Traffic Signal
Coordination Handbook

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 to communicate with each other therefore allowing them to work together. When traffic 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 such as Danbury, Norwalk, Stamford, Hartford, Bridgeport, and New Haven 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 coordination. The reader should note that a glossary of traffic signal terminology is available later in the document for reference. All terms shown in bold italics are also defined in the glossary.

II. 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 movements. 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 vehicular traffic.

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 phase.

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 time signal.

In an actuated signal control, the signal is designed to adjust its timing within specified limits to respond to traffic conditions 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 go to 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 fire, ambulance, and 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 of establishing relationships between adjacent traffic control signals using offsets.

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 of 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 (Figure 4). Travel speeds are lower when traffic signals are closely spaced and operate under a short cycle length. Conversely, higher 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 by minimizing lost time. As a result, cycle lengths of 90 to 120 seconds are commonly used in those areas. A spacing of 0.5 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 safety concerns, 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 to one 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, curves, and 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 signal 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 operation, an 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 below.

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
One-Way

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 network are one-way in operation.

Under these circumstances, functional classification and traffic volumes play an important role in selecting a roadway for traffic 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 roads.

Traffic signal coordination on roadways serving the downtown area should be designed to favor the heavier traffic flow direction. Typically, these roadways carry heavy traffic volumes into 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 (TBC) Systems, and traffic adaptive signal control systems. The TBC system operates on a time clock that is used to take actions automatically based upon the time of day and day of week. In contrast, both UTCS and the Closed-Loop systems react to real-world 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 or communication mechanisms. 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

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 central 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 are:
   • 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; and,
   • Ability to monitor daily traffic operations (UTCS and Closed-Loop).

Some of the disadvantages of traffic signal coordination are:
   • Increase in travel speeds may have a negative impact in the community;
   • 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 daily operations.

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 are actuated from vehicle 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 9).

Crosswalk – Any portion of a roadway distinctly designated for pedestrian crossing by lines or other markings on the surface.

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. Detectors 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 are allowed to cross in all directions.

Functional classification – Grouping of highways based on the character of service they provide. Freeways, arterials, collectors, and local roads fall under different functional classifications.

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 downstream 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 geometry, and other factors.

Pre-emption Control – A change in traffic signal operation from normal to a special mode. This type of control is most 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 at a 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 function 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 of time.

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 the red interval is displayed.

REFERENCES

   • 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, 1996.

   • Meyer, Michael D., “A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility”, Institute of Transportation Engineers.

   • Traffic Engineering Handbook, Institute of Transportation Engineers, 5th Edition, 1999.

   • Traffic Engineering Handbook, Institute of Traffic Engineers, 3rd Edition, 1965.