems, operating on a common cycle length. These links, which determine the offsets
between the subsystems, may be permanent or may link and un-link. This ensures that where
traffic flow between subsystems is sufficient to warrant coordination the link is enforced but
when one or more subsystems can operate more efficiently at a lower cycle time, the link is
Degree of Saturation
The basic traffic measurement used by SCATS for strategic control is the degree of saturation
on each approach or, more accurately, a measure analogous to degree of saturation. Inductive
loop vehicle detectors placed in important approach lanes at the stop line of the critical
intersections (and some detectors at other intersections) are defined in the regional computer
data base as strategic detectors. The local controller collects flow and occupancy data during
the green of the approach and, after pre-processing, it is sent to the regional computer and
used (together with automatically self calibrated saturation flow data for each detector) to
calculate the SCATS “degree of saturation” (DS).
DS is defined as the ratio of the effectively used green time to the total available green time on
the approach. The effectively used green time is the length of green which would be just
sufficient to pass the same platoon of vehicles had they been travelling at optimum headways
as in saturation flow conditions. The algorithm is capable of producing values of DS greater
than unity in congested conditions, enabling SCATS to deal effectively with over saturated
Effect on Cycle Time
Cycle time is increased or decreased to maintain the degree of saturation around 0.9 (user
definable) on the lane with the greatest degree of saturation. A lower limit for cycle time
(usually 30 to 40 seconds) and an upper limit (usually 100 to 150 seconds) are specified by the
user. Cycle time can vary by up to 21 seconds each cycle but this limit is substantially
reduced unless a strong trend is recognised.
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Effect on Phase Splits
Phase splits are varied by a few percent each cycle in such a way as to maintain equal degrees
of saturation on competing approaches, thus minimising delay. The minimum split which can
be allocated to a phase is either a user definable minimum or, more usually, a value
determined from the local controller’s minimum phase length. The current cycle time and the
minimum requirements of the other phases limit the maximum split, which can be allocated to
Offsets are selected for each subsystem (ie., the offsets between intersections within the
subsystem) and between subsystems which are linked together on the basis of traffic flow. In
this way, the best offsets are selected for the high flow movements. Other links carrying
lower flows may not receive good coordination if the cycle time is inappropriate. However,
when traffic conditions permit the use of a cycle time which can provide good offsets on a
majority of links, the system tends to maintain this cycle time even though a smaller cycle
time would provide sufficient capacity. Optimal offsets on the heavy flow links minimise the
total number of stops in the system, reducing fuel consumption and increasing capacity of the
SCATS “tactical” control refers to the lower level of control which is undertaken by the local
controllers at each intersection. Tactical control operates under the strategic umbrella
provided by the regional computer but provides local flexibility to meet the cyclic variation in
demand at each intersection. Tactics essentially provide for green phases to be terminated
early when the demand for the phase is less than the average demand and for phases to be
omitted entirely from the sequence if there is no demand. Conditional signal group
introduction is also provided. The local controller bases its tactical decisions on information
from the vehicle detector loops at the intersection, some of which may also be strategic
Tactical Control is the Responsibility of the Controller
The tactical level of control is carried out in the local controller using exactly the same
operational techniques as described for isolated operation for a local controller. The degree to
which tactical control is able to modify the signal operation is entirely under the control of the
Tactical Control different to Isolated
A basic difference from isolated operation is that one phase, usually the main road phase,
cannot skip and cannot terminate early by action of gap and waste timers. This is because all
controllers in a linked group must share a common cycle time to achieve coordination. Any
time saved during the cycle as a result of other phases terminating early or being skipped may
be used by subsequent phases or is added on to the main phase to maintain each local
controller at the system cycle length.
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Strategic and Tactical Control Together Equals Efficiency On The Street
The combination of strategic control which varies the split, cycle time and offsets in response
to gradual changes in traffic demand patterns together with tactical control which handles the
rapid but smaller changes in demand cycle by cycle results in a very efficient operation of the
signals on the street.
SCATS provides the operator with a range of manual functions to override the normal
automatic operation. These functions allow manual control of signal lamps to “on”, “flash” or
“off”; manual selection of link mode to Masterlink, Flexilink or Isolated mode; manual
selection or alteration of split, cycle time or offset either on an individual intersection or for a
whole subsystem; a dwell facility which allows any signal to be held on a nominated green
phase for as long as required.
Variation by Timetable
SCATS also allows for system operation to be varied by a timetable. Almost any function
which can be executed manually can also be set up to occur at specified times on specified
days. For example, in a central business district, pedestrian walks may be automatically
introduced on business days, late shopping nights and other periods of high pedestrian
A range of special routines is also available in SCATS which allows the user to define special
operations to occur under special conditions. These routines are used to address requirements
not covered by the general operation of SCATS. It is features of this type which enable every
detail of signal operation to be tailored to meet the operational needs of each individual
intersection. SCATS is the only system to offer such a feature.
The theoretical capacity of SCATS regional traffic control software is 250 intersections.
Software is available in a variety of increments as follows:
Subsequent increases are in multiples of 32 Intersections
In the event of regional computer failure, loss of communications between the computer and
any local controller, failure of all strategic detectors or certain other local malfunctions, the
affected intersection(s) will “fallback” to a user defined mode of operation which may be
either Flexilink (time based coordination) or Isolated operation.
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Coordination Maintained During Fallback
If specified by the user, fallback at one intersection will also cause other intersections in the
subsystem to fall back and, optionally, intersections in adjacent linked subsystems. In this
way, if Flexilink is specified as the fallback mode, coordination can be maintained between
intersections affected by the failure.
The Controller is the Key
All data necessary for fallback operation is held in the local controller, ie., local signal timings
for Isolated operation and plans and schedules for Flexilink operation. A copy of this data is
held in the regional computer so that it may be “downloaded” from the regional computer to
the local controller in the event of it being lost. The clocks in the local controllers are
periodically checked by the regional computer and adjusted as necessary.
SYSTEM HARDWARE – COMPUTER PLATFORM
Distributed, Hierarchical System
SCATS has been designed in a modular configuration to suit the varying needs of small,
medium, and large cities. Personal Computers are used. In its simplest form, a single regional
computer can control signals at up to 250 intersections. Expansion of the system is achieved
by installing additional regional computers. For large systems, it is usual to add a Central
Management Computer which provides centralised access for data input, monitoring and
traffic data collection, improved system management support, data analysis, data backup, fault
logging and analysis and a system inventory. These features ease the logistic burden of
managing larger systems. A typical large system SCATS computer configuration is shown in
Personal Computers operating under the Windows NT operating system are used for the
SCATS regional traffic control function. This software operates on standard PCs and The
Digital Alpha range of computers. Asynchronous serial (multiport) interfaces and modems
(one channel per intersection controller) interface the Regional Computer to the intersection
communications lines. Regional computers are usually located near the centre of the group of
signals to be controlled in order to minimise the cost of communications lines.
Central Management Computers
SCATS Central Management Computer can be a Personal Computer operating under the
Windows NT operating system. Communications with Regional Computers and
Workstations is via an Ethernet LAN or, for serial communications with remote Regional
Computers, via a PC based Communications Processor.
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Operator interface to SCATS is normally by a Personal Computer (PC) acting as a
workstation terminal and running RTA interface software. Minimum requirement is 486 DX
or better with a 32 bit operating system (Windows 95 or Windows NT V4). The graphical
user interface requires a minimum screen resolution of 800 x 600 (Super VGA).
PC workstations are able to operate in the following modes:
Local mode (as a free standing PC)
Local network mode (accessing any computer on the LAN)
SCATS workstation, providing access to the traffic control system and management
subsystems provided by the Central Management Computer.
Workstations may be connected via the LAN (eg thin wire Ethernet), via a terminal server
or direct to a Regional Computer.
Field terminals (eg laptop PC) connected to a local controller are also supported.
MONITORING AND CONTROL FACILITIES
The full range of operator commands and monitoring functions is available from all
workstations subject to the security access afforded to each operator as defined in the
database. Passwords are provided for security purposes. These facilities are provided from
workstations at the control centre, any regional computer, and any intersection controller or
remotely via modem. The data displayed includes:
Current phase demands
Time in Phase.
Current offset plan
System cycle length
System detector data
Graphical User Interface
A graphical user interface (GUI) is now the standard user interface, replacing the previous
character based screen. The intersection monitoring window with a small intersection graphic
display is illustrated in Figure 2. Data entry is by forms, an example of which is shown in
Figure 3. All alarms are logged and can be viewed with the Alarm management window as
seen in Figure 4.
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The PC workstations support full colour graphics in a sizeable window. Four levels of colour
graphics display are provided:
The Server window: Figure 5 shows a map of the whole system indicating by colour the
boundaries of each region and the traffic conditions at the six most heavily trafficked
subsystems in each region.
The Graphics window Regional display: Figure 6 shows a map of the selected regional area
with an on-line representation of traffic flow conditions by means of colouring the roads
with five different colours representing traffic conditions in the range from very light traffic
to heavy congested conditions.
The Graphics window Subsystem display: Figure 7 shows the selected subsystem layout
together with an on-line graphical bar chart representation of traffic flow and density as
measured by the strategic detectors in the subsystem.
The Graphics window Intersection display: Figure 8 shows the selected intersection layout
and phasing design with real time display of detector operation and phase greens.
It is possible to display and/or change all adaptive control parameters from any system
workstation while the regional computer is on-line both by operator command and
automatically by time of day. There is no need to take the regional computer off-line when
altering data or re-configuring the dimensions of any data array. Manual control of any
intersection is also possible from any system workstation.
The system provides a comprehensive set of alarm conditions to warn the operator of all
unusual or fault conditions. These alarms are logged automatically on occurrence and
clearance and can be queried at any time. Alarms are also provided for congested traffic
conditions in each subsystem.
SYSTEM HARDWARE – LOCAL CONTROLLERS
Four Controller Modes
SCATS local controllers can operate in four modes. These modes can be invoked manually or
automatically by the regional computer or at the local controller.
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In the adaptive mode the regional computer determines the phase sequence and the maximum
duration of each state and the duration of walk displays. The local controller may terminate
any phase under the control of the local vehicle actuation timers or skip an undemanded phase
unless prohibited by instructions from the regional computer.
The regional computer controls the phase transition points in the local controller subject to the
local controller safety interval times being satisfied (eg. minimum green, pedestrian clearance
etc.). On completion of the transition to a new phase, the local controller times the minimum
green, and minimum walk intervals and then waits for a phase termination command from the
regional computer. On receipt of the command to move to the next phase, the local controller
then independently times the necessary clearance intervals (eg. yellow, all-red) for the phase
Communications errors or faulty operation of the traffic control computer cannot cause the
local controller to produce dangerous signal displays such as short greens, short pedestrian
clearances, short yellows or short reds as would be the case if the local controller depended on
the regional computer for the timing of all intervals.
The termination of pedestrian walk signals is also under the control of the regional computer
so as to allow the walk timing to be varied to match prevailing traffic conditions. The
duration of the walk signal cannot, however, be less than the prescribed minimum walk.
In Flexilink (time based coordination) mode the phase sequence and the maximum duration of
each phase and the duration of walk signal displays is determined by the current plan. The
local controller may terminate any phase under the control of the local vehicle actuation
timers (gap, headway and waste) or skip an undemanded phase unless prohibited by
instruction within the plan. Flexilink is the usual fallback mode of operation.
In Isolated mode the state sequence and the maximum duration of each phase is as specified in
the local controller time-settings. The local controller may terminate any phase under the
control of the local vehicle actuation timers (gap, headway and waste) or skip an undemanded
phase unless prohibited by the local controller personality. Isolated mode may be specified as
the fallback mode of operation.
In Flash mode the signals display flashing yellow to all approaches. Other flashing displays
can be provided eg., flashing red/yellow.
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The signal cycle is divided into phases called A, B, C, etc., and these can be introduced in any
defined sequence (eg., A-B-C-A). Any phase can be skipped if no vehicle is waiting for a
green on that phase (eg., if no vehicle is waiting for B phase the sequence would be A-C-A).
In Isolated and Flexilink modes, the sequence is as defined in the local controller personality.
In Masterlink mode, the sequence is determined by the regional computer.
Stop Line Detection
All detectors (both strategic and tactical) are normally located at or near the stop line (one in
each lane). The calculation of DS relies on the detector being of sufficient length in the
direction of traffic flow to ensure that large values of space are not measured under conditions
of slow moving, closely spaced traffic (which would appear to be the same as light traffic
widely spaced). The detector must not, however, be too long as it would not measure any
spaces when traffic moves freely. Research has shown the optimum length of the detection
zone to be 4.5 metres.
Strategic detectors are located at the stop line in order to enable measurement of the use made
of the green time by traffic at a point at which the traffic is controlled by the signal. If the
strategic detectors were placed remotely from the stop line, assumptions would have to be
made about the flow rate actually achieved during the green period. At any time when these
assumptions were not valid, an incorrect green time would be allocated to the approach.
Tactical detectors located at the stop line enable differentiation between the left turn, straight
ahead and right turn movements at the intersection both by knowledge of the lane usage in
lanes of exclusive use and by speed differential in lane shared by two or more movements. If
the detectors were remote from the stop line it would not be possible to identify the intended
movement (direction) of detected vehicles due to subsequent lane changing. Additional
detectors may be installed in advance of the stop line but this has, in general, been found
Tactical detectors should be provided on all lanes of an approach (or movement) which will
benefit from tactical control, the more minor movements being the most suitable.
It can be seen that approaches most requiring strategic detection are those least requiring
tactical detection and vice-versa, resulting in the need for detection on most approaches. In
general, the approach lanes which can be left undetected are lightly used curb lanes on
approaches which otherwise require strategic detection and at minor intersections on the
“main road” approaches which are not immediately upstream of a major intersection.
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Two Wire Circuits
The local controllers are connected to SCATS by standard voice grade telephone lines or a
dedicated cable network. In both point to point and multi-drop configurations, a single pair of
wires is required.
Messages are sent to, and a reply message received from, each intersection controller, every
In point to point mode data is transmitted at 300 bps full duplex, asynchronous, FSK.
The low speed rate required for SCATS communications allows for a high degree of tolerance
in the reliability of the local communications network.
Roads and Traffic Authority of NSW Contact
Traffic Systems Branch
RTA Transport Management Centre
25 Garden St Eveleigh NSW 1430
Traffic Systems Branch
RTA Traffic & Transport Directorate
PO Box 1927
Strawberry Hills NSW 2012
Attention: Jim Giffin
Phone: +612 8396 1605
Mobile: + 61412 251 860
Fax: + 612 8396 1600
SCATS Distributor Worldwide Contact
Tyco Electronics Products Group
2-8 South Street
Rydalmere NSW 2116
Attention: Allen Yip
Phone: +612 9638 8212
Fax: +612 9638 8113
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Typical SCATS Computer Configuration
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Monitoring window with small graphic window
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Data entry form
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