Traffic Control Systems Handbook: Chapter 2. Available and Emerging Traffic Control System Technology
This Handbook assists users in defining, evaluating, and selecting systems that match their needs. This chapter presents a broad overview of system functions and available options for both hardware and software.
Since the 1996 edition of the Traffic Control Systems Handbook, surface street traffic control systems technology has seen significant advances in the following areas (1):
- Improved traffic signal controllers.
- Increased use of CCTV and changeable message signs (CMS) on surface streets.
- Increased use of non-pavement-invasive detectors.
- Improved transit priority strategies and equipment based on the use of GPS technology.
- Increased use of fiber optic cable for interconnection of traffic signal controllers and communication with other field devices.
- Increased use of standardized protocols to migrate data between intersection controllers and field master controllers or traffic management centers.
The period since the last edition of the Traffic Control Systems Handbook has also witnessed the following improvements in control strategies and operations:
- Greater information migration among adjacent and nearby traffic management centers.
- Increased coordination of signals across neighboring jurisdictions and traffic control systems.
- Increased use of adaptive traffic control systems.
- Improved coordination of surface street and freeway operations.
- Provision of traffic control systems with software that facilitates the automatic migration of signal timing plan data derived from signal timing programs into the traffic control system database.
2.2 Control and Management Functions
Control and management functions may include the following:
- Collection of data for development of signal timing plans and other functions (identification of control section boundaries and provision of parameters in the traffic control systems).
- Development of timing plans and the remainder of the traffic control system database.
- Implementation of signal timing plans, such as:
- Traffic responsive.
- Operator selection of timing plans based on data provided by the traffic control system, CCTV and other information sources.
- Implementation of motorist information by means of changeable message signs, highway advisory radio, independent service providers, media and websites.
- Management of incidents on surface streets.
- Evaluation of system performance.
2.3 Integrated Transportation Management Systems
The National ITS Architecture (2) provides the methodology to coordinate transportation related services in a region (Regional ITS Architecture). Integration is achieved through:
- Information sharing among agencies and management centers.
- Coordination of operations among agencies.
Information Sharing Among Agencies and Management Centers
Information is commonly shared among agencies by means of voice communications and data communications. The Regional ITS Architecture establishes the general data flow requirements between agencies and from each agency's management center to the field equipment or other equipment that it communicates with or controls. To perform data communication between management centers, a common language and frame of reference is required. Protocols for the sharing of transportation related information are being established at the time of this writing by the National Transportation Communications for ITS Protocol (NTCIP) and are available on its website (http://www.ntcip.org).
In essence, the information may be put into the proper high level language by the use of the Traffic Management Data Dictionary (TMDD). The TMDD provides the definition and format for the data and the Message Sets for External Traffic Management Communications (MS / ETMCC) which organizes the TMDD elements into relevant messages. Different protocols are included in the NTCIP standards for transmitting these messages between management centers.
Coordination of Operations Among Agencies
Agencies and TMCs may each partly contribute to the improvement of transportation issues in a region. For example, information flow between a transit property and traffic control system may result in the provision of signal priority to transit vehicles. Similarly, a transit property might be able to detect and confirm traffic related incidents on the surface street system, and communicate this information to the TMC operating the signal system. Coordination of traffic signals across jurisdictional boundaries is a common interagency issue.
An appropriate way to address these issues when system design requirements are being established is the development of a Concept of Operations. This document establishes the way agencies will relate their operations to each other and also establishes how the agency will manage its internal traffic system operations. The following material is extracted from Reference 3.
In its simplest definition, a concept of operations for a TMC defines what the center accomplishes, and how it goes about accomplishing it. Thus, it defines functions (what is accomplished) and processes (how they are accomplished). The concept of operations ideally addresses both operations and maintenance of the TMC, and the resources for which it is responsible. It describes the interactions that occur within the TMC, and between the TMC and its partners (firms and agencies) and customers (motorists, media, etc.) in managing transportation. As a tool developed primarily in the planning stage, it often works at a summary level. It is not intended to serve as an operations manual, although it may follow a similar outline.
The outline for a TMC concept of operations contains the following topics:
- The Systems
- Operational Facility Needs
- Integration and Testing
- Performing or Procuring Operations and Maintenance
- Workload and Performance
- Nonstandard Operations
- Fault Detection and Correction
- System maintenance
- Training and Documentation
- Operational Procurement and Contracting
2.4 Range of Agency Needs and Range of Available Options
Although each agency with responsibility for traffic control systems represents a unique entity, with no two exactly alike, many similarities exist. Table 2-1 lists typical system functions.
|System Type||Potential Systems||Special Features|
|Urban and Suburban Street||
|Integrated||Combinations of freeway control and urban street systems||Communication compatibility with freeway control, urban streets and traveler information systems|
In most jurisdictions, traffic control hardware represents an accumulation of different age equipment from multiple manufacturers, purchased over an extended time, using low-bid procurement techniques. Maintenance level of effort and quality greatly affect the hardware's current condition. For example, though the hardware's characteristics permit integration into a system, its physical condition and reliability may preclude continued use. In other cases, existing field hardware might not prove compatible with the desired control system, again precluding continued use. Recent years have seen significant effort in standardizing traffic controller hardware and facilitating equipment interchangeability. This will simplify future replacement.
The conditions which create a need to upgrade or install new traffic systems include:
- Growth and / or changes in traffic demand create a need to examine the adequacy of existing traffic control systems. When traffic flows well below available capacity and no recurrent congestion (or motorist perception of congestion) occurs, little public sentiment develops for control system improvement. However, as flows approach capacity and congestion becomes evident, demand for, and need of, improved control system operation increases. Although agencies should not postpone control system improvements until congestion appears, significant funding for major improvements generally follows public perception of need. Changes in traffic demand may also highlight the functional inadequacy of the existing control system in achieving full use of available capacity.
- Frequent failure of older equipment that results in degraded and inefficient on-street operation.
- The need to obtain improved traffic control through the use of modern hardware and software technologies not supported by existing equipment.
Experience has shown that the agency should establish specific traffic control system objectives in the following areas:
- Traffic Operations - The ability to respond to existing and anticipated future traffic operations requirements. Specific goals might include:
- Obtain maximum efficiency in terms of minimum delay, minimum stops, and maximum capacity utilization consistent with the safety of operation,
- Improve vehicle, pedestrian and bicycle safety, or
- Provide motorists with real-time traffic or routing information.
- System Reliability - Issues include:
- Minimizing control system downtime,
- Minimizing cost of maintenance, and
- Improvement of automated detection and reporting of equipment failures.
- Adaptability - The ability to satisfy traffic operation requirements over a long period of time under changing conditions.
- Implementation - Ease of installation or changeover from an old system to a new one with minimum technical problems and disruption of traffic flow.
- Ease of Operation - The ability to easily develop and maintain system databases including generation and maintenance of timing plans.
Transportation Systems Management (TSM) Relationship
Since first introduced in the mid-1970s, transportation systems management (TSM) has evolved from a list of about 150 low-cost actions to the productive use of existing transportation resources through their coordinated operations and improved management. TSM implies "a philosophy about planning, programming, implementation, and operations that calls for improving the efficiency and effectiveness of the transportation system by improving the operations and / or services provided" (4). TSM, then, provides an umbrella philosophy that aims to:
- Analyze the total system, and
- Improve operation and safety before capital-intensive projects add significant capacity.
Roark (5) classifies TSM actions within 9 different urban operating environments, including:
- Freeway corridor,
- Arterial corridor,
- Central business district (CBD),
- Regional operating environment,
- Major employment site (non-CBD),
- Outlying commercial center,
- Major activity center, and
- Modal transfer point.
In contrast, Wagner uses two primary strategies - supply and demand (6). Supply strategies focus on changing the quality of vehicular flow, whereas demand-oriented strategies target decreasing the quantity of vehicular travel. Supply actions include:
- Arterial signal coordination,
- Signal removal or flashing operation,
- Freeway monitoring and control,
- Incident management,
- Parking prohibition,
- Turn controls, and
- Bottleneck-removal programs.
Demand actions include:
- High occupancy vehicle (HOV) priority treatments, and
- Variable work hours.
In both classification schemes, traffic control systems and their effective operation predominantly affect TSM and prove vital to the full realization of several other TSM actions. For example, it does little good to entice drivers to ride the bus or join a vanpool if inefficiently operating traffic signals stop or delay all vehicles (including buses and vans).
Control System Options
Operational objectives of traffic control systems include making the best use of existing roadway and freeway network capacity and reducing trip times, without creating adverse environmental impacts (7).
Controlling the movement of vehicles through signalized intersections provides the major effect on traffic flow in urban areas. The control strategies shown in Table 2-2 can achieve signalized intersection control. Table 2-2 provides a summary of the features of different categories of traffic control systems. These categories and their characteristics are discussed in greater detail in Section 3.8.
|Categories||Main Characteristics||Application||Control Technique||Method|
|Isolated Intersection Control||Does not consider timing for adjacent signalized intersections||
Intersection sufficiently isolated from adjacent signalized intersection so that arriving vehicles do not exhibit strong platooning characteristics
Intersection timing requirements inconsistent with remaining signal section
|Fixed Time (Pretimed)||Assigns right-of-way according to a pre-determined schedule.Computer programs used with average demand volumes for period to compute timing off line.|
|Traffic Actuated||Adjusts green time according to real-time demand measured by detectors on one or more approaches|
|Time Base Coordination||Coordinates based on common time synchronization.||Signals sufficiently closely spaced to require coordination||Pretimed coordination||Computer programs used with average demand volumes for period to compute timing off line.|
Signals are networked together using wireline or wireless techniques
Provides field equipment status
Downloads timing plans from traffic management center
Pretimed coordination commonly used where variation in day-to-day demand is not excessive
Operator selection used for special situations
Operator selection of timing plans
Computer programs used with average demand volumes for period to compute timing off line.
Operator selection based on special events or external information on incidents or traffic conditions
|Traffic Adjusted Control||Conventional traffic adjusted operation||Traffic adjusted capability employed where variations in day-to-day demand may vary significantly at a particular time||Use of traffic sensors to provide traffic adjusted capability||
Traffic adjusted selection of timing plans
Often provides more timing plans than for interconnected control
|Traffic Responsive Control||Timing plans generated rapidly and automatically using system sensors||Where variations in day-to-day demand may vary significantly or where variations result from unusual traffic patterns or events||Changes split within a cycle. Changes cycle offset within a few minutes||Uses upstream sensor data to optimize objective function such as delay or controls to level of congestion|
|Traffic Adaptive Control||Phase change based on prediction from traffic measurement at each signalized approach||Same as traffic responsive control. Also responds to random variations in traffic flow||Uses predictive data change phase. Does not use explicitly defined signal cycles, splits or offsets||Predicts vehicle flow at intersection from sensor data|
Criteria for Selection
The previous discussion describes the range of alternative systems available to meet a jurisdiction's traffic control needs. Making the most appropriate selection requires critical self-examination and consideration of life-cycle issues concerning:
- System acquisition,
- Operation, and
Matching a control system's capabilities to a set of identified agency requirements proves the most crucial element in system selection. Viable candidates should satisfy these requirements in a cost effective way.
The agency should also match the system's sophistication to the staff's anticipated ability to operate and maintain it. Similarly, to assure system success, the agency must demonstrate its commitment to ongoing staffing and maintenance costs. As described in Federal regulations (see chapter 11), the availability of funds to procure traffic control systems must also include a similar commitment to provide adequate resources for operation and maintenance.
Chapter 11 provides a more detailed discussion of a suggested system selection process that uses an effectiveness-analysis approach. The chapter also describes a utility / cost analysis approach.
2.5 Available Technology
Available control system technology has progressed to the point where current hardware and software capabilities provide the designer with a wide range of control concepts. The transportation engineer or control system designer now has a large array of hardware and software options from which to choose in defining alternative control systems. The challenge is to use them effectively in achieving improved on-street traffic performance.
Subsequent chapters in this Handbook describe in-depth the various hardware elements of a control system. Components and subsystems include: detectors, local controllers, changeable message signs, CCTV, operator displays, central computers and field masters.
Chapters 3, 4, and 8 describe software used in traffic control systems. This includes real-time control software, optimization software and simulation software.
Real-Time Control software developed for local controllers allows the controller to function as a signal switching unit by:
- Receiving detector inputs,
- Processing status data,
- Computing timing, and
- Driving signal lamp load switches.
Manufacturers of standard NEMA controller units provide such software (or firmware) as a part of the device. By contrast, both manufacturers and users have developed software for the Model 170, Model 2070 and advanced transportation controllers.
Many conventional traffic systems feature the UTCS First Generation (1-GC) signature matching algorithm for real-time traffic-responsive control. Unlike earlier UTCS, these contemporary systems usually store signal timing plans at the intersection and select a plan based on detector data patterns. An alternative strategy selects the cycle, split and offset individually based on detector data for each of these parameters. Conventional systems often feature the ability to update timing plan databases from signal timing programs with a minimum of manual operation.
Traffic adjusted systems are being installed in increasing numbers. Chapters 3 and 8 describe both conventional systems and traffic adjusted systems.
Chapter 3 describes the use of offline timing plan development programs. These include TRANSYT 7F, the PASSER family, SYNCHRO and aaSIDRA.
2.6 A Look to the Future
Current research as well as emerging trends are likely to lead to the following areas for changes in traffic systems in coming years.
Hardware in the Loop
Recent research (8,9) has resulted in the development of systems that enable traffic controller equipment to be tested under simulated traffic conditions. Figure 2-2 provides an example of the implementation of this concept. A microscopic simulation program such as CORSIM is interfaced to a physical traffic controller by a controller interface device (CID). A software link in the form of a dynamic link library (d11) transfers information between the computer on which the simulation is running to the CID. A network of traffic controllers may be interfaced to the simulation in this way.
Figure 2-2. Hardware in the Loop Configuration
This technique provides the capability to:
- Achieve a high level of fine-tuning after controller settings prior to implementing the settings in the field.
- Prove out controller software.
- Study the performance of new or modified traffic control algorithms.
Non-Pavement Invasive Traffic Detectors
These detectors generally have a number of desirable operational features including low maintenance cost, ease of maintenance, ability to more easily change detected locations, and a variety of traffic data preprocessing capabilities. Nevertheless, their accuracy for detection of traffic responsive control parameters has generally not yet approached that of inductive loop detectors. It is anticipated that continued development of these devices will result in performance improvement in this regard.
Support of Emergencies and Evacuations
Regional architectures increasingly require the interchange of data and video between traffic management centers and other agencies responsible for emergency operations. Traffic control equipment capability such as the ability to change timing plans to support emergencies, to make CCTV images available, and to communicate with motorists provides an increasingly useful tool to support emergency operations.
Advanced Transportation Controller
With the recent development of standards, the Advanced Transportation Controller (ATC) will assume traffic signal control functions as well as other transportation system control and management functions. The ATC is an open architecture platform. By employing an applications programming interface (API), the same applications software may be implemented in ATC controllers with different processors and operating systems. ATC will provide the capability to migrate control software into controllers using different microprocessors and operating systems. This will enable operating agencies to employ different controller models interchangeably, while still achieving the same performance capability with each unit. This will facilitate maintenance by avoiding the issue of equipment obsolescence and discontinuance of support by controller and component suppliers.
Advanced Signal State Transition Logic
The objective of this research program (NCHRP Project 3-66) is to make use of the ability of advanced traffic sensors to develop additional traffic state information that might be used to improve the control of traffic signal states.
Improved Transit Priority Systems
The increased use by transit vehicles of advanced equipment such as on board processors, terminals for drivers, GPS equipment, passenger counters and door position monitors in conjunction with computer aided dispatch systems enables the development of signal priority strategies for transit vehicles.
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2. "The National ITS Architecture, Version 5.0." Federal Highway Administration, 2003.
3. "Transportation Management Center Concepts of Operation, Implementation Guide." Federal Highway Administration, December 1999.
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5. Roark, J.J. "Experiences in Transportation System Management." National Cooperative Research Program Synthesis of Highway Practice 81. Transportation Research Board, Washington, DC, November 1981.
6. Wagner, Frederick A. "Energy Impacts of Urban Transportation Improvements." Institute of Transportation Engineers, Washington, DC, August 1980.
7. "Assessment of Advanced Technologies for Relieving Urban Traffic Congestion." National Cooperative Highway Research Program Report 340. Transportation Research Board, Washington, DC, 1991.
8. Bullock, D. and A. Catarella. "Real-Time Simulation Environment for Evaluating Traffic Signal Systems." Transportation Research Record 1634. pp. 130-135, 1998.
9. Engelbrecht, R., C. Poe, and K. Balke. "Development of a Distributed Hardware-in-the-Loop Simulation System for Transportation Networks." Proceedings of the 78th Annual Conference of the Transportation Research Board. Washington, DC, 1999.
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