The SCOOT Urban Traffic
Control System
1
Contents
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Introduction of SCOOT
SCOOT System Architecture
The SCOOT Traffic Model
The SCOOT Signal Optimiser
Several New Functions
Discussion: Deficiencies of SCOOT
2
What is SCOOT?
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The Split Cycle Offset Optimization
Technique (SCOOT), is an online signal
timing optimizer.
It was developed in 1973 by the
Transport Research Laboratory in the
United Kingdom
It has been implemented into realworld application since 1979.
3
Development of SCOOT
Early
1980’s
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Now
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Early work - developed off-line software (TRANSYT) to calculate
optimum signal settings for a signal network.
Based on TRANSYT, SCOOT has been continuously developed as an
on-line signal control system at early 1980’s.
Version 3.1 included bus priority, database facilities and incident
detection.
Version 4.2 added estimates of the emission of pollutants.
Version 4.5 enabled the bus priority to differentiate between different
buses. (e.g. : to give more priority to late buses.)
MC3 has enabled the Kernel software to safely use data supplied by
packet switched communications systems, provided a congestion
supervisor and increased the priority available to buses by allowing
state skipping where it is appropriate.
4
Introduction
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SCOOT: it is designed for general application within a computerized
Urban Traffic Control System.
Methodology: it is a method of coordination that adjusts the signal
timings in frequent, small increments to match the latest traffic
situation.
Traffic Model: data from vehicle detectors are analyzed by an online computer which contains programs that calculate traffic flows,
predicted queues.
Optimisers: three optimizers which are adapting – the amount of
green for each approach (Split), the time between adjacent signals
(Offset), and the time allowed for all approaches (Cycle Length).
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System Architecture
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How the SCOOT Works?
Upstream
Detectors
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C
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C
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On-line Traffic
Model
Up-stream
Cycle Flow
Profile
Down stream
Predict
Requirement
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C
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Signal Optimiser
Signal Timing
Plan
Local Controller
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SCOOT Traffic Model
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Both SCOOT and TRANSYT employ the traffic model
to predict the delay and stops caused by particular
signal settings;
In the case of TRANSYT, the model is off-line and it
predicts the average delays that resulted from
specified average flows;
The SCOOT model is on-line that the predictions of
delay are re-calculated every few seconds from the
latest measurements of traffic behaviors
8
SCOOT Traffic Model
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1.
2.
3.
4.
5.
Vehicle Detection
Cycle Flow Profile
Prediction of Queues
Congestion
Measurement of Travel Behavior
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Vehicle Detections
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The vehicle detectors are placed at the upstream of the stop-line;
The detectors are located as far upstream as possible from the stopline;
Normally, the distance between detector and stop-line is larger than
the maximum potential queue length;
If the actual queue length is larger than the distance, then the system
would get the warning of congestion, and the corresponding function
would be effective;
For the specific bus priority control, a set of bus detectors should be
installed.
10
Cyclic Flow Profiles (CFP)
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The data from detectors are stored in the SCOOT
system as the form of “Cyclic Flow Profiles”.
The profile patterns tend to be repeated and
coupled with new data in a cyclic sequence to avoid
large random fluctuation in the profile.
The cycle flow profiles contain the information
needed to decide how best to coordinate adjacent
pair of signals and cause the signal optimiser to
search for a new best timing.
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CFP Example (1)
Profile A shows that most of the
traffic crosses the detector as a
dense “platoon” during the first
half of the signal cycle time;
Traffic platoon
Without other considerations, a good
progression could be achieved by ensuring
that the downstream signals remain green
while the platoon crosses the stop-line.
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CFP Example (2)
Profile B shows no marked tendency
for traffic to travel in platoons;
Profile C shows that traffic has
formed two distinct platoons within
each signal cycle. In this case, the
green time at the downstream may
be arranged to give progression to
either the first or second platoon.
13
Prediction of Queues
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The cruise time is used to predict when the vehicle
flows that are recorded in the profile are likely to
reach the stop line.
Vehicle arrivals at the stop line during the red time
are added onto the back of queue, which usually
continuous to grow in the next green time until the
queue clear.
Vehicle discharges from the front of the queue at
the specified saturation rate
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Prediction of Queue
Time “Now”
Flow
Current CFP
0
1 Cylcle
Detector Data
Travel
Distance
Cruise
speed
Saturation Flow
Rate
Flow adds to the back of the
queue
Actual
queue
Predicted queue
Red
Time “Now”
Past
Green
Future
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Prediction of Queue
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It will be apparent that these predictions of queue
lengths cannot be completely accurate.
The prediction errors may become serious and so
various validations have been incorporated into
SCOOT.
As long as the validation has been accepted, the
queue model could be used continuously if the
traffic condition is stable in the future.
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Congestion
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Widespread congestion may occur when the queue
grow in length and extend backwards into its
upstream junction.
The traffic model measures the proportion of cycle
time that the detector is occupy by a queue,
moreover, the optimiser use this information to
reduce the likelihood of queue blocking the
upstream junction.
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Measures of Traffic Behavior
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To estimate current size of queue at link with the
control area.
From these estimates, SCOOT calculates an average
value for the sum of the queues; this value is used
as a measure of the inefficiency of traffic movement
and is called Performance Index (PI).
The SCOOT optimiser continuously searches for
signal settings that make the PI as small as possible.
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Measures of Traffic Behavior
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The total number of stops can be
weighted and summed with the
average queue into the PI.
The proportion of a cycle time that
vehicles are stationary over detectors
can be weighted and summed into the
PI (indicate congestion).
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Measure of Traffic Behavior
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The degree of saturation is defined as the ratio of
the average flow to the maximum flow which can be
pass through the intersection from a particular
approach and calculated by the traffic model.
Traffic model measure average travel demand (the
sum of the average flows across all the detectors in
the area) and the average value of PI.
If a large increase in PI without much change in the
demand, it suggests that abnormal event, such as
accident, has occurred.
20
SCOOT Signal Optimiser
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The system contains a set of parameters that control
all the signals settings in the area.
If this set of parameters unchanged, then the
associated signals will be controlled by a fixed time
plan.
However, in normal operations, the optimiser makes
frequent changes by small alternations to the
parameters so as to adapt the fixed time plan to
variations in the traffic behavior.
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SCOOT Signal Optimiser
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A few seconds before each stage change, it estimate
every junction whether it is better to make change
earlier, latter, or as scheduled.
To implement the alternation to minimize the “max
DS” on the approach of that junction.
Calculation is to take account of current queue
length, approach congestion measurement and
minimum green time constrains.
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SCOOT Signal Optimiser
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The sequence of installing process:
fix time plan,
split optimiser,
offset optimiser,
cycle time optimiser.
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SCOOT Signal Optimiser
Signal Optimiser
Minimization
Max. DS Approach
Calculation
Parameters
Current
Queue
Length
Constrain
Minimum
Green Time
Sequence
1. Fix Time Plan
2. Split Opt.
3. Offset Opt.
4. Cycle Opt.
Approach
Congestion
Measure
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Split Optimiser
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A few seconds (5 secs) before each stage change at
every SCOOT intersection is scheduled to occur, the
optimizer estimates whether it is better to make the
change earlier or later;
Any one decision by the optimizer may alter a
scheduled stage change time by no more than a few
seconds (Maximum allowed changed time)
The signal optimizer will minimize the maximum
degree of saturation on the approaches at each
intersection.
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Split Optimiser
Change of green duration:
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Temporary change (e.g. 4 secs): it is made to the change of
green durations to take account of the cycle-by-cycle random
traffic variations.
Permanent change (e.g. 1 sec): it is made to the stored values
of green duration so that longer term trends in the traffic
demands can be followed.
Over a period of several minutes, the proportions of green time
can be completely revised by SCOOT to meet a new pattern of
traffic flows.
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Split Optimiser
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Each junction will be threaded by the split optimiser
independently and performed more frequently than
other optimisers.
For example, in a network of 50 junctions with an
average of 3 stages per junction, there will be 150
decisions per cycle.
If some decisions are missed, then the split plan will
remain unchanged.
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Split Optimiser
Cycle by Cycle
Missing Decision
Junction
Independent
Stage
Specification
Making Decision
No Change
Stage Change
Stage Change
As Schedule
Early
Latter
Temporary
Change
Account Each
Temporary
Change
Account Cycle by
Cycle
Permanent
Change
Random Variation
Flow
Change Stage
Value
After Several
Minutes
Follow Long
Term Traffic
Demand
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Split Optimiser(example)
95%
55%
90%
90%
Without Optimization
-4
With Optimization
0
+4
1. Based on the CFP data, the degree of saturation at major road and minor
road would be 95% and 55% follow the scheduled.
2. With the objective of minimize the maximum degree of saturation, a
temporary change are made to increase the green duration of major road
by 4 secs and reduce the green time of minor road;
3. The corresponding degrees of saturation achieved a balance and both are
changed to 90%.
29
Offset optimiser
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At each intersection, the offset optimizer make the
offset decision once every cycle.
Since the offset of one intersection is altered relative
to adjacent intersection, the offset between an
adjacent pair of intersections may alter twice per
cycle.
Decisions are taken during a predetermined stage
within every cycle time.
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Offset optimiser
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To use cycle flow profile information to estimate
overall traffic progression in those street which are
immediately upstream and downstream.
To compare the “sum of PI” : scheduled offset /
occurring earlier or latter.
Offset optimiser are modified when congestion
occur: to improve the coordination on shorter street
at the expense of longer street, since the longer
street has a larger queue storage space to prevent
potential spillback.
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Offset optimizer
How the offset optimizer operate?
The offset decisions at A~E are
influenced only by the movements
represented in the “mini-area”.
Offset decision are taken once per
cycle for each “mini-area”;
Totally new signal offsets may
evolve where timing alterations
accumulate over several cycles of
the signals.
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Offset Optimiser
Estimation
Each Junction Cycle by
Cycle
Congestion
Comparision
Cycle Flow Profile Inform.
Schedule & Early & Latter
All Adjacent Junction
Adjacent
Prevent
Spillback
Minimum Sum of PI
Log Street
Fine Space for
Queue
Decision
Change Relative
Offset
Amend All Stage
Change Times
After Several
Cycle
Evolve New
Offset Plan
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Cycle Length Optimiser
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The signal controlled intersections are grouped into
“sub areas” which have pre-set boundaries.
All signals within a sub area are operated by SCOOT
on a common Cycle Length.
Also, some intersections can be operated on one
half of the common cycle time of the sub-are; this is
referred to as “double-cycling” and is of particular
value for signal controlled pedestrian crossings.
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Cycle Length Optimizer
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Constraints:
The cycle length would be re-optimized in increments of a few
seconds at interval of not less than 2.5 minutes;
Each sub-ease is independent of other sub-areas, between preset upper and lower bounds;
The lower bound is determined by the considerations of safety,
pedestrian crossing time and min green time (30-40 secs);
The upper bound is set to give maximum traffic capacity but
without unduly long red times (90-120 secs).
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Cycle Length Optimizer
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In SCOOT, the cycle length will increase when the traffic
demand is increasing:
Traffic
Demand
Time
Cycle Length
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Cycle Length Optimizer
Optimization Strategy
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Optimized interval: 2.5-10 mins;
Cycle Length increment: 4s, 8s, 16s, 32s (depends on the length of
scheduled cycle);
Objective: to provide maximum traffic capacity of the intersection;
Optimization Method: the cycle length is incremented or decremented
to ensure that the most heavily loaded intersection operates at a
maximum degree of saturation of about 90%.
1) if all stop-lines are less than 90% saturated, the CL decreased;
2) if the degree of saturation exceeds 90%, SCOOT will increase the
cycle length to increase the capacity.
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Cycle Time Optimiser
Common Cycle Length
Operate 90% DS
DS < 0.9
DS > 0.9
Cycle Time Reduce;
Demand Fix; Capacity
Decrease
Cycle Time Increase;
Demand Fix; Capacity
Increase
DS Increase
DS Decrease
DS Critical Intersection = 0.9
Minor Intersection < 0.9
Mini Area
Save Delay
System DS – 0.9
Upper Bound
Lower Bound
* Pedestrian
Crossing
* Minimum
Green
Single &
Double CL
Decision
New Cycle Plan
* Traffic
Capacity
Implement within 2.5
Min
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CL Optimizer (Example)
CL: 96; s: 87%
CL: 52; s: 43%
CL: 52; s: 43%
Most heavily loaded
intersection
CL: 64; s: 55%
CL: 104; s: 85%
CL: 100 s: 92%
CL: 52; s: 25%
CL: 104; s: 25%
Most heavily loaded
intersection
CL: 104; s: 32%
Scheduled
Cycle
Length
Optimized
Cycle
Length
CL: 104 s: 90%
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Control with users’ adjustment
Sometimes, the users may not satisfy with the optimized result and want to adjust
the optimization process to face some particular issues.
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For example: to optimize the signal along an arterial, traffic engineers prefer to
provide the arterial flows a better level of service. Then, a higher weight is
given to the corresponding directions.
SCOOT Result
Adjusted by engineer
90%
90%
70%
95%
70%
70%
50%
85%
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Congestion Management
SCOOT includes functions to deal with the occurrence
of congestion:
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It will automatically move the offset to these
congestion directions, to provide the congestion
movements with a larger green band.
Gating facility is also used to limit the flow of traffic
into a particularly sensitive area by restraining traffic
on user specified roads.
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Congestion Management
Upstream Control Strategy
Restrict traffic
Sensitive Area
At the upstream, the
gating facility is
used to limit the
traffic flows into the
sensitive area
Downstream Control Strategy
Sensitive Area
Vehicle discharge
At the downstream,
a proper signal
settings is provided
to discharge vehicles
ASAP.
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Bus Priority
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Bus Detection
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The SCOOT allows for buses to be detected either by selective
vehicle detectors, i.e. using bus loops and transponders on
buses, or by an automatic vehicle location (AVL) system.
The best location for detection will be a compromise between
the need for detection as far upstream as possible and the
need for accurate journey time prediction.
Bust detectors need to be located downstream of any bus
stops, as SCOOT does not attempt to model the bus dwelling
time.
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Priority Techniques
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Priority can be given to individual buses: extension to prevent a bus
being stopped at the start of red and recalls to start the bus green
earlier than normal.
In SCOOT MC3, intermediate stages between the current stage and
the bus stage can be skipped.
Differential priority allows different levels to be given to certain buses,
e.g., limited priority to late buses and high priority to very late buses,
but no priority to those ahead of schedule.
Controlled by user set parameters to prevent the
priority causing undesired extra delay to other vehicles.
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Bus Priority
If a bus is detected towards the end of Stage 1 (Orange), it will
receive an extension of green time.
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Bus Priority
If bus is detected during a red period, it will receive a
recall (the stage 2 and 3 are shorten so that stage 1 starts
earlier)
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Bus Priority
If the bus is detected during a red period, the following
stage 3 would be skipped so that the stage 1 starts earlier.
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Applications
Over 50 cities allover the world have implemented SCOOT into
applications.
In North America:
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Anaheim, California, USA

Santiago, California, USA

Oxnard, California, USA
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Minneapolis, Minnesota, USA

Red Deer, Alberta, Canada

Toronto, Ontario, Canada
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Deficiencies of SCOOT
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Due to the limitation of local controllers,
SCOOT cannot skip phase or change
the phase sequence.
The objective function is not sufficient
for use in various traffic conditions.
SCOOT does not provide good results
to deal with congestions, especially for
those caused by incidents.
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References
Introductions:
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Hunt, P. B. A Traffic Responsive Method of Coordinating Signals. Report No.
LR1014. Transport and Road Research Laboratory, Crowthorne, Berkshire,
England, 1981.
Greenough, J.C. and W.L. Kelman. Metro Toronto SCOOT: Traffic Adaptive
Control Operation. ITE Journal, Vol. 58, No. 5, May 1998.
Jhaveri, C.S., J. Perrin and P.T. Martin. SCOOT: An Evaluation of Its
Effectiveness Over a Range of Congestion Intensities. Presented at the 82nd
Annual Meeting of the Transportation Research Board, Washington, D.C.,
January 2003. TRB, National Research Council, Washington, DC, 2003.
SCOOT - The World's Leading Adaptive Traffic Control System. Peek Traffic
Limited, Siemens Traffic Controls and TRL Limited. www.scootutc.com/index.php. Accessed April 27, 2007.
51
References
Algorithm and Functions
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Hunt PB, Robertson DI, Bretherton RD, Royle MC. “The SCOOT on-line traffic
signal optimisation technique”. Traffic Engineering and Control, Vol 23, Issue
NO. 4., 1982.
Ian Day, Siemens Ag, Ron Whitelock. “SCOOT - Split, Cycle & Offset
Optimization Technique “. Transportation Research (1998)
Dennis I. Robertson and David Bretherton. “Optimizing Networks of Traffic
Signals in Real Time – The SCOOT Method”. IEEE Transaction on vehicular
technology, Vol. 40, NO.1, 1991.
R D Bretherton, M Bodger and N Baber. “SCOOT – Managing Congestion
Communications and Control.” Proceedings of ITS World Congress. San
Francisco (2005).
Hansen, B.G., P. T. Martin, and H. J. Perrin. 2000. SCOOT real-time adaptive
control in a CORSIM simulation environment. Transportation Research Record
1727. National Research Council, Washington, D.C. p.27-31.
52
References
Updated Versions
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Bretherton, R.D., and G.T. Bowen. Recent enhancements to SCOOT - SCOOT
version 2.4. Paper presented at the 3rd IEE International Conference on Toad
Traffic Control. 1990
Bretherton, R.D. Current developments in SCOOT: Version 3. Transportation
Research Record 1554. National Research Council, Washington, D.C.
Bretherton R D and G T Bowen. “Latest Developments in SCOOT – SCOOT
version 3.1” proceedings IEE 6th International Conference on Road Traffic
Control. London (April 1996).
Bretherton R D, K Wood and G T Bowen. “SCOOT Version 4.” Proceedings IEE
9th International Conference on Traffic monitoring and control. London (April
1998).
Siemens Traffic Controls Ltd., Peek Traffic Ltd. and TRL Ltd. Bus Priority in
SCOOT, 2001, http://www.scoot-utc.com/SCOOTFacilities/busprior.htm,
accessed June 12, 2002.
53
References
Materials Provided by the Company
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Advice Leaflet 1: The “SCOOT” Urban Traffic Control System
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Advice Leaflet 2: Congestion Management in SCOOT
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Advice Leaflet 3: SCOOT control of pedestrian facilities
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Advice Leaflet 4: Bus Priority in SCOOT
54
References
Other Evaluation Papers
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Mazzamatti, M.V., D.V.V.F. Netto, L.M. Vilanova and S.H. Ming. 1998. Benefits
gained by responsive and adaptive systems in São Paolo. Paper presented at
the IEE Road Transport Information and Control, Conference Publication No.
454.
Taale, H., W. C. M. Francis and J. Dibbits. 1998. The second assessment of the
SCOOT system in Nijmegen. Paper presented at the IEE Road Transport
Information and Control, Conference Publication No. 454.
Quan, B.Y., J.C. Greenough, and W.L. Kelman. 1993. The Metropolitan Toronto
SCOOT demonstration project. Paper presented at the 72nd Annual Meeting of
the Transportation Research Board at Washington, D.C.
Peck, C., P.T.W. Gorton, D. Liren. 1990. The application of SCOOT in
developing countries. Paper presented at the 3rd International IEE Conference
on Road Traffic Control at London, U.K. p.104-109.
Chandler, M.J.H., and D.J. Cook. 1985. Traffic control studies in London:
SCOOT and bus detection. Paper presented at the PTRC Annual Summer
Meeting at Seminar M, P269. p.111-128.
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