OVERSEAS ROAD NOTE 13
THE USE OF TRAFFIC SIGNALS IN DEVELOPING CITIES
Copyright Transport Research Laboratory 1996. All rights reserved.
This document is an output from a project funded by the UK Overseas Development Administration (ODA) for the benefit of
developing countries. The views expressed are not necessarily those of the ODA.
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Urbanisation/Transport, energy efficiency
Project title: The Use of Traffic Signals In Developing Cities
Project reference: ODA R6016 Theme: Main Subject area:
Transport Research Laboratory
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Crowthorne, Berkshire, RG45 6AUOverseas Development Administration
94 Victoria Street
London, SW1E 5JL ACKNOWLEDGEMENTS
This Note was prepared by A. Cannell of Transcraft Consultants, Curitiba, Brazil and G Gardner of the Overseas Centre,
Transport Research Laboratory (TRL). Useful advice and assistance was given by D. Singh and J. Cracknell.
First Published 1996
OVERSEAS ROAD NOTES
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Centre, Transport Research
Laboratory. Total Lost Time per Cycle (L)
Degree of Saturation
Junction Capacity Analysis
Traffic Signal Calculation Sheet
Stage/Phase Sequence Diagram
Check List for Signal Design
6. COORDINATION AND LINKING
OF TRAFFIC SIGNALS
Simple Progressive System
Mechanisms for Linking Signals
Fixed Time Coordinated Signals
Area Traffic Control (ATC)
Fixed Time ATC Systems
Fully Responsive Control
7. THE TRANSYT PROGRAM
8. SYSTEM AND ECONOMIC ANALYSIS
OF TRAFFIC CONTROL
9. SPE CIFI CATI ONS
Traffic Signal Controllers
General Road Traffic Signals
Inductive Loop Detectors
Associated Electrical Works23
Traffic Signal Controller Civil Works 33
26 Time Settings
2. CRITERIA AND WARRANTS
FOR SIGNAL INSTALLATION
3. BASIC TRAFFIC COUNT SURVEYS
4. JUNCTION DESIGN AND LAYOUT
Siting of Signal Equipments
Approaches and Lanes
Signal Design Techniques
Right Turning Vehicles
Early Cut Off
No Pedestrian Signal
Full Pedestrian Stage
Staggered Pedestrian Facility
Pedestrian Signal Displays
Pedestrian Push Buttons.
Pedestrian Signal Sequences and
Vehicle-actuated (V.A.) Traffic Signals
Traffic Signals on High Speed Roads
Speed-related Green Extensions
5. CALCULATION OF TRAFFIC SIGNAL
TIMINGS - WEBSTER'S METHOD
Minimum Green Period
Estimation of Saturation Flow
Width of Approach
Parking, Waiting and Bus Stops THE USE OF TRAFFIC SIGNALS IN DEVELOPING CITIES
1.1A traffic signal installation is a power-operated device
which informs motorists or pedestrians when they have the
right of way at a particular intersection.
1.2The first traffic signal was installed in London in 1868
and used semaphore 'arms' together with red and green gas
lamps. Unfortunately, it exploded, putting an end to this sort of
control for 50 years.
1.3However, in 1918 the first three coloured light signals
were installed in New York and in 1925 they started to be used
in Great Britain.
1.4At the beginning of the 1930's an attempt at making the
signals more `intelligent', or vehicle responsive, was tried in
America, using microphones at the side of the road, requiring
drivers to sound their horns. This was obviously not too
popular and the first traffic detectors - electrical and pneumatic
- were invented.
1.5Traffic signals are now used throughout the world, using
the three light signals of Green, red and amber. Also, by
convention, these are normally arranged vertically with the red
signal at the top and the green light at the bottom. This also
helps people who are colour blind - both drivers and pedestrians
- to identify the differences between the lights.
1.6Traffic signals are used at intersections to reduce
conflicts to a minimum by time sharing of right of way. This
actually reduces the capacity of the intersection, but greatly
1.7Conflicts at intersections are illustrated in Figure 1 which
shows the potential conflict points at the junction of two roads,
both with two way traffic flows, at which all crossing and
merging movements are permitted.
1.8With the provision of traffic signal control the number of
potential conflicts can be reduced from 64 to zero.
1.9The object of this report is to give traffic engineers or
technicians in the cities of the emerging world a brief
introduction to traffic signals, together with some practical
guidelines on how to use them to obtain good and safe results.1.10There is no doubt that signals are one of the most
powerful tools for urban traffic control available to city
authorities and their correct installation can improve both
traffic flow and the safety of all road users. In comparison to
other traffic improvements, signals are also relatively low
capital intensive and in recent years the advancement in
informatics and telecommunications has led to a new
generation of low cost controllers and systems that have made
modern signalling an even more attractive and powerful tool.
1.11 Essentially, traffic signals form part of the "soft-
ware" of a city as opposed to the roads and bridges that are
part of it's "hardware". As such they have the advantage of
being cheap and often the disadvantage of being so cheap
that no local lobby is interested in them, especially when
city mayors fail to see the political advantages in changing
an old signal for a new one.
1.12 It is thus part of the traffic engineer's task to prove to
city authorities that a modern and well designed traffic
signal system will bring real and visible benefits to the city.Figure 1 Conflict points at an intersection
1 2. CRITERIA AND WARRANTS FOR
2.1When two or more traffic flows are competing for the
same road space at a junction, some form of control - or set of
rules - is needed to minimize delays and the risk
accidents. In some countries, a simple rule of preference states
that the traffic coming from the left (or right where there is
right-hand drive) has priority to enter the junction. As few
people tend to know - or obey - this rule, unsignalled junctions
can come under "popular control" and users have to consider
that the larger vehicle, or the one that sounded the horn first, or
a public transport bus, etc., may have priority.
2.2This is obviously inefficient and dangerous, so with
higher flows some form of stop or priority sign is used to
inform to the user on one or more approaches that the other
road has right of way. At even higher flows this form of control
breaks down when the delay on the minor road becomes too
high, forming queues and forcing drivers to run the risk of
accepting gaps in the major road traffic that are too small for a
safe crossing. At this point, time must be allocated for the right-
of-way to traffic on the various approaches.
2.3However, the introduction of traffic signals (or lights)
into a city often runs the risk of these equipments being
considered a panacea for all traffic problems. The engineer or
technician in charge of the traffic comes under political and
popular pressure to install too many signals, thus leading to the
even greater risks of red-running - as the users `learn' to
disrespect the red lights that they consider to be unnecessary.
2.4To avoid this problem it is essential that the engineer or
traffic department has a clear set of warrants to justify the use
2.5If possible, these warrants should be approved by the
local government bodies (elected and executive) so that
requests for signals on sites that do not need them can be
refused according to pre-discussed rules - and not just on the
personalized decision of the head of the traffic depart-ment.
2.6Traffic signals may be justified if, usually two, of the
following criteria are present:
- where there is a minimum major-street/minor-street
conflicting vehicle volume;
- where there may be need to interrupt continuous flow
on the major road to allow traffic to exit from the
minor road without excessive delay;
- where a minimum pedestrian volume conflicts with a
minimum vehicle volume;
- where a schoolchildren crossing is present;
- where there is a need to maintain progressive movement
of vehicles along an otherwise signalled route; and
- where there is a record of accidents of the type which
could be reduced by the use of traffic signals.
2.7A rough and ready set of warrants might be:
traffic flows - when there is a minimum of 1000 pcu's per hour
entering the junction during the peak hours.
visibility - when drivers on the minor road have poor visibility
for judging gaps.
accidents - when three or more accidents (collisions or
pedestrians) are registered per year.
2.8Figure 2, for example, shows the relationship between
major-road/minor-road flows and the type of control
recommended at a junction in the UK. For a major road flow of
20,000 pcu's per day and a minor road flow of 6,000, a
roundabout would be a good solution for eliminating the
conflicting traffic movements - if space were available. If,
however, the junction is in a built-up area, then traffic signals
probably represent the best solution.
2.9It should be stressed, however, that traffic signals if
located or timed wrongly can INCREASE delays and accidents
and their maintenance and electrical supply represents an
ongoing cost of around US$1000 to 2000 per year.
2.10To minimize the need for signals, the road hierarchy
should try to conform to the network shown in figure 3, which
offers the most efficient and safe layout.
Figure 2 UK practice for intersection control selection
based on combinations of traffic flow 2.11A method of reducing conflicts on local distributors and
access roads is to physically separate traffic flows, allowing
access but avoiding the pressure to install new lights.2.12Figures 4 and 5 show how, in some cases, conflicting
flows may be avoided - provided that no economical or
environmental restrictions exist.
2.13 If, however, traffic lights are to be installed, the
engineer and police forces should be in agreement on how
the flows are controlled. In many developing cities, the
police will often take manual control, assuming that they
can reduce traffic queue lengths. Research has shown that
this is not true (Walker et al, 1988). Police are reluctant to
stop a traffic stream even when it is no longer saturated, as
shown in figure 6. It is preferable to allocate police to
control illegal parking, removal of breakdowns and enforce
Figure 6 Typical flow/saturation relationship for
police control - inefficient use of the
end of the green period
Figure 5 Eliminating the need for traffic
signals -"7 esquinas", Arequipa, Peru Figure 3 Ideal urban road network
Figure 4 Elimination of conflicts at a junction pair 3. BASIC TRAFFIC COUNT
3.1For each site where traffic signals are being contem-
plated it is fundamental to obtain adequate data on the traffic
flows at the junction. Normally, surveys would be carried out
during the peak hour periods. However, it may be important to
have a broad view of the flows in the city throughout a normal
working day, especially when Area Traffic Control or linked
signalling are being considered.
3.2Traffic counts are likely to be divided into two types:
- all day counts (normally during 16 hours of a work day)
usually mid block on key roads, with the objective of
defining the duration of the peak periods and general
vehicle composition; and,
- specific junction counts carried out with the objective of
providing the data for evaluation and design of the
3.3The classification of vehicles might be cars, taxis, light
vans, trucks (heavy and medium) and public service vehicles.
In some cities it will be necessary to include motorcycles,
cycles or other common vehicle types. The counts should be
made in periods of about 15 minutes, during at least two
working days. If the counts are not similar (as demonstrated in
figure 8), then the counts should be repeated on another
A simple 16 hour survey form could look like figure 7.
3.4Specific junction counts are aimed at providing the data
for detailed evaluation and design. The peak periods can be
identified from the all day (16h) counts and the junction counts
should be undertaken in the peaks - including the "shoulders"
just before and after the peaks. Unless a city is subject to
excessive congestion, this usually means a count period of
about two hours for each peak. If an ATC scheme is under
consideration, counts should also be carried out at weekends.
3.5Each surveyor can usually manage to count two
independent flows. For a simple junction involving two
one-way streets, two surveyors (normally temporary staff) will
be needed, as shown in figure 9.
3.6Each site should also be carefully checked to make sure
that pedestrian volumes during the peak hours that might
require special phases are also considered.
3.7Counts in congested areas often suffer from the spillback
of upstream queues which means that surveyors will not count
the real demand of the traffic that wants to go through the
junction, but only the traffic that actually manages to pass. This
can lead to the classic case, in which
4Figure 7 Simplified traffic survey form
a survey is made during a widespread "gridlock"; reported by
the surveyors in terms of near zero flow on all approaches.
3.8TRL ORN 11, "Traffic Surveys in Developing Cities"
should be consulted for further reference.
3.9 The warrants used and/or approved by the city to
justify the installation of signals are likely to include
accidents. It must be stressed that an updated accident data
base is essential for completing the traffic surveys. Figure 8 16 hour traffic count on Peru Street, Mendoza, Argentinia, During two working days
Figure 9 Survey forms for a simple junction of
two one-way streets
5 4. JUNCTION DESIGN AND
4.1The aim of any junction layout is to provide for the safe
movement of traffic, both vehicular and pedestrian, without
undue delay or congestion. Various alternative layouts may be
considered and the ultimate choice will be governed by such
factors as the nature and volume of traffic using the junction,
the availability of land and the cost.
4.2The overall capacity of a road network is limited by the
capacity of individual junctions. Failure to provide the correct
type of layout at one particular junction may result in accidents,
congestion and delay to an extent which may impair the
efficiency of the road system over a wide area.
4.3The following descriptions of junction layout and design
procedures are based mainly on UK practice. Other standards
are of course possible. For example, in the UK signals are
located on the kerb, at the roadside with the "primary" signal
close to the stop line. In many countries overhead signals on the
"far side" of the junction are the norm. Both methods have their
merits, however, a country will generally have it's own
standards and such standards have to be adopted in designs.
The important requirement is that signals should be consistently
designed, located and operated throughout the city and clear
unambiguous indications given to all road users.
SITING OF SIGNAL EQUIPMENTS
4.4The minimum requirement is one traffic signal in-stalled
I m from the stopline, on the nearside of the carriage-way. If
possible a second primary signal should be installed if there is a
central island or divider, or more than three approach lanes.
Minimum visibility distances from the primary signals are
given in Table 1
right of way first. The secondary signal in this case should not
be placed beyond the nearside of the junction.
Approaches and lanes
4.7It is essential that approaching drivers are made fully
aware of the nature of the junction by adequate signing.
Carriageway markings and/or channelized islands should be
used to guide users on the correct path, and visibility should
not be impaired.
4.8Approaches should be marked out in lanes. Lane widths
at signalled junctions should normally be between 3 and 3.6m,
although 2.7m is acceptable in some instances where speeds
are low and there are few large vehicles (trucks or buses).
4.9On roads where land is available the saturation flow and
capacity of an approach can be increased by widening the road
to the vicinity of the junction to provide more ahead lanes. An
example of this is shown in figure 10. Another option, where
there are large turning movements is to divide the road space
available to favour the turning lanes, as shown in figure 11.
4.10 Perhaps the most important factor affecting the
capacity of a junction approach is the need to avoid
obstruction to traffic flow, either temporary (a taxi or bus
stopping for passengers) or permanent (a parked car). Plate
1. clearly shows the problem caused by a (very) long term
parked car which has eliminated a lane of traffic. In a
situation such as the example in plate 2, even the most
sophisticated traffic signals will not improve the traffic
TABLE 1: VISIBILITY DISTANCES
85 percentile visibility distance
approach speed (m)
50 km/h 70
60 km/h 95
70 km/h 125
85 km/h 165
100 km/h 225
4.5A secondary signal is normally installed diagonally
opposite the first primary signal, as shown in figure 12.
4.6When the signal method of control contains a special
right turn phase, extreme care should be used in the siting of
secondary signals for the direction of flow which losesPlate 1. Parked car obstructing the approach
- a severe capacity loss Figure 10 Flared junction a pproachFigure 11 Extra road space given to approaches
TABLE 2: APPROACH LANE WIDTHS
Approach Lane width (m)
Lane 1 Lane 2 Lane 3 Lane 4
5.50 2.75 2.75
6.00 3.00 3.00
8.00 4.00 4.00
8.50 3.00 2.75 2.75
10.00 3.40 3.30 3.30
11.50 3.10 2.80 2.80 2.80
Lane 1 is nearest the kerb
4.11Each signal face normally has three vertical lights with a
nominal diameter of 200mrn. The height of the centre of the
green lens from the surface of the carriageway (where light
signals are placed at the side of the carriageway) should be not
less than 2.1 metres nor more than 3.5 metres. If signals are
placed over the carriageway, this distance should not be less
than 5.0 metres nor more than 9 metres.4.12Traffic control is by means of red, amber and green
signals, supplemented by additional green arrow light signals,
tram signals, etc.
4.13The signal sequence at junction traffic signals in British
practice countries is red, red + amber and green, amber and red.
Most Panamerican standard countries, however, use the
sequence red, green, amber and red and some countries adopt
other variations, eg. flashing green in place of amber.
7 8Figure 12 Typical layout of a signalled controlled junction 4.14 the red light signal indicates the prohibition that
vehicular traffic shall not proceed beyond the stop line provided
in conjunction with the light signals, or if the stop line is not
visible (or there is no stop line), beyond the light signals.
4.15 the amber light signal when shown alone, indicates the
prohibition that vehicular traffic shall not proceed beyond the
stop line, or if the stop line is not visible (or there is no stop
line), beyond the signals, except in the case of any vehicle
which when the light signal first appears is so close to the stop
line or light signals that it cannot be safely stopped before
passing the stop line or light signals. The time for the amber
signal is normally fixed for the city or region at 3 or 4 seconds.
4.16 the red and amber light signals together indicate an
imminent change from red to green. However the red light still
prohibits forward movement.
4.17 the green light signal means that traffic may proceed, if
safe to do so.
4.18 the green arrow signal indicates that traffic may
proceed only in the direction indicated by the arrow.
4.19 a flashing amber signal in some countries means that
drivers must proceed with caution. Normally displayed on all
approaches with a frequency of 1 hertz (1 flash per second),
this signal is sometimes used from midnight to 4 or 5 o'clock in
towns with notorious night time red-running.
4.20Pedestrian signals are red and green, either with a green
walking man and a red standing man, or with "WALK/ DON'T
4.21There are two alternative concepts used in describing the
control of traffic by means of light signals. One, known as
stage control, is concerned with the sequentialsteps in which the junction control is varied. The other, phase
control, refers to the periods of time allocated to each traffic
4.22In UK practice a phase is used to describe a set of traffic
movements which can take place simultaneously or the
sequence of signal indications received by such a set of
movements. A stage is that part of the cycle during which a
particular set of phases receives a green indication.
4.23In USA based practice, a phase is that part of a cycle
allocated to any combination of traffic movements receiving
the right-of-way simultaneously during one or more intervals.
An interval is a period of time during which all signal
indications remain constant.
4.24 The cycle is the complete series of stages during which
all traffic movements are served in turn. The cycle time is the
sum of each of the stage times.
SIGNAL DESIGN TECHNIQUES
4.25Conflicts are reduced at signal controlled junctions by
holding certain traffic streams stationary while others are
allowed to pass. To hold all streams and release each in turn
would remove all conflicts but would not be satisfactory since
delays to all traffic would be high and effective capacity of the
junction would be low.
4.26The art of designing an installation is to reduce delay
and increase capacity while still maintaining a high degree of
4.27Reduction in total delay and improvement in capacity
can be achieved by:
- utilizing the lowest practicable number of stages in
any signal cycle.
- ensuring that each approach is capable of carrying the
maximum predicted traffic flow for that approach.
- ensuring that the time allotted to each stage is appro-
priate to the actual traffic flow.
- if appropriate, coordinating the control of adjacent
junctions to maintain the flow of traffic `platoons'.
- allowing simultaneous non-hooking right turns.
- separating left turn movements with an exit lane
controlled only by a "give-way" priority sign.
- where the degree of conflict is acceptable and move-
ments can be executed safely with the exercise of due
care, a conflicting move may be accepted (e.g. a right
turn on full green).
- restriction of movements, e.g. banned right turns,
where conflicting manoeuvres are forbidden.
- separation of traffic streams which conflict, assigning
them to different stages.
Plate 2. Street markets: a safety risk as well as
a huge capacity restraint - considering different stage sequences for different
times of the day.
- providing extra lanes for turning traffic or flares on
- combining the green periods for vehicles and pedes-
trians when this can be done safely.
- providing two separate green periods in a cycle (re-
peated greens) for important movements.
4.28As an example of these principles, figure 12. shows a
four arm junction with two stages with all movements
permitted. This is a very common junction and two stage
operation forms the basis of signalling techniques. Traffic on
opposite arms flows simultaneously, while traffic on the other
two arms is stopped. Each arm may have one or more lanes on
approach but the right turning traffic may impede vehicles
wishing to proceed over the junction if the road width is
restricted. Where there is a relatively minor right turn flow the
capacity of the junction is reduced by the road space occupied
by such traffic waiting to turn right and by the time which has
to be provided to this movement in the cycle. If the right turn
manoeuvre is removed then reduced delay and improved
capacity can be expected. An alternative route may often be
indicated to traffic before the junction is reached. Usually
motorists can turn left before the junction, make two right turns
to appear at the junction on the left hand arm (known as a `g'
turn). Alternatively motorists can pass through the junction,
turn left and make two further left turns to appear at the
junction on the left arm (known as a `q' turn). Such "q" and "g"
turns should be carefully evaluated as there will be increased
costs to set against savings injunction delay. In the case of "q"
turns, the use of the junction twice by former right turn traffic
may adversely affect junction capacity and thus delays and
RIGHT TURNING VEHICLES
4.29The usual practice is for opposing right-turners to turn on
the nearside of each other. With this arrangement locking of
turning movement cannot occur but driver visibility may be
4.30On high speed roads or where right turning movements
are heavy (above 300 pcu's/h), separately signalled and
segregated lanes are strongly recommended.
4.31Another very common situation is the four arm junction
with three stages. The types of control are known as either early
cut-off or late start.
EARLY CUT OFF
4.32To facilitate a heavy right turn movement from one
approach, the green time of the opposing approach can be cut
off some seconds before the approach with the right turn.4.33The approach which is permitted to flow over two stages
should have a three light primary signal. The secondary signal,
placed beyond the junction, should have four lights, including a
right turn arrow of 300mm diameter (in addition to the full
green signal) illuminated on the second stage when the
opposing traffic has been signalled to stop, as shown in figure
13 and 14.
Figure 13 Early cut off stage sequence
Figure 14 Green filter arrow for right turn
4.34An alternative way of dealing with right turning traffic is
to delay the start of the opposing traffic by a few seconds. This
method causes difficulty at the start of the following stage if
the right turn flow is heavy and the opposing traffic cannot
establish precedence. For this reason a late start stage is usually
4.35When both right turn movements are heavy, another
option available is to hold both right turns with a red signal
while the ahead and left turn traffic flows unhindered. All
traffic is then stopped before the right turn traffic on both
approaches is released together on the same stage. It is usual to
separate the right turn traffic onto exclusive lanes with separate
signals on each approach. This method should be employed on
high speed roads.
4.36When a traffic signal installation is being designed or
modified, the nature and extent of pedestrian flow has to
10 be taken into account as well as that of vehicular traffic. The
object of providing pedestrian facilities is to assist pedestrians
to cross in safety, with the minimum delay to both pedestrian
and vehicular traffic.
4.37There area number of alternative methods of achieving
this aim and the engineer has to consider which of these
methods can be best applied to individual sites, knowing the
pedestrian flow pattern, degree of saturation and site layout.
4.38Each junction should be considered on its own merits,
taking into account factors such as infirm or handicapped
pedestrians, junction capacity and any available accident
4.39 If full pedestrian stages are new to the local traffic
culture, great care should be taken to introduce them only
when accident data and high pedestrian flows justify their
NO PEDESTRIAN SIGNAL
4.40The presence of traffic signals at an intersection provides
assistance to pedestrians in crossing the arms of a junction,
especially where refuges are available, and in many cases no
further facility is necessary. An extended all red period between
two traffic stages to assist pedestrians is not recommended. This
practice leads to increased delays to traffic and to
driverdisobedience since the extended period will always be
present even when there are no pedestrians.
FULL PEDESTRIAN STAGE
4.41With this facility, all traffic is stopped while pedestrian
movement is signalled across all arms of the junction. This
method will cause delay to traffic. However, the stage can be
programmed only to operate during certain hours or by demand
from push buttons. Where the crossing is across a dual
carriageway, additional push buttons on the central reserve
should also be considered.
4.42Although pedestrians may be allowed to cross any of the
approaches to an intersection there will usually be one approach
upon which the pedestrian problem is most acute. The
pedestrian stage should immediately follow the end of the
vehicle stage on this approach. The signal sequence should be
arranged to ensure that on termination of the pedestrian period,
the right of way will revert to a nominated stage in the absence
of other demands.
This is shown in figure 15.
PARALLEL PEDESTRIAN STAGES
4.43Where it is possible to prohibit permanently some
turning movements a combination of pedestrian and vehicle
stages can be installed. By virtue of banned turns, pedestrian
facilities can be provided across appropriate arms. In order to
reduce the possibility of vehicles turning illegally, kerb radii
should be kept as low as possible.
Figure 15 Full pedestrian stage
STAGGERED PEDESTRIAN FACILITY
4.44Where carriageway widths permit, a large island in place
of the normal refuge may be provided. Pedestrians can
negotiate one half of the carriageway when traffic on that
approach is held on red at the junction signals. Normal
pedestrian signals are shown during this period. The other half
of the road is controlled by separate signals which are located
at the opposite end of the island. Normally the stagger should
be at least one crossing width in order to alert pedestrians that
the crossing is in two sections. A right-hand stagger may
reduce junction intergreen times by placing approach stop lines
closer to a junction. A left-handed stagger, as shown in
figurel6, is normally preferred as pedestrians stepping on to the
central refuge will turn towards the approaching traffic stream.
PEDESTRIAN SIGNAL DISPLAYS
4.45Normally each signal face has two lights arranged
vertically (the upper red standing man and the lower green
walking man) of 300mm nominal diameter. An alternative size
of 200mm nominal diameter may be used when specified.
4.46The red stationary man, when illuminated by a steady
light, indicates to a pedestrian that he should not cross or start to
cross the carriageway at the crossing. tone for the green walking man period and an intermittent tone
for the flashing green period.
4.51Each proposal for use of audible signals at junctions
should be considered on individual merits and carefully
checked against real demand, safety aspects and potential risks,
technical feasibility of the equipment or supplier, local layout
and environment (these signals are not popular with nearby
residential blocks of flats).
4.52An additional benefit to the visually handicapped can be
given by fixing metal plates with the street names in Braille
onto traffic signal posts in the vicinity of schools or other
buildings frequently used by them.
4.53It is desirable in some cases to restrict the crossing of
pedestrians to certain approaches at an intersection and guard
rails can be used to prevent pedestrians crossing at dangerous
places (for example where filtering traffic may be moving at
times unexpected by pedestrians). Guard rails should always be
provided on large islands where staggered pedestrian
movements are allowed. Normally minimum length of guard
rails provided at each side of a crossing should be 15m.
PEDESTRIAN SIGNAL SEQUENCE
4.54Pedestrian time should be sufficient to enable pe-
destrians to cross the full width of the road with relative ease at
normal walking speed. An assumed walking speed of 1.2 m/s
for the measured crossing distance is satisfactory in
determining the minimum times. A staggered crossing can be
considered as two separate crossings.
4.55Normally, minimum green periods of less than 5
seconds are considered too short and are not recommended.
4.56Provided that the above minimum requirements are met,
the green period of a parallel pedestrian stage may be
determined by the predominant traffic flow running in parallel.
4.57 The vehicle clearing times before the start of all
pedestrian stages should be checked to ensure that the last
vehicle clears the crossing by the time the pedestrian green
signal is lit.
A summary of pedestrian facilities is given in table 3.
4.58With vehicle -actuated (VA) signals the duration of
the green periods and the cycle time will vary in relation to the
traffic flow into and through the controlled area. A vehicle-
actuated signal responds to demands recorded for
Figure 16 Left-handed stagger stage sequence
4.47The green walking man signal, when illuminated by a
steady light, indicates to a pedestrian that he may cross the
camgeway at the crossing.
4.48The green signal, when illuminated by an intermittent
light (flashing green man) indicates that a pedestrian who is
already on the crossing should proceed to complete the crossing
with reasonable speed; and/or a pedestrian who is not already
on the crossing should not start to cross.
PEDESTRIAN PUSH BUTTONS
4.49Pedestrian push buttons units mounted on signal posts
may be used for calling up pedestrian stages. Additional push
buttons are also necessary on wide refuges where pedestrians
may be trapped at the end of the pedestrian stage. It is advisable
to have push buttons located at each side of the pedestrian
crossing, so that pedestrians approaching from either direction
can pass a push button before reaching the crossing.
4.50Audible warnings, in the form of pulsed tones, are
intended for the benefit of visually handicapped pedestrians.
The set up consists of a post-mounted audible device which
emits different patterns of audible signal, representing different
pedestrian signal indications e.g. a slow hammering tone during
the red standing man period, a quicker the various directions of flow. Once a green has been given to a
particular direction of flow, the length of green for that
direction will be extended until all the traffic has passed
through the junction, or the maximum green time for that
direction has been reached.
4.59Vehicle actuated signals will be most appropriate for
isolated junctions where coordination with other signals is not
important and for locations with fluctuating light or medium
4.60On the approach to a red signal, a green signal will be
demanded on the arrival of a vehicle on that approach. This
demand is stored in the controller which will serve stages in
cyclic order omitting any stages for which no demand has been
received. Where it is essential that one stage must always
follow another, the appearance of the first stage will
automatically insert a demand for the second stage.
4.61When a stage loses right of way on a maximum green
period change, then a demand is inserted for a reversion to that
stage after other demands have been met.
4.62When a green signal is displayed, the period for which it
is displayed may be extended by vehicles detected moving
towards the signal. The purpose of this extension, or the sum of
several extensions, is to permit vehicles to pass the stop line
before the maximum green period is reached.
4.63With some semi-vehicle-actuated signals, detectors are
installed on the side roads only (i.e. not all approaches) and the
right-of-way normally rests with the main road, being
transferred immediately or at the end of a preset period to the
side road when a vehicle passes over the side road detector.
The green period on the side road can be extended in the
normal way by successive demands up to a preset maximum.
After right-of-way has been returned to the main road, it cannot
be taken away from the main road until the preset period has
4.64Another modified form of V.A. signals is to operate one
or more demand-dependent stages within a fixed cycle time.
The demand dependent stages which may consist of vehicle
phases (such as right turn traffic, minor flows) or pedestrian
phases may be slapped or extended in accordance with the
prevailing situation detected. The advantage of this type of
control is that a fixed cycle time can be maintained for linking
with surrounding controllers.
13 TRAFFIC SIGNALS ON HIGH
4.65When traffic signals are installed on roads where the 85
percentile approach speed at a junction is between 60 km/h and
105 km/h on any arm, drivers have a difficult decision to make
when green changes to amber: they are often faced with a
choice between attempting to brake to a halt at the stop line, or
continuing at the same speed through the junction and clearing
4.66They may fail to achieve either, thus putting themselves
and others at great risk.
4.67Because of the increased braking distances required at
high speeds, drivers need adequate warning that they are
approaching a signalled junction. High approach speeds also
result in drivers misjudging the lengths of gaps in opposing
traffic when making a right turn at the junction -again leading
to increased risk.
4.68On high speed roads, the use of right turn clearance
phases should be avoided. Right turning movement, across
high speed flows should be channelized and controlled with a
separate vehicle phase, or preferably banned.
SPEED-RELATED GREEN EXTENSIONS
4.69To assist drivers and minimize risk it is necessary to
provide green extensions, the extensions being related to the 85
percentile approach speed. Normal approved vehicle detection
equipment is used within 40m of the stop line on each approach
and in addition approved speed discrimination or speed
assessment equipment can be used.
4.70 Advance warning signs are necessary on each
approach, according to local or regional standards.
4.71 When the 85 percentile approach speed on any arm
exceeds 105 km/h it is recommended that traffic signals
should not be installed.
4.72The great majority of passengers in the cities of the
developing world travel by bus. Although these road users
normally have less political influence than the more affluent car
owners, the traffic engineer should consider how to improve
bus flows at signalized junctions.
4.73The simplest form of priority is to guarantee that
saturation on the approaches most used by buses is kept as low
as possible, even if this means additional waiting times for the
other stages. 4.74In ATC systems, the TRANSYT program (see section 7)
permits bus flows to be treated separately thus providing
optimum settings for buses.
4.75 rity to buses, not necessarily within ATC systems has been
achieved at traffic signals by a number of methods. These
- the selective detection of buses using on-bus trans-
ponders and detectors in the approaches to signals;
- the use of segregated lanes, exclusively for buses on
approaches to junctions, within which detectors are
installed to actuate the signals; and
- the use of pre-signals on the approaches to junctions.
These enable traffic queues to be relocated upstream
of the junction and control traffic and bus flows to an
advance area so that all vehicles are able to clear the
junction. (TRL ORN 12, 1993). Figure 17 Phase and stage sequence for early cut off operation TABLE 3: SUMMARY OF PEDESTRIAN FACILITIES
Type of facility Characteristics
No pedestrian signal - Traffic signals, even without signals for pedestrians, can help
pedestrians to cross by creating gaps in traffic streams.
- Especially applicable where there are refuges and
on one-way streets.
Full pedestrian stage - All traffic is stopped.
- Demanded from push buttons.
- More delay to vehicles than combined vehicle/pedestrian stages.
Parallel pedestrian stage - Combined vehicle/pedestrian stage often accompanied
by banned vehicle movements.
- Useful across one-way streets.
Staggered pedestrian facility - Pedestrians cross one half of the carriageway at a time.
- Large storage area in the centre of the carriageway required.
- Stagger preferably to face on-coming traffic.
Displaced pedestrian facility - For junctions close to capacity.
- The crossing point is situated away from the junction but within 50m.
- Normal staging arrangements as above apply.
15 distance 'x' should be determined from the position of the
pedestrian crossing. Where pedestrians are losing right-of-way
the start of the following stage should be delayed until the
crossing area is clear.
MINIMUM GREEN PERIODS
5.8Minimum Green Periods cannot be overridden by any
demands, whether emanating from vehicles, manual control
devices or received remotely from central computers or linked
controllers. Such a period is built into signal controllers. The
shortest minimum green period normally used for vehicle
stages is six to eight seconds.
5.9Site conditions may require a longer period where large
numbers of heavy vehicles have difficulty in starting, or the
approach is on a steep gradient.
5.10Where pedestrians and traffic share the same stage,
minimum green times may be governed by the time required
by pedestrians to clear the crossing.
ESTIMATION OF SATURATION FLOW
WIDTH OF APPROACH
5.11The Road Research Technical Paper No. 56 suggested
that the Saturation Flow (S) be expressed in terms of passenger
car units (pcu's) per hour and with no turning traffic or parked
S = 525w ,
where w is the width of the approach road in metres and
5.1520pass. 2.25 2.0 1.75 2.62 3.6 2.5 1.52 2.5
Auto Rickshaw - - - 0.52 0.6 - - 0.5
Pedal Rickshaw - - - 0.93 1.4 - - 1.0
Pedal Cycle - 0.3 0.2 - 0.4 - - 0.3
Horse &Cart - - - - 2.6 4.0 - 3.0
Bullock Cart - - - - 11.2 4.0 - -
TOTAL LOST TIME PER CYCLE (L)
5.27With a 2 second red/amber start and 3 second amber
finish, the Intergreen Period for two conflicting stages is 5
seconds plus any all red time.
5.28The lost time for a single phase during the green and
amber period is normally about 4 seconds, in other words, the
effective green time is thus the green time + 1. Then, per cycle:
Total lost time = 4*number of stages + all red
5.29Full pedestrian stages should be treated as all red tunes
and included for the calculations.
5.30The flow factor `y' for each phase is given by the
design flow for an approach (q)
saturation flow for an approach (S)
5.31Where more than one approach is operating during a
phase, the maximum `y' should be used. Where an early cut-off
or late start is to be used in connection with a right turn the `y'
values for the right turn and for the approach with shortened
time should be added together to represent one phase, unless the
straight on traffic on the same approach as the right turn has a
higher value, in which case this latter figure should be taken.
5.32The sum of these higher `y' values for all the phases is
the `Y' value, which is in fact a measure of the congestion, and
which will be used for calculating the optimum signal setting.
PARKING, WAITING AND BUS STOPS
5 23A parked car on an approach causes an effective loss in
width, which can be expressed as:
loss = 1.68 - 0.9 (z - 7.62)m
where z (>7.62) is the distance from the stop line of the parked
car and g the green time in seconds.
5.24In practice, the handling of parked or waiting vehicles in
developing cities is extremely complex and involves a lot of
assumptions on the part of the engineer. Traffic signals are
generally put on the busiest junctions - and these are exactly the
favorite street corners for commerce and irregular bus stops.
5.25Road signs and road markings may prohibit parking, but
enforcement is normally difficult or ineffective. Inmost cases
the engineer will be left with the choice of reducing the
effective carriageway width by lm (for the odd parked car, a
bus stop or loading), by 2m (a row of parked cars with some
double parking and occasional bus stop) or even 3m (next to a
school where double parking is almost the rule) - or in
situations such as the street market shown in Plate 2.
5.26One of the other main causes of reduced saturation flow
is the proximity of a signal controlled junction down-stream.
This may result in the need for vehicles to slow down because
of poor linking or vehicles backing up from another congested
signalized junction downstream. The adverse effect of adjacent
signals on one another can be particularly serious during peak
hours. In these circumstances site observations should be made
to evaluate the actual effective green usable by various vehicle
phases and to determine the necessary adjustments on signal
timings i.e. green splits and offsets.
18 Figure 18 Variation with time of discharge rate of a queue in a fully saturated green period
5.33For an isolated signal installation, where the mean traffic
level is constant and where vehicle arrivals are at random, the
U.K. Transport Research Laboratory (TRL) has shown that the
optimum cycle time for minimum delay (Co) is given by:
1.5L + 5
Co = secs.
1 - Y
5.34The cycle time which is just sufficient to pass the traffic
(Cm) is given by:
Cm = secs.
1 - Y
5.35This is the minimum possible cycle time which may be
associated with excessively long delays. In designing linked
signals a cycle time should be chosen which provides a margin
over this minimum cycle time for the key intersection.
5.36In practice it will be generally appropriate to choose a
practical cycle time (Cp) which will allow the installation to be
loaded to 90 per cent of its capacity:
Cp = secs.
0.9 - Y
5.37Where pedestrian crossing volumes are high it will be
desirable to use as short a cycle time as practicable to minimize
delays imposed on pedestrians. In these cases, recommended
practice normally limits cycle times to below 90 seconds. Cycle
times lower than 50 seconds, on the other hand, tend to waste
too much of the cycle time in lost time.5.38Even when pedestrian movement are low, a practical
upper limit to cycle times is around 120 seconds.
5.39Signal setting for the effective green periods (g) should
be in proportion to the y values on each approach, with an
allowance for lost time:
y (c – L)
g = effective green period
y = flow factor
G = actual green period
c = cycle time
L = total lost time
Y = sum of y flow factors
c-L = total effective green time
y1 (c – L)
y2 (c – L)
5.40To simplify the calculations, for most cycle times of
around or above 60 seconds, the same value can be used for
19 5.46Most of the useful formula have been incorporated so
that the engineer may perform the operations directly, without
referring back to this guide. This particular model was
composed using a spreadsheet and can be adapted according to
local conditions and the program most suitable.
STAGE/PHASE SEQUENCE DIAGRAM
5.47The method of signal control should be fully illustrated
by the Stage/Phase Sequence diagram, complete with the
- diagrammatic junction layout
- signals operation sequence
- design flows in pcu/h
- pcu. factor (if necessary), for converting unclassified
counts from veh/h to pcu/h
- intergreen periods required
- actual green times (G = g - 1 )
5.48 All traffic signal calculations should be regarded as a
good first estimate. There is no substitute for on-site checks
and the `fine tuning' that should take place once the
changes or installations have been implemented.
5.49 This section has outlined the "Webster" method of
traffic signal design, which, although manual, has great merit in
that it is simple to apply and enables a clear understanding of
the principles used. However, there have been great advances
in PC based programs for signal design, such as TRANSYT,
OSCADY, LINSIG, and others which enable calculations to be
canned out rapidly and accurately.
CHECK LIST FOR SIGNAL DESIGN
Step 1 - Identify Traffic Flow Volumes
Traffic flow volumes are identified, including turning
Step 2 - Identify Junction Layout, Lane
Geometry and Site Characteristics
The junction layout, including lane geometry and site
characteristics are identified.
It may be necessary, if revealed in Step 4 or Step 7, to modify
the layout to cater for turning movements, pedestrians or to
enhance capacity and/or safety.
Step 3 - Identify Signal Phasing and Method of
The method of control to be used for analysis is identified. both G and g. In other words, if the total lost time for the
junction is 8 seconds and the desired cycle time is 60 seconds,
both G and g can be taken as 52 seconds.
5.41If parallel pedestrian facilities are included in the
junction method of control, the minimum green times for the
minor movements could well be dictated by parallel pedestrian
crossing green times. This could distort the green split
calculation and in situations where pedestrian signals are being
introduced the engineer is faced with the dilemma of either
reducing the minimum pedestrian time or oversaturating the
junction capacity, in turn leading to disrespect of the pedestrian
signal. In many developing cities this problem still remains.
DEGREE OF SATURATION
5.42Degree of saturation (X) for individual approaches may
also be expressed as;
q = design flow
c = cycle time
S = saturation flow
g = green time for approach
5.43The degree of saturation should be the same for all the
predominant arms of an intersection when the signal timings are
at optimum settings and is given by the equation:
1 + Y
JUNCTION CAPACITY ANALYSIS
5.44The ultimate capacity of an intersection may be defined
as the maximum
flow which can pass through the intersection
with the same relative flows on the various approaches and with
the existing proportions of turning traffic. Capacity will
normally increase as the cycle time increases, since the ratio of
lost to useful time decreases. In practice, for maximum reserve
capacity assessment, a maximum cycle time of 120 seconds
should be adopted. However it should be noted that for new
installations, the maximum operating cycle time should be
limited to about 90 seconds.
TRAFFIC SIGNAL CALCULATION SHEET
5.45A `Traffic Signal Calculation Sheet' is shown as an aid
for performing traffic signal calculations.
20 21 Step 4 - Check Turning Movements and
Adequate provision for turning movements and pedestrians
should be checked. It may be identified at this stage that the
assumed method of control would need adjustment before
Adequate allowance in calculations for parallel pedestrian
minimum green crossing times should be made.
Step 5 - Estimate Saturation Flows
The saturation flows for various approaches/movements are
identified. In critical cases the saturation flows for important
movements may have to be measured on site.
Step 6 - Compute Y, L,
The lost times, flow factors and sum of the critical flow factors
Step 7 - Compute Reserve Capacity
The maximum reserve capacity of the intersection is then
calculated as a measure of operating performance. If this is not
satisfactory, then it may be necessary to go back to Step 2,
modify data and layouts and recalculate. A minimum provision
of 25% reserve capacity should be provided wherever possible
for new junctions. A lower standard may be adopted for
existing junctions where further improvement is restricted by
Step 8 - Compute Co, Cm, and Cp.
The optimum, minimum and practical cycle times for operating
the junction are then computed for further analysis, if
Step 9 - Select C
It is then necessary to select a cycle time for operating the
intersection. Sometimes, for reasons of linking, the selected
cycle time may be different from the values calculated in the
Step 10 - Compute Green Times, Degree of
The green times of the various phases are then computed.
Degree of saturation may be computed as well if detailed
analysis of signal operation is required. If good linking to other
junctions requires a cycle time that results in very low degree
of saturation and a very high reserve capacity, consideration
should be given to double cycling this junction within the
linking group, i.e. running it at half the linking cycle time.
22Step 11 - Determine Offset and Other Controller
Offset and other controller settings such as minimum green,
maximum green ,etc. are then finalized. Offsets for linking
signals may be prepared with the aid of time-distance
diagrams. or programs such as TRANSYT. (see section 6).
Step 12 - Prepare Documentation
For record purposes, drawings showing the junction layout,
method of control, stage/phage diagram, traffic flow, etc. need
to be prepared and maintained. Standard symbols should be
used wherever applicable.
Step 0 - The Proven Need for traffic signals at the
junction according to approved warrants, should
always come first and be part of the documentation. 6. COORDINATION AND LINKING
OF TRAFFIC SIGNALS
6.1 Many cities in the developing world have grown to huge
dimensions in a very short period and lack the large scale road
infrastructure needed for modern traffic, as well as the capital to
build it. This leads to the inefficient use of the existing road
network, often in the form of extensive one-way systems, in
turn leading to the need for a large number of traffic signals -
mostly fairly close to one another.
6.2 The effect of vehicles stopping at a signalised junction is
to form the vehicles into a queue behind the stop line. When
this queue is released as the green is given, it will discharge
initially at its maximum rate (i.e. saturation flow) and move
forward as a `platoon'. If, as this platoon approaches another
signal controlled intersection, its arrival is made to coincide
with the start of the green period, the vehicles will experience
no delay at this new junction. If the platoon has to stop, a queue
may form leading to spillback, which in turn may block the
6.3 The objectives of coordination of signals are to:
- prevent the queue of vehicles at one intersection from
extending back and interfering with an upstream
- increase the capacity of the linked route.
- enhance driver comfort by offering less stops and
smoother flows in a controlled manner.
- offer minimum overall delay for road users, reducing
overall travel time.
- reduce fuel consumption - and hence pollution -within
- impose on drivers a safer behaviour, as they normally
bunch together at the speed designed for green
coordination (or waves). Speeding is thus minimized,
accident risk and severity are reduced as is the asso-
ciated risk for pedestrians, as they can judge oncoming
speeds with greater accuracy.
SIMPLE PROGRESSIVE SYSTEM
6.4 The most commonly-used linking system works with a
cycle time common to all intersections and the signals are so
timed that the `go' periods are staggered in relation to each
other according to the road speed to give a 'progression' of
green periods along the road in both directions. Thus road speed
should be considered "reasonable" by drivers; if speeding is
common before linking, then a measured speed will be too high
for safe operation. In thiscase, a desired speed should be used to ensure that the platoon
conforms to the legal limit.
6.5The timings of the signals in a simple progressive system
can be prepared with the aid of a tune-distance diagram,
examples of which are shown in figures 19 and 20.
6.6On these diagrams, distances between junctions along
the route are plotted along the abscissa (y axis) and the travel
times are plotted along the ordinate (x axis). The slope of
diagonal lines represent the chosen speed of progression and
green stages of successive junctions are offset in time.
Normally the problem is one of determining, by trial and error,
the optimum through-band speed and width for a fixed cycle
time. For one-way roads, the green bands follow each other in
sequence. The driver, having passed one intersection, will then
receive right of way at the others.
6.7When the flow of traffic is two-directional and where the
intersections are not equally spaced, the situation is more
complex and it may be necessary to come to a compromise on
progression between the two directions. It may also be
necessary to take into account other requirements such as
demands from cross-street traffic.
6.8The time-distance diagram method can be used to bias in
favour of a particular direction of flow e.g. to favour a heavy
inbound flow in the morning peak at the expense of increased
delay to the fewer vehicles travelling in the opposite direction.
The situation may be reversed for the evening peak.
6.9Cycle time for a coordinated signal system is normally
dictated by the timings of a key junction, i.e. the junction which
is most heavily loaded Spare green time should be allocated as
required to clear traffic turning into the main route from side
roads in order not to delay the through platoons.
6.10To minimize congestion, opportunities for leaving the
system should also be greater than for entering.
6.11In heavy city centre traffic a design `speed' of about
l0m/s ( about 40km/h) usually gives good results. For suburban
traffic, where traffic is lighter and signals are about 300m apart,
a design velocity of 15m/s (about 60km/ h) can be used as a
first estimate, provided this does not conflict with local speed
6.12On two-way roads, good coordination can usually be
obtained by using a common cycle time equivalent to twice the
average travel time between junctions.
6.13For example, using a design speed of l0m/s on a two-way
road between nodes A.B.C, with links lengths AB+ 260m and
BC = 300m. The travel times are then 26 and 30 seconds with
an average of 28s. A good estimate of cycle time would then be
2x28 =56seconds, or in practice a cycle time of 55 to 65 would
be adequate (with the lower option possibly leading to higher
Figure 19 Co-ordinated signals for one-way traffic
Figure 20 Co-ordinated signals for two-way traffic
Red (amber omitted, only north-south phase shown
Green MECHANISMS FOR LINKING SIGNALS
6.14 Local traffic signal controllers at intersections
working in a linked group may be cable-linked to a master
controller. The master controller generally ensures that the
local controllers, or `slave controllers' as they are termed,
operate in synchronization by sending control pulses or
instructions down the link cable to every slave controller.
6.15Linking of signals may also be achieved by cable-less
means such as radio, mains synchronization, etc. Originally
conceived to offer the benefits of wide area coordination while
avoiding the expensive costs of underground ducting, these
systems are now less competitive than the simpler ATC
systems, as they do not allow for supervision of faults or such
basic Responsive System facilities such as queue control.
FIXED-TIME COORDINATED SIGNALS
6.16These systems are based on assumptions that traffic
flows are repetitive over a weekly (24h, 7day) cycle and that
the appropriate predetermined signal settings can be prepared
to cope with predictable traffic flows. The traffic plans, or
signals settings can be selected according to the day of week
and time of day. These systems require controllers with
synchronized internal clocks and, typically, a minimum of 6
plans to choose from.
6.17These would normally include plans for the:
- Morning Peak.
- Off Peak (work day).
- Evening Peak.
- Off Peak (night, holiday or Sunday).
- Nighttime Yellow Flashing, etc.
AREA TRAFFIC CONTROL (ATC)
6.18Area Traffic Control, (ATC) is the centralized control of
traffic signals on an area-wide basis using micro-processor and
6.19Usually the traffic controllers on street are linked to one
or more central computers in the control centre, via data
transmission cables. The cable network can either be provided
as a dedicated network or private circuits leased from the
telephone company (or a mixture of both, according to cost).
Urban Traffic Control, (UTC) involves central coordination of
signals as an ATC, but will include other facilities such as car
park space control and variable message signing.6.20The main facilities provided by ATC Systems are
- Optimized signal coordination. With the aid of
centralized computer control, signal settings can be
optimized on an area basis to provide minimum overall
delay and reduced journey times.
- Control flexibility. Changing traffic conditions can be
catered for by vehicle actuation or predetermined multi-
plan operation. The time settings of traffic signals can
also be altered very quickly by manual intervention at
the control centre, such as modifying the existing signal
timing plan or replacing it with a new plan.
- Fault monitoring. One of the most important facilities
offered by ATC systems is the continuous monitoring
of the operation of the traffic signal equipment linked to
the computer. Any fault condition detected is reported
to the Control Room immediately and fault repairs can
be carried out quickly.
- Priority for emergency and public transport vehi-
cles. For fire engines which always start from a certain
fire station, special plans may be prepared for
predetermined `preferred routes' and stored in the
central computer. Priority arrangements can be given to
public transport routes, busways, etc.
- Accident reduction. ATC systems improve road safety,
especially in the traffic conditions of some developing
cities. Table 6 shows some results of traffic accidents
before and after the construction of an ATC system.
TABLE 6: ACCIDENTS IN ATC SYSTEMS
ATC System Accident
Witchita Falls 8.5
West London 18
6.21On certain corridors these improvements may be even
greater. For a well designed system on downtown streets
together with suburban corridors, an accident reduction of
about 20% (all types) may be expected.
25 FIXED TIME ATC SYSTEMS
6.22 These ATC systems operate on a strategy of fixed time
traffic signal plans for each controller. Signal timings and plans
for junctions alter in accordance with a preset timetable held in
the central computer or in a microprocessor inside the
controller itself. These systems work very well for traffic
patterns which are predictable and which change quite slowly.
Traffic signal timings are developed from historical traffic flow
data collected for the junctions and the timings require updating
periodically, depending on the traffic fluctuations at each site.
6.23 For large urban areas with possibly several hundred
traffic signal controlled intersections, a large system, complete
with a special computer, monitor rooms and possibly closed
circuit television is an entirely appropriate arrangement.
Usually this type of maxi system will demand (and can justify)
its own dedicated team of hardware and systems engineers,
traffic engineers and technicians and system operators.
6.24 For medium-sized cities in the developing world,
however, the benefits accruing from the installation of a maxi
system cannot be justified in terms of the initial costs and staff
resources demanded to run it. Experience has revealed that
fixed time systems are cheap and simple to install and maintain
and recent developments in electronic technology have made
the Mini ATC, or Compact ATC systems almost as efficient as
the traditional 'centralized' systems. The features of these
systems are fairly standard, and can be summarized as:
- Computer and data transmission system in a stand-
alone unit which does not require a specially controlled
- "off the shelf" system operating software.
- automatic printout of faults; can be left to run virtually
- usually these systems are purchased as a complete
package to a standard specification. This minimizes the
staff input at the outset of the scheme.
6.25Standardization usually means that the costs in-volved to
the purchasing of such a system are modest. As the system
requires no special architectural/environmental arrangements,
and will virtually 'run itself,' the staff input on the part of the
purchaser/local administration is also small. Much of the
flexibility and monitoring facilities of the maxi systems are,
however, maintained and signal timings can be kept up to date
by a small team, possibly on a part-time basis in addition to
normal traffic engineering duties.
6.26 One of the major difficulties and costs of ATC
systems is in installing an adequate data transmission
network. This item should be given special attention,
examining all possible alternatives, during the evaluation of
any ATC or UTC system.
6.27To overcome some of the rigidities inherent in the fully
fixed time strategy, whilst avoiding some of the sophistication
and complexities of fully responsive strategies, some degree of
traffic responsive control can be introduced into the fixed time
6.28This can take two main forms
- Using strategic detectors to detect fluctuations in traffic
flow, the system automatically changes from one fixed
time traffic plan to another. Basically, this is equivalent
to the introduction of a flexible timetable into the
- Installing vehicle presence detectors and pedestrian push
buttons at junctions and introducing a VA 'window' into
the traffic signal timing plans. The difficulty with fully
traffic actuated signals is that the concept of signal
linking within a fixed cycle time breaks down under
fully traffic-actuated control. The VA-window concept
allows a certain amount of 'local discretion' in the traffic
signal plan. This permits the controllers at certain times
in the signal cycle to decide whether to introduce a
certain stage or whether to remain on the stage running,
or to control the extension of queues by allowing for
green extensions, etc.
6.29The former is very effective in overcoming difficulties
in defining accurately plan change points. Also, as only a
relatively small number of detectors may be required, a heavy
ongoing maintenance commitment on detectors is unnecessary.
And in developing cities detectors are often a problem.
6.30The second type of system can be very effective in
coping with minor fluctuations in traffic flow, queues and with
the problems of light vehicle and pedestrian flows in the off
peak period. Though semi-responsive systems do require more
complex signal controllers on street.
FULLY RESPONSIVE CONTROL
6.31The logical extension of computerised traffic control
philosophy is the fully automated system, where the computer
automatically calculates and adjusts signal timings to suit
actual traffic conditions on street.
6.32The SCAT (Sydney Coordinated Adaptive Traffic)
System is based on a limited distributive intelligence system
with a main computer performing certain monitoring functions
and several satellite computers controlling a group of
controllers on street.
6.33All intersections within a subsystem operate on a
common cycle length and are equipped with inductive loop
vehicle detectors on all approaches. These are located in each lane immediately in advance of the stopline and perform
the dual functions of providing traffic flow data for strategic
control and local or `tactical' vehicle actuation.
6.34 The SCOOT (Split Cycle Offset Optimization
Technique) System consists 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 from the approach to the junction as
possible and are then calibrated to strike a balance between
flow and occupancy.
6.35In 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.
6.36 It must be remembered that in order to operate any
fully responsive system at least 80% of detectors need to be
fully operational. This requires constant maintenance and
adequate continuous funding --exactly the areas which are
often a problem for transport projects in the developing
DUET – DIAL-UP EQUIPMENT TESTING
6.37It is arguable whether the greater benefit obtained from
an ATC system is derived from signal coordination or from the
ability to monitor continuously the performance of signal
equipment. For signals in remote locations or where the cost of
a dedicated ATC telecommunication network is not justified,
DUET may provide a compromise solution. With the aid of this
technology the equipment can be interrogated over public
telephone lines as and when necessary.
6.38Each local controller is connected to the public telephone
system. Communication is achieved by dialing a telephone
number from the Central Office, which connects the local
controller via modem to equipment in the Central Office for
testing/fault reporting purposes.
6.39This arrangement avoids the need for a dedicated private
circuit but provides only a limited fault monitoring facility as
equipment faults will only be detected when the interrogation
takes place, which may be some time after the fault has
6.40An alternative is for the local controller to have the
ability to `ring-up' the Central Office when a fault is detected.
At the same time, it should be possible to change and load
plans from the Central Control Room via a DUET setup.6.41The curve shown in figure 21 represents the cost-benefit
relationship for the various options of control out-lined in this
section. It should be noted that the principal gains are obtained
with less complex measures and that high-tech solutions -
although extremely useful options in big cities - are normally
only recommendable as a final stage in the ongoing process of
traffic signal improvements.
Figure 21 Cost/benefit relationship for different
types of signal co-ordination 7. THE TRANSYT PROGRAM
7.1The TRANSYT program was developed by the U.K.
Transport and Road Research Laboratory in a series of
experimental applications in Glasgow, Scotland, where an ATC
System had been installed.
7.2The network being modelled in TRANSYT is repre-
sented by `nodes' inter-connected with `links'. Each signalised
junction is represented by a node; each distinct one-way traffic
stream leading to a node is represented by a link. TRANSYT
takes the flows on each link and models traffic behaviour from
7.3Figure 22 represents a small network of three junctions
together with the traffic flows for one of the peak hours.
Figure 22 Simple road network and traffic flows
7.4This is then transformed into a "TRANSYT Dia-
gram" such as figure 23; note that turning movements are given
separate link numbers.
Figure 23 Transyt diagram of the network
7.5TRANSYT requires the user to input a variety of
information about the network. The data required is basically of
four types; network data, signal optimisation data, link data,
and node data. Network data includes such thingsas the common cycle time for the network and the cost of
delays to vehicles. Signal optimisation data allows the user to
specify what type of optimisation is to be carried out. Link
data requires the user to specify information such as the flow
and saturation flow on a link, as well as during which stages
the link receives a green signal. The node data requires the
user to specify the staging of the signals at each node,
minimum greens and intergreens.
7.6This data is resumed in the input table as follows:
7.7TRANSYT will produce an optimised set of signal
timings for each node within a network. The signal timings
are calculated by a method which works by varying both the
signal offsets and the duration of the individual stage green
times. Together, these signal timings result in delay being
reduced to a minimum within the network.
7.8The program can also be used to advise on the best
cycle time for a network. This area of TRANSYT will also
advise the user about the benefits to be gained from double-
cycling any of the nodes.
7.9For the optimised signal timings calculated by
TRANSYT, a table of results, relating to performance and
delay on each link, is given. From this table the user can
deduce how saturated each link is, the potential size of any
queues, the cruise time for vehicles, the amount of delay each
vehicle can expect to suffer, as well as a series of cost related
indices in terms of fuel consumption and delays.
7.10This program has been immensely successful and is
recognized as one of the most important traffic signal tool
available to engineers. Versions are now available for use in
P.C.'s and these also allow increased emphasis to be attached
to key links in the network and the whole process can, if
desired, be biased either towards minimum delay or
minimum stops to vehicles. The average speed within the
network and total fuel consumption are also given and these
provide the basic data for evaluating the operating benefits to
be gained from a new plan or system. Special routines are
also available to model bus movements separately and aid in
the selection of optimum cycletimes.
7.11 In practice, the TRANSYT Performance Index
may be improved in "Grid" type networks by building
into the initial offsets simple green waves on the main
streets of the network. The output is then normally a
much improved version of the desired coordination. If left
to optimize from an all zero setup, the program may opt,
when signals are close together, for a solution of the `open
all lights north-south, then east-west'. This leads to
speeding as drivers note that all the lights along the road
go green at the same time.
28 TABLE 7: TRANSYT OUTPUT FOR A SIMPLE NETWORK
NO. OF ENTRIES TO SUBPT = 10
NO. OF LINKS RECALCULATED= 115
1 LINK FLOW SAT DEGREE MEAR TIMES --------------DELAY--------------- --------STOPS-------- -------QUEUE-------- PERFORMANCE EXIT GREEN TINES
NUMBER INTO FLOW OF PER PCU UNIFORM RANDOM+ COST MEAN COST MEAN INDEX. NODE START START
LINK SAT CRUISE OVERSAT OF STOPS OF MAX. AVERAGE WEIGHTED SUM END ENE
DELAY (U+R+O.HEAN Q) DELAY /PCU STOPS EXCESS OF ( ) VALUES 1ST 2ND
(PCU/H) (PCU/H) (%) (SEC) (SEC) (PCU-H/H) ($/H) (%) ($/H) (PCU) (PCU) ($/H) (SECONDS)
10 1200 2400 87 10 25 5.3 + 3.1 ( 68.1) 86 ( 9.9) 27 + 77.9 1 38 89
11 550 35005 81 8 31 3.6 + 1.2 ( 38.7) 98 ( 5.2) 24 43.8 1 4 33
12 400 11L 81 8 24 1.7 + 0.9 ( 21.3) 94 ( 3.6) 24 24.9 1 4 33
20 1150 2600 87 10 29 6.2 + 3.1 ( 74.6) 91 ( 10.0) 28 + 84.5 2 42 87
21 1101 38005 86 12 31 7.2 + 2.4 ( 77.3) 87 ( 9.1) 31 86.4 2 2 37
22 199 21L 86 12 39 1.7 + 0.4 ( 17.4) 108 ( 2.1) 31 19.4 2 2 37
30 500 1600 76 10 34 3.2 + 1.6 ( 37.9) 92 ( 4.4) 12 42.2 3 12 48
31 1000 36005 67 13 11 2.2 + 0.8 ( 24.1) 38 ( 3.7) 13 27.8 3 53 7
32 200 31L 67 13 5 0.1 + 0.2 ( 2.4) 22 ( 0.4) 13 2.8 3 53 7
39 100 1600 15 10 20 0.5 + 0.1 ( 4.4) 61 ( 0.6) 2 5.0 3 12 48
40 800 1600 82 10 23 3.0 + 2.2 ( 42.0) 80 ( 6.1) 18 + 48.1 4 63 27
41 550 3200S 73 10 27 3.1 + 1.0 ; 33.1) 90 4.73 17 37.8 4 32 58
42 150 41L 73 10 16 0.4 + 0.3 ( 5.3) 81 ( 1.2) 17 6.5 4 32 58
49 100 1600 10 10 9 0.2 + 0.1 ( 2.1) 40 ( 0.4) 1 2.5 4 63 27
TOTAL TOTAL MEAN TOTAL TOTAL TOTAL TOTAL PENALTY TOTAL
DISTANCE TIME JOURNEY UNIFORM RANDOM+ COST COST FOR PERFORMANCE
TRAVELLED SPENT SPEED DELAY OVERSAT OF OF EXCESS INDEX
DELAY DELAY STOPS QUEUES
(PCU-KH/H) (PCU-H/H) (EM/H) (PCU-H/H)(PCU-H/H) ($/H) (S/H) ($/H) (S/H)
843.0 79.8 10.6 38.2 17.5 ( 448.6) + ( 61.2) + ( 0.0) $09.8 TOTALS
****************************************************************************************************************** CRUISE DELAY STOPS TOTALS
LITRES PER HOUR LITRES PER HOUR LITRES PER HOUR LITRESPER HOUR
FUEL CONSUMPTION PREDICTIONS 59.7 + 78.0 + 60.7 = 198.3
29 8. SYSTEM AND ECONOMIC
ANALYSIS OF TRAFFIC
8.1In many cities throughout the developing world there is
enormous scope for introducing modern traffic controllers
capable of offering linked signal timings compatible with the
different traffic volumes found during the day. The TRANSYT
Program can now be run on a 486 PC and similar advances in
micro processors have given local controllers the "intelligence"
that used to occupy large main frame computers, thus
simplifying the need for expensive telecommunication
8.2ATC systems improve road safety, they no longer
require expensive "command centres", with a large public
service staff and they normally have exceptionally high
Benefit/Cost ratios that make them potentially attractive to
international lending agencies and banks.
8.3Warrants for an ATC System vary from city to city and
there are no hard and fast rules that can be followed, however,
some basic guidelines would cover:
- the city centre, where there is a high density of signals
- the streets used by public transport vehicles
- one-way suburban roads signals are up to 600m apart
- the main corridors and on the side streets up to 300m
from the corridor
- the access roads to the city from major highways
- isolated junctions (using DUET or similar) that are
overloaded during peak hours.8.4An analysis of existing traffic signals, with or without
coordination, plus those sites where counts or other data
suggest the installation of signals, using the above criteria will
tend to show the areas or sub-areas that would probably benefit
most from a modern traffic signal scheme, such as those
indicated in figure 34.
8.5Obviously, if the city has a population of several mullion
with a large and fast growing fleet of cars and buses, then some
form of responsive control, such as SCOOT should be
considered. In the city centre, important isolated junctions can
also benefit from the application of an intelligent vehicle
responsive system, such as MOVA (TRL. 1993.). Most cities in
the developing world, however, will obtain the highest benefits
from updating their traffic control system for a simpler model
using prefixed timings.
8.6In this case, TRANSYT is a good tool for evaluating a
8.7Having arrived at the study area (or areas) a TRANSYT
simulation for each area should be carried out, using real traffic
data for each junction. If this data is not available, then new
counts have to be made.
8.8If the number of junctions in the initial proposal leads to
a cost that is unacceptable to the city authorities, then the
difficult task of reducing the proposal has to be undertaken,
eliminating (or transferring to a later stage) those junctions or
sub-areas that show least benefits - providing that neither the
safety nor the basic concept of the system is impaired.
8.9TRANSYT simulations of these new areas, scaled to
meet budget requirements, will then provide
- the existing operating costs and travel delays (using
existing flows and timings) for each peak period
Figure 24 Definition of ATC system sub-areas
Cit y centre - optimized operating costs and delays (using the
optimizing process), for each peak period
- existing operating costs and delays for the off peaks
- optimized operating costs and delays for the off
8.10This same process can be repeated for a three, five and
ten year horizon, using the relevant traffic growth factors
(based on recent traffic or vehicle growth data). The yearly time
and operating benefits of the proposed system can then be
estimated over the project's useful life.
8.11According to Table 6, a 20% reduction on the accident
costs within the proposed area(s) could also be included.
8.12In developing cities it should be remembered that bus
(public transport) normally plays a major role and so these
travel benefits should be calculated, if possible, separately. The
value of time savings is always a delicate point - especially as
these are aggregated in small amounts for a very large number
of people. However, it is standard practice to allocate a
reasonable value to these for economic evaluation.
8.13On major public transport trunk routes, the extra fleet
capacity gained during the peak hours may also be relevant and
could be included as a real benefit if this means the
postponement of additional fleet capacity during the project's
8.14Table 8 below shows the costs of installing a new
traffic signal layout to different parts of the world. A rough
estimate of junction cost for a modern, simple ATC System
would be around US$20.000 - excluding the costs of data
transmission and any detectors.8.15DUET systems require the cooperation of the local
telephone service - normally by contracting the lines, including
their maintenance and installation costs. This requires a written
proposal providing annual costs per junction.
8.16Software, engineering and system maintenance costs
vary greatly from city to city according to the size and
specifications of the proposed system. However, an additional
30-40%o of the total would give a rough estimate.
8.17Considering the speed of advances in electronics and
telecommunications a useful life of 10 years can be expected
for ATC/UTC projects, allowing for a residual value of 20% of
the total project cost after year 10.
8.18For two projects in Latin America, both nonresponsive
using prefixed timings, the economic data for the systems can
be summarized as:
8.19Project a), Cost = US$ 5,794,000. Benefit/Cost Ratio =
2.41. Internal Rate of Return = 39.88
8.20Project b) Cost = US$ 5,000,000. Annual time savings =
US$ 2.000.000. Annual fuel savings (excluding national and
local taxes) = US$ 3.709.548. Annual accident cost savings =
8.21In both cases it is clear that projects of this kind have an
extremely high economic return. In fact, when a city proposes
to upgrade its traffic signal system from isolated junctions with
just one cycle time and plan to a simple ATC network it is no
exaggeration to claim that the scheme will pay for itself in a
question of months.
TABLE 8: ESTIMATES OF TRAFFIC SIGNAL INSTALLATION COSTS PER JUNCTION
city pop. junctions cost/simple cost/complex
with junction junction
(106) signals (US$) (US$)
São Paulo 12.0 4000 4.150 36.000
Santiago 4.5 980 14.300 28.600
Curitiba 1.6 350 12.000 18.000
Nairobi 1.5 27 18.000 26.500
Harare 1.0 152 8.000 16.000
Bombay 11.0 305 12.000 18.000
Istanbul 4.0 300 14.000 55.000
Islamabad 0.4 41 10.450 13.550
31 9. SPECIFICATIONS
9.1Most countries have developed their own national
specifications for traffic signal materials, based either on local
experience and production or on international standards used in
the larger markets of North America, Japan, European
Community, etc. Detailed specifications of equipment and
services are beyond the scope of thus report, however, some
basic requirements can be resumed as follows
TRAFFIC SIGNAL CONTROLLERS
9.2The increasing demand for better traffic control
mechanisms and the advancements in technology have
introduced the microprocessor-based traffic signal controller -
now a fairly standard, cheap and reliable form of control.
9.3A major difference arising from the use of
microprocessors is that the operating parameters such as
timings are stored in memory rather than by switches or
potentiometers. Changes in operation are accomplished by the
use of hand-held terminals, although safety requirements are
met by storing the basic site configuration data and control
program on programmable read only memories (PROMS),
alterable only by signal maintenance personnel. High reliability
is also achieved by removing as many moving parts as possible
and by using inbuilt self-monitoring and automatic fault
9.4Essential features of this type of controllers are:
- ` intelligence' and flexibility inherent to microprocessor
systems offers the possibility of easy modification as
- capability of operating in any of the following modes:
9.5 Hurry Call. This mode maybe requested by the local
detector or by switch input to allow priority to be given to a
particular movement for the passage of emergency vehicles
such as fire engines or rapid transit vehicles.
9.6 Manual. This mode will allow the controller to operate
under emergency manual control.
9.7 Local Coordinate Mode. Multi-plan operation in this
mode shall be possible in accordance with plans and timetables
stored in the memory.
9.8 Local Isolated Vehicle Actuated Mode. The con-troller
in this mode may operate with a mixture of fixed time, demand
dependent or vehicle actuated phases.
9.9 Amber flashing on all approaches.
- capability of linking to a pedestrian controller for
- capability of incorporating pedestrian push buttons,
pedestrian wait indicators and pedestrian audible signals.
9.10Most controllers offer a minimum setup of 4 stages
and/or 8 phases, allowing for expansion to 8 stages and/or 16
phases, without the need to change the cabinet or internal
architecture of the equipment.
GENERAL ROAD TRAFFIC SIGNALS
9.11 Vehicular signals should contain three optical systems
arranged vertically each having a nominal diameter of 200mm.
9.12Pedestrian signals should contain two optical systems
arranged vertically and shall normally each have a nominal
diameter of 300mm - an alternative size of 200imm nominal
diameter may also be used where necessary.
9.13Where an optical system incorporating a green arrow is
used, it should normally have a nominal diameter of 300nim -
an alternative size of 200mm nominal diameter may also be
used if necessary.
9.14Signal heads should have adequate mechanical strength
and durability to withstand the conditions of installation and
normal use operations. It should be dust proof and
weatherproof against corrosion and action of direct sunlight
without significant deterioration. The signal head assembly
together with the attachment of a backing board should be
capable of withstanding wind velocities up to 160 Km/hour in
any direction and temperatures over a range of -25°C to 70°C.
9.15Normally, the signal heads will be made from
polycarbonate or cast aluminium.
9.16High intensity lamps should be used for enhancing
INDUCTIVE LOOP DETECTORS
9.17The basic system consists of a loop of wire (typically 2
or 3 turns) buried approximately 5mm below the road surface.
The ends of the loop are returned to the vehicle detector,
usually housed some distance away in the controller cabinet. A
small electric current is passed through the loop and this causes
an electric field to be built up.
9.18A change in the inductance of the loop occurs when
a vehicle is positioned directly over it - or is passing over the
loop. The change in inductance is sensed by the unit and a
signal change indicates the presence of a vehicle.
9.19Provided that loop detectors are properly installed, tested
and maintained, they will work reliably and offer efficient and
accurate detection as well as providing congestion, counting
and vehicle classification information. ASSOCIATED ELECTRICAL WORKS
9.20 These works include
Laying of aspect cables, linking cables (or telecom-
munications cables in an ATC area).
necessary cable jointing work. signal wiring and
TRAFFIC SIGNAL CONTROLLER
9.21Again, the specifications of civil works vary from
country to country, however, it is recommended that all
cabling should be underground using ducts as shown in
figure 25. Inspection boxes, normally 60cm on each side
should be built every 50m (maximum). In developing
cities the pavement is often dug up for new services, so to
avoid the severance of cables, the ducts should be
protected by a plastic strip in bright colours (normally
yellow with black
Plate 3. Microprocessor controller suitable for
pole installation at simple junctions Fi
gure 25 Cross section of cable duct Early cutoff overlap
Condition in which one or more traffic streams are permitted to
move after the stoppage of one or more other traffic streams,
which during the preceding stage had been permitted to move
Effective green period
The period in the green and amber periods throughout which flow
could take place at saturation flow levels.
A request for the continuation of the green signal to a
predetermined maximum, made by a vehicle which, when the
request is made, has the right of way.
Fixed-time traffic signals
Traffic signalling equipment in which the stages and their
duration in each cycle are preset to suit predictable traffic
The flow factor or `y' value of an approach is the ratio of the
design flow to the saturation flow of the particular approach.
Green filter arrow
An additional green arrow mounted by the side of the three light
display to indicate early movement in the direction of the arrow
and is terminated by a full green light signal. It is normally used
for early discharge of left turners ahead of other movements at
the same approach.
The ratio of green time allocated to each of the conflicting phases
in a signal sequence.
The state in which downstream traffic completely blocks the
junction, which then forms queues that blocks other junctions,
and so on until the whole area is blocked with
Indicative green arrow
An additional green arrow mounted on the right of the three light
display of the secondary signal only, to indicate the early cutoff
of an opposing flow.
The period between the end of the green display on one stage and
the start of the green display on the next stage is known as the
Late start overlap
Condition in which one or more traffic streams are permitted to
move before the start of one or more of the traffic streams which,
during the subsequent stage are permitted to all run together. 10. GLOSSARY
A short glossary of terms used in this volume in alphabetical
order is given below:
All red period
Period when red signals are shown to all approaches simul-
taneously, usually of short duration to allow vehicles to clear
Signals in the form of pulsed tones provided for the benefit of
visually handicapped pedestrians.
Area Traffic Control (ATC)
Also called Urban Traffic Control (UTC), this is the cen-
tralized control of traffic signals on an area wide basis by
means of computer.
The Background board is usually coloured black with a white
border which is placed behind signal lanterns to make them
more visible against a bright sky or other street or shop lights.
A cycle is a complete series of stages during which all traffic
movements are served in turn. The cycle time is the sum of
the stage times.
Degree of saturation
The degree of saturation at an approach is the ratio of the
design flow to the actual capacity of a particular approach,
weighted by the amount of green the approach receives in a
Traffic Delay is the lost time by vehicles due to traffic
`friction', congestion or control devices.
A request for right of way for traffic on a phase which has no
right of way when the request is made. The demands normally
being stored in the controller and served in a prearranged
Demand - dependent stage/phase
A demand-dependent stage or phase is one which appears
only on demand from a vehicle detector or pedestrian push-
button. i.e. it is skippable.
A device to detect the presence or passage of a vehicle in the
34 Lost time
Lost time in the green and amber periods is the wastage time
during which no flow takes place. Total lost time per cycle is
the sum of these lost times for the critical phases plus other lost
times due to red-amber periods, all red periods and pedestrian
green and flashing green times.
Maximum green running period
The maximum time that a green signal can run after a demand
has been made by traffic on another phase.
Minimum cycle time
The minimum cycle time which is just sufficient to pass the
Minimum green running period
The duration of the green signal following the extinction of a
red/amber signal during which no change of signal lights can
The time difference or interval in seconds between the start of
the green indication at one intersection as related to the start of
the green interval at another intersection from a synchronized
system time base.
Optimum cycle time
The theoretical cycle time for attaining minimum vehicle delay.
Passenger car units
Passenger car units for a given type of vehicle are expressed in
terms of the number of moving passenger cars it is equivalent
to, based on headway and delay characteristics.
An average pcu value derived for the convenience of signal
calculation to convert unclassified (by type) vehicle counts
from vehicles per hour units to pcu per hour units.
The sequence of conditions applied to one or more streams of
traffic which during the cycle, receive identical signal light
conditions. Two or more phases may overlap in time. A series
of phases is usually arranged in a predetermined order but some
phases can be omitted if required.
Practical cycle time
The cycle time at which the traffic signal installation will be
loaded to 90 per cent of its capacity.
A measure of the spare capacity of a signal controlled junction,
expressed in terms of the percentage of the current total flow
factor value (Y) which will be available for further increase of
The act of disobeying, consciously or not, the red signal
requiring vehicles on a determined approach to stop and remain
Right of way
The condition which applies when a green signal is displayed
to traffic at the approach thereby permitting that traffic to
The maximum flow which could be obtained if 100 percent
green time was awarded to a particular approach.
Modified vehicle-actuated signals whereby detectors are
installed on the side roads only. Some semi-vehicle-actuated
signals also operate one or more demand-dependent stages
within a fixed cycle time.
A condition of the signal lights which permits a particular
movement of traffic. Stages usually, but not always contain a
green period. They are arranged to follow each other in a
predetermined order but .stages can be skipped, if not
demanded, to reduce delay.
Queue from a downstream junction which affects the traffic
flow at the junction being examined.
A system of different coloured lights, including arrow-shaped
lights, for controlling conflicting streams of
traffic and pedestrians.
Traffic signal controller
The electronic control equipment which activates the signal
phases at an intersection.
Vehicle-actuated (V.A.) signals
Traffic signalling equipment in which the duration of the green
time and cycle length varies in relation to the traffic flow on its
Vehicle extension period
A vehicle extension period is the additional duration of the
green signal which is secured by the actuation of a detector.
An abbreviation of `Traffic Network Study Tool', a program
developed by the Transport and Road Research Laboratory for
optimizing signal timings.
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