I. PURPOSE OF THE HANDBOOK
The purpose of this report is to document, advance and build
support for implementing traffic signal coordination in the
Greater Danbury-New Milford area. The Housatonic Valley Planning
Region has grown rapidly over the years, and there is increased
need for improving traffic flow operations through its corridors.
The Housatonic Valley Council of Elected Officials (HVCEO),
as the regional transportation-planning agency, recommends
which signal systems should be interconnected.
The basic function of most arterial streets and roadways is
to move traffic safely and efficiently with minimum delay.
The main source of delay and congestion along most arterial
streets and roadways are traffic signals. Too often motorists
are required to make unnecessary stops because adjacent traffic
signals bear no relationship to each other. This results in
longer travel times and increased vehicle emissions and fuel
consumption. Additionally, increased driver frustration related
to unnecessary stops or undue delay may also result in a potential
increase in accidents.
Traffic signal coordination provides a means
for alleviating these problems. It enables traffic signals
with each other therefore allowing them to work together.
signals work together (or are coordinated), they provide
a greater opportunity for motorists to travel through
adjacent traffic signals without making unnecessary stops.
Incorporating traffic signal coordination
is an accepted and proven practice throughout the United
States and certainly
in Connecticut. The Connecticut Department of Transportation
has implemented coordinated traffic signal systems on various
state highways; for example, along the Berlin Turnpike
(Route 15) in Berlin, Newington, and Wethersfield. Cities
Danbury, Norwalk, Stamford, Hartford, Bridgeport, and New
operate their own coordinated traffic signal systems, as
do towns such as Greenwich, Hamden, and Wallingford.
The following sections highlight the definition,
need, factors, advantages, and disadvantages of traffic signal
The reader should note that a glossary of traffic signal
terminology is available later in the document for reference.
shown in bold italics are
also defined in the glossary.
BASICs of Individual TRAFFIC SIGNAL OPERATION
To understand traffic signal coordination, it is first necessary
to review the basic operations at an individual signal. This
section therefore defines terms commonly used in signal operations
and shows several examples of how they operate.
An interval is defined as the part of a cycle during which the traffic signal indications (red,
yellow or green) do not change. A signal
phase is a group of three intervals (the right-of-way
(green), change (yellow), and clearance (red)) that are assigned
to an independent traffic movement or combination of movements.
A signal cycle is a
combination of signal phases in which different approaches
of vehicles are permitted to go through the intersection.
Each interval is assigned a discrete amount of time in seconds.
The combination of all the interval times for a given cycle
is called a cycle length.
These terms are demonstrated on Figure
1. Figure 1(a) is a simple two-phase operating traffic
signal, which operates as follows:
Phase 1 – All movements on Main Street are permitted
to travel through the intersection. Left-turners are required
to wait for a gap in the opposing direct traffic before they
can make their turn;
Phase 2 – All movements on Elm Street are permitted
to travel through the intersection. As with Main Street left-turners,
Elm Street left-turners must wait for a gap in opposing flow
to complete their turn.
This treatment of left-turn traffic is called permissive operation;
left-turns may be made after yielding to oncoming traffic
In a protected operation,
left or right turns are protected from oncoming vehicular
traffic. An example of protected
operation is shown in Figure 1(b). In this example, the
left-turners from Main Street are protected from oncoming
Pedestrian movements are accommodated at intersections
by providing either a concurrent or an exclusive pedestrian
phase. In a concurrent pedestrian phase, pedestrians may cross
parallel with the vehicles that have a green signal. In an
exclusive pedestrian phase, vehicular traffic is stopped in
all directions and pedestrians are allowed to cross in all
directions. The cycle length can be varied to accommodate
pedestrian phases as part of a signal cycle. Figure
2(a) shows an example of a concurrent pedestrian phase,
and Figure 2(b) shows an example of an exclusive pedestrian
III. BASIC Types of Traffic Signal Control
The overall operation of a traffic signal is coordinated
through a traffic signal controller.
The controller is an electrical
device housed in a cabinet that directs which signal phase
is to be called and for how long. The traffic signal control
can be either pre-timed or actuated. In a pre-timed signal
control, a signal cycle follows a fixed order of signal phases
and each of the intervals is a fixed length of time. A pre-timed
signal is also called as a fixed
In an actuated signal control, the signal
is designed to adjust its timing within specified limits
to respond to traffic
at the moment, as registered with the controller via
traffic detectors located in the street. Actuated controllers have
the ability to alter their sequences to skip phases on which
no vehicle demand is registered by the detectors. In other
words, if no vehicles are stopped for a red light on a particular
intersection approach, the detector would not be actuated
and the controller would skip this phase of the cycle and
the next phase. If none of the approaches with detectors
are actuated by vehicles then the “green” stays
on the green interval of the phase in operation.
An actuated operation can be either fully-actuated or semi-actuated. In
a fully-actuated signal operation, all approaches to an intersection
are equipped with detectors. In a semi-actuated operation, at least one approach to an intersection does not
have a detector and is usually the major street. A semi-actuated
operation is best suited for traffic signal coordination.
Figures 3(a) and 3(b) show a fully-actuated and semi-actuated
signal control operation respectively. In Figure
3(a), detectors are provided on all approaches to the
intersection (fully-actuated operation). In Figure 3(b), detectors
are provided on the Elm Street approach and the left turn
lane on Main Street. The Main Street through and right turn
movement does not have any detectors (semi-actuated).
In a flashing operation, a flashing yellow
light is displayed on the main street and a flashing red
light is displayed
along the side street. This type of signal control is used
when traffic volumes are very low or when a signal is inoperative.
Another feature of traffic signals is signal pre-emption which is used for emergency vehicles such as
police to give them priority in an emergency. Occasionally,
the term “pre-emption” is also associated with
at-grade railroad crossings.
IV. DEFINITION OF Traffic Signal Coordination
An offset is defined as the time difference in the beginning
of green between adjacent traffic control signals and is
expressed in seconds. Traffic
signal coordination is a method
relationships between adjacent traffic control signals using
Traffic signal coordination reduces delay
and unnecessary stops at traffic signals. The benefit of
traffic signal coordination
is based on the relationship
Figure 4: - Optimum Signal spacing as
a Function of Speed and Cycle length.
between the prevailing speed
vehicles on the main street, the spacing of/distance between
traffic signals, the hourly traffic volume on a major street,
hourly traffic volumes on the side streets, and number of
non-signalized intersections along the roadway system.
Travel speed along a roadway system is dependent
on the signal spacing and the cycle length at traffic signals
Travel speeds are lower when traffic signals are closely
spaced and operate under a short cycle length. Conversely,
travel speeds are a result of long cycle lengths and large
spacing between intersections.
Traffic signal coordination can be achieved
at short signal spacings, such as at 0.25 mile, as long as
the traffic volumes
are low and short cycles (70 second or less) can be used.
As arterial and cross-street traffic volumes increase,
longer cycle lengths must be used in order to increase capacity
minimizing lost time. As a result, cycle lengths of 90
120 seconds are commonly used in those areas. A spacing
miles will enable traffic flow at a wide range of speeds,
with cycle lengths ranging from 60 to 120 seconds.
V. NEED for Traffic Signal Coordination
Traffic signal coordination is typically needed to process
traffic efficiently through a group of intersections. This
is an attempt to utilize the existing roadway infrastructure
by insuring optimum travel speeds while reducing delay. Traffic
coordination may delay or even eliminate the need for roadway
widening. Since traffic signal coordination attempts to reduce
the number of stops and slow down of traffic, there is a
reduction in accident potential. In addition to traffic and
the need for signal coordination may be justified by high
levels of vehicle emissions and poor air quality.
An engineering study may be required to determine
the need for traffic signal coordination. The need is based
on a detailed
investigation of the existing conditions which include travel
speeds and delay, traffic volumes and accident experience.
VI. factors INFLUENCING Traffic Signal Coordination
To maximize the effectiveness of traffic signal coordination,
the following factors should be considered: traffic signal
spacing, traffic flow characteristics, and traffic signal
cycle lengths. Although these factors are closely related
another, they should be considered independently for evaluation.
Traffic Signal Spacing
The Manual of Uniform Traffic Control Devices (MUTCD), the
official national standard, states that traffic control signals
within 800 meters (0.5 miles) of one another along a major
corridor or in a network of intersecting routes should be
considered for coordination.
Figure 5: - Closely
Spaced Intersections in downtown New Milford.
Other factors such as grades,
operating speeds may also need to be considered in conjunction
with signal spacing.
The goal of traffic signal coordination is
to establish platoons or tight groups of vehicles that can
move easily from one
intersection through another without stopping. The ideal
condition for establishing
these platoons is to have the traffic signals uniformly spaced.
When signals are spaced too far apart, traffic may not form
these platoons thereby undermining the effectiveness of signal
coordination. In addition, uneven or closely spaced traffic
signals can also reduce the effectiveness of platoon formation
therefore reducing arterial travel speeds, resulting in an
excessive number of stops, even under moderate traffic volumes.
Traffic Flow Characteristics
The operations of traffic along a street can be influenced
by the volume of total traffic, the directionality of the
traffic, the time of the day, and the amount of traffic entering,
exiting or crossing from a side street. These traffic flow
characteristics can influence the effectiveness of traffic
signal coordination. For example, on a roadway corridor serving
a downtown area, traffic flows may be heavy inbound in the
morning and outbound in the evening peak hour periods. In
such a case, the traffic signal coordination should be designed
to favor the heavier traffic flow movement.
Traffic Signal Cycle Lengths
Traffic signal coordination requires the cycle lengths at
each of the intersections to be the same. Without traffic
coordination, cycle lengths can vary and are determined based
on traffic volumes using the intersection. If these uncoordinated
cycle lengths vary widely, then traffic signal coordination
may not be appropriate or the corridors may be subdivided
into multiple systems, each operating on its cycle length.
Sometimes, however, certain intersections
in the system may operate on a half or a double cycle. In
a half cycle operation,
an intersection operates at twice the cycle length of the
remaining intersections of the system. In a double cycle
intersection operates at one-half the cycle length of the
remaining intersections of the system.
VII. METHODS OF SIGNAL COORDINATION
When considering traffic signal coordination, there are generally
two environments that require different approaches, traffic
signals located either along a corridor or in a downtown
area. The technique differs in each case and is explained
Corridor Signal Coordination
Traffic signal coordination is provided along a linear study
corridor to improve vehicle progression.
Figure 7: - Linear Study Corridor.
Traffic signal coordination
depends upon the length of the corridor and the spacing of
intersections in the corridor. It is critical when developing
a signal coordination plan to consider traffic operations
of the side streets. For very long corridors, it may be possible
to divide the corridor into sub-areas and establish a coordination
plan for each sub-area.
Downtown Signal Coordination
Typically, downtown roadway networks represent a closed grid
structure with heavy traffic flow
Figure 8: - Downtown Grid System.
patterns in various directions.
It is often difficult to design effective signal coordination
within a downtown due to the directionality of the heavy
traffic flow patterns. Sometimes roadways in a downtown
one-way in operation.
Under these circumstances, functional
classification and traffic volumes play an important role in selecting a
signal coordination. Functional classification is the grouping
of highways based on the character of service they provide.
There are four main functional classifications of highways:
freeway, arterial, collectors, and local access roads. Arterials
and collectors can be further divided into major and minor
Traffic signal coordination on roadways serving
the downtown area should be designed to favor the heavier
direction. Typically, these roadways carry heavy traffic
the downtown during the weekday morning condition and out
of the downtown area during the weekday evening condition.
VIII. TYPES OF COORDINATED SIGNAL SYSTEMS
A signal system can be defined as a group of traffic signals
that are coordinated. The selection of a type of signal system
is based upon the available budgetary resources and the applicability
of that system in the given area.
The most common signal systems are Urban Traffic
Control Systems (UTCS), Closed-Loop Systems, Time-Based Coordination
Systems, and traffic adaptive signal control systems. The
TBC system operates on a time clock that is used to take
automatically based upon the time of day and day of week.
In contrast, both UTCS and the Closed-Loop systems react
conditions as they are happening, based on actual traffic
volume and signal timing data stored in the system.
In UTCS and Closed-Loop systems, traffic
signals are interconnected using different types of cables
Electrical cables are the most commonly used method of signal
system interconnection. Fiber-optic cables are slowly getting
recognition in signal systems. Connecting cables are not
needed in the TBC system, as adjacent intersections are coordinated
by the timing of their individual controlling clocks.
Traffic-adaptive signal control systems are
designed to develop coordination patterns in real-time based
on traffic flow
data gathered, processed, and communicated to a central computer.
The traffic flow data is gathered using a detector located
in each lane at the signalized intersection.
IX. Advantages and disadvantages OF Traffic
Signal coordination is perceived by many agencies as an advantageous
improvement to the community or corridor in consideration.
In many cases, signal coordination techniques have proven
to be successful in improving the quality of life and mobility
through the area. Project experience from around the United
States has indicated that interconnecting previously un-coordinated
signals and providing newly optimized timing plans and a
master control system can result in a reduction in travel
time ranging from 10 percent to 20 percent.
Some of the advantages of traffic signal coordination
• Improves mobility and access through the area;
• Reduces vehicle accidents in the area;
• Reduces energy and fuel consumption;
• Reduces stops;
• May control travel speeds;
• Provides environmental benefits from reduced vehicle emissions;
• Ability to monitor daily traffic operations (UTCS and Closed-Loop).
Some of the disadvantages of traffic signal
• Increase in travel speeds may have a negative impact in the
• May attract additional traffic through the corridor;
• Maintenance and equipment costs may be high based on the type
of hardware and software used; and,
• Requires qualified staff for maintenance and monitoring of
GLOSSARY OF COMMONLY USED TERMS
This glossary contains some of the most common terms needed
to understand traffic signal coordination.
Figure 9: - A Controller Cabinet.
Actuated Operation – Type of traffic
signal control operation in which some or all signal phases
detectors in the pavement.
Concurrent Pedestrian Phase – A signal phase where
pedestrians may cross parallel with the vehicles that have
a green signal.
Controller – An electrical device mounted in a cabinet
for controlling the operation of a traffic signal (Figure
Crosswalk – Any portion of a roadway distinctly designated
for pedestrian crossing by lines or other markings on the
Cycle Length – The time required to complete a full
sequence of traffic movements.
Detector – A sensing device (usually either embedded
in the pavement or from video camera locations) used for
determining the presence or passage of vehicles or pedestrians.
are used in an actuated or semi-actuated operation.
Exclusive Pedestrian Phase – A signal phase where
vehicular traffic is stopped in all directions and pedestrians
allowed to cross in all directions.
Functional classification – Grouping of highways
based on the character of service they provide. Freeways,
collectors, and local roads fall under different functional
Green Band – The amount of green time available to
a group of vehicles in a progressive signal system.
Interval – A portion of a signal cycle where signal
indications do not change.
Offset – The time duration between the initiation
of the progressed movement (phase) common to any two signals
at the two intersections. It is generally measured at the
intersection relative to the upstream intersection.
Patterns of Operation – A set of cycle lengths, splits,
and offsets part of a signal coordination plan.
Permissive Mode – A mode of traffic control signal
operation in which, when a green light is displayed, left
or right turns
may be made after yielding to oncoming traffic and/or pedestrians.
Phase Sequence – The order of appearance of signal
phases during a signal cycle.
Platoon – A group of vehicles traveling together
as a group, because of traffic control signals, roadway
and other factors.
Pre-emption Control – A change in traffic signal
operation from normal to a special mode. This type of control
commonly used for emergency vehicles such as fire, ambulance,
and police to give them priority in an emergency.
Pre-timed Operation – Type of signal control operation
where a signal cycle follows a fixed sequence, the intervals
of which are of fixed length.
Progression – A time relationship between adjacent
signals permitting continuous operations of groups of vehicles
planned rate of speed.
Protected Mode – A mode of traffic signal operation in
which left or right turns are protected from oncoming vehicular
traffic. Under this operation, a “GREEN ARROW” is
displayed and opposing traffic must stop.
Red Interval – A very short period in a signal phase
where traffic is stopped in all directions and all signals
display a “RED BALL” or “RED ARROW”.
Semi-actuated Operation – A type of traffic control
signal in which at least one, but not all, signal phases
on the basis of actuation.
Signal Coordination – The establishment of timed
relationships between adjacent traffic control signals.
Signal Phase – The portion of a signal cycle that
serves a combination of traffic movements.
Signal System – Two or more traffic control signals
operating in signal coordination.
Signal Timing – The amount of time allocated for
the display of a signal indication.
Split – A portion of the cycle length allocated to
each phase that may occur.
Time-Space Diagram – A two-dimensional representation
of the spacing of various signals along a roadway and the
signal indications of each of these signals as a function
Walk Time – The time provided for a pedestrian, crossing
in a crosswalk, to safely cross the roadway. A “WALK” and “DON”T
WALK” signal is displayed to direct pedestrians to
cross the roadway.
Yellow Interval – This interval follows the green
interval and is a warning for motorists to slow down before
interval is displayed.
• Fred L. Orcutt Jr., “The Traffic Signal Book”,
Prentice Hall, Englewood Cliffs, N.J., 1993.
• Homburger, W.S., Hall J.W., Loutzenheiser R.C., Reilly W.R., “Fundamentals
of Traffic Engineering”, 14th Edition, Institute
of Transportation Studies, University of California, Berkeley,
• Meyer, Michael D., “A Toolbox for Alleviating Traffic
Congestion and Enhancing Mobility”, Institute of
• Traffic Engineering Handbook, Institute of Transportation Engineers,
5th Edition, 1999.
• Traffic Engineering
Handbook, Institute of Traffic Engineers, 3rd Edition, 1965.