With the ever-increasing usage of roads by motorists and pedestrians, an efficient traffic signal control system becomes inevitable. This guide looks into the basics of a traffic signal control system.
A traffic signal control system is a network of electrically or mechanically control electronic traffic lights that regulate the flow of vehicles and pedestrians at an intersection.
It is mainly composed of the main controller, vehicle detectors, control circuit, timer, decoder drive circuit, decoder, clock signal generator, and a digital decoder display drive circuit. All these works together in detecting traffic and relaying this information to the main controller which transmits the messages to the traffic signals.
A traffic signal control system consists of different signals directing different routes in and out of an intersection being controlled by a central command controller. The main controller is the brain that takes traffic trends sent by traffic detectors embedded in the road into consideration and then directs the signals.
The controller can be programmed to operate on a fixed time or vehicle actuation module.
To motorists as well as pedestrians, a traffic signal control system offers several unmatched benefits. &#; A well-coordinated traffic signal control system allows for the orderly movement of traffic to and from a specific intersection.
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Figure 8-1. Traffic Management Center (TMC) (1).
Section 3.8 describes functional categories for system control of traffic signals. This chapter concentrates on the architectures and the physical implementation of these categories. Table 8-1 briefly identifies these categories and their major architecture characteristics.
Table 8-1. Performance Levels for Traffic Signal Systems Category Of System Operation Architecture Characteristics Traffic Adaptive ControlSignal timing plans for the lower four levels of Table 8-1 are stored in the controller's database. Interconnected Control and Traffic Adjusted Control require interconnection by means of wireline or wireless techniques and provide the capability for time of day operation and operator selection of signal timing plans. The following architecture variations may be used to implement these two levels.
Figure 8-2 shows how the computation, control and equipment functions are distributed among the three levels. The TMC equipment complement consists of servers, communication equipment, local area network, and peripheral equipment such as printers. The database for the system is stored in the servers and downloaded to the field master controller and then to the local intersection controllers.
Figure 8-2. Typical Architecture for Three Computation Level
Distributed (Closed Loop) System
The architecture contains one or more field master controllers. These units may be located in controller cabinets in the field or may be located at the TMC. Their function is to break down the information from the TMC into the communication channels appropriate to each local intersection controller, and to assemble data from these controllers into an information stream for transmission to the TMC. In the case where the system is traffic adjusted, system detector data from the local field controllers is processed in the field master controller and is used to select the appropriate timing plan. The traffic responsive control algorithm for closed loop systems is described in Section 3.8.
The local intersection controller controls the traffic signal displays. It stores the library of timing plans available for control and implements the selected timing plan (the information for the selection of traffic responsive control timing plans originates at the field master controller and the information for operator selection originates at the TMC). Time of day control uses schedules downloaded from the TMC and stored in the local intersection controller.
The local intersection controller preprocesses the system detector data into volume and occupancy parameters that are uploaded to the higher levels at intervals that might range from several seconds to a minute. Based on data received from the local detectors, the local controller terminates actuated phases at the appropriate time.
Three level systems are often provided by traffic equipment suppliers.
The functions for this architecture are identical to those identified for the three distributed computation level architecture discussed above; however, all of the functions performed by the field master controller are performed by the TMC equipment complement. The functions of the local field controller remain unchanged. Two level systems are often provided by traffic system software suppliers.
Figure 8-3 shows the signal flow for this architecture, which is currently used less frequently than in the past. While the TMC performs all of the computation functions as for the previously described architectures, it also performs the following major functions:
Figure 8-3. Typical Arrangement for Central Control Architecture
A remote communication unit (RCU) or, in some cases, an intelligent remote communication unit (IRCU) may be used to establish communication between the TMC and the intersection controller. Its information processing functions are generally minimal, and it primarily serves to match the interfaces and protocols required by the field controller to those used for communication with the TMC.
Under normal conditions, the computation functions in the local intersection controller are minimal. Commands from the TMC at precisely the proper time in the traffic signal cycle terminate the green interval for each phase. Indication of the state of the green interval is transmitted to the TMC for monitoring purposes. Backup timing plans are also stored in the local intersection controller to be used in case the TMC fails or communication is lost.
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Since this architecture directly controls the phases (or in some cases the intervals) of the traffic cycle from the TMC, it can, in principle, facilitate the use of control strategies that may not be based on the use of stored timing plans such as certain types of transit priority strategies.
The conceptual aspects of these signal systems are discussed in Section 3.8. This section discusses the physical aspects of representative adaptive control systems as well as some of the performance tests.
Conventional traffic signal systems were discussed in Sections 8.2 and 8.3. These systems represent the largest number of systems currently in use in the United States. Timing plans for these systems are stored in the local controller, and may be selected from the traffic management center or by a field master controller in the following ways by:
Since the intent of traffic responsive and traffic adaptive systems is to rapidly respond to changes in traffic flow by analyzing the measured flow of upstream vehicles, the required intracycle information (from the traffic management center or in some cases from adjacent intersections) must be appropriately processed by the controller software. Thus, standard NEMA TS2 controllers generally require software modification to operate with adaptive systems. Type and ATC controllers with appropriate software are often used for adaptive systems. More intensive deployment of traffic detectors is generally required for adaptive systems as compared with conventional traffic responsive systems.
A wide range of performance improvements have been reported in the literature for these systems. While test techniques vary, the baseline for these tests has generally been pretimed systems, with considerable variation in the quality of the time of day plans. Table 8-3 provides a representative sample of results obtained in this way. The greatest benefits occur for those situations where traffic experiences significant variation from a pretimed plan. Examples include special events and arterials that support diversion from freeways during incidents.
Table 8-3. Representative Results of Performance Testing Adaptive Control Algorithm Test Location Results OPAC Northern Virginia 5-6% improvement in stops and delays1 SCATS Oakland County, Michigan Average of 7.8 % reduction in delay2 Newark, Delaware Travel time reduction up to 25%2 SCOOT Minneapolis, Minnesota Delay reduction of up to 19% during special events2 Toronto, Ontario 8% decrease in travel time, 17% decrease in delay3 Durban, South Africa 7% travel time reduction4 1 Gartner, N.H., F.J. Poorhan, and C.M. Andrews. &#;Implementations and Field Testing of the OPAC Adaptive Control Strategy in RT-TRACS.&#; Presented at the 81st Annual Meeting of the Transportation Research Board, Washington, DC, .Table 8-4 identifies some advantages and disadvantages of traffic responsive and adaptive systems.
Table 8-4. Advantages and Disadvantages of Traffic Responsive and Traffic Adaptive Control Systems AdvantagesACS-Lite, a FHWA research project, adapts certain principles developed during adaptive control system research and development to use by closed loop systems. This discussion is adapted from Reference 2.
ACS-Lite employs the concept that a TOD schedule is an appropriate way to manage traffic demand over the day and by days of the week. Within the context of a TOD schedule, ACS-Lite will adapt the particular plans that are implemented at each time of day based on the overall performance of that plan for the similar previous day. This approach to adaptive behavior uses the traditional traffic engineering assumption that average behavior of traffic on, for example, Tuesday at 3 P.M., is roughly the same on every Tuesday at 3 P.M., but drifts slowly with long-term changes in population, construction, new routes, etc.
If the performance of the baseline plan is determined to be improvable by changing cycle, splits, or offsets, then those changes will be made to the "optimized" plan stored in ACS-Lite and downloaded to the local controllers for use on the next day. The goal of being appropriately adaptive at this level is the maintenance of the timing plan over long periods of time to address the typical degradation of plan effectiveness (e.g., 4% worse per year) and replace the very expensive task of re-timing signals on a periodic basis.
The next level of adaptivity used by ACS-Lite is on-line modification of the TOD plan parameters as the plan is running. With the assumption that the baseline optimized TOD plan is a good starting point, ACS-Lite will adapt the cycle, split, and offsets of the plan within some neighborhood of the baseline settings over the plan's intended implementation duration. ACS-Lite may also adapt the start and end time of the plan from the baseline TOD schedule according to the current conditions, considering the effectiveness of the new plan versus the one that is currently running. ACS-Lite also identifies and selects the best strategy to transition between timing plans.
Signal coordination requires a common time reference shared by all controllers in the coordination group. They must each reference their offset to the same background cycle - a background cycle that is of the same duration and starts at the same time at all controllers. This can be achieved by a master controller transmitting a synchronization pulse or message to all the controllers at the start of the background cycle. However, such schemes fail if the communications link breaks down.
Modern controllers most commonly use time base coordination as the means of synchronizing the start of the background cycle in all controllers. This scheme uses a time-of-day clock in each controller. The clock enables each controller to know the current time to at least the nearest second. The controller considers the background cycle to have started at a particular time of the day, such as midnight, called the offset reference time. At any time during the day, the controller can determine where it is now in the background cycle by calculating the number of seconds since the offset reference time (say midnight) and dividing by the cycle length. The remainder is used to calculate when the current background cycle started. The local cycle zero point is then calculated by adding the offset time to the background cycle zero point.
Time-base coordination works regardless of the mix of controller types and software in the system. However, it only works if the clocks in all controllers are well synchronized. Each controller counts the passage of time and automatically adjusts for daylight saving time. Clocks in controllers tend to drift over time and need to be reset periodically. If the controller is connected to a master or central computer, the clock can be reset automatically.
Standard time can be obtained by computers, masters and laptops computers used by technicians using transmissions from various sources such as cellular systems, radio broadcast from the National Institute of Standards and Technology (radio stations WWV and WWVH), time servers on the Internet, and the Global Positioning System. Some agencies have installed wireless time-source receivers in each signal cabinet. Section 7.5 provides additional information on time base references.
A traffic management center (TMC) is the hub of a traffic control system. The TMC brings together human and technological components from various agencies to perform a variety of functions. TMCs may deal with freeway traffic management, surface street traffic management, transit management or some combination of these functions. Chapter 14 of the Freeway Management and Operations Handbook (3) gives detailed information on freeway TMCs. This section is concerned primarily with traffic signal system TMCs.
Typically, surface street TMCs perform the following functions (3, 4):
One of the primary purposes of a traffic signal system TMC is to manage the timing of traffic signals in urban networks and on arterial streets. Special software allows an operator at a workstation in the TMC to communicate directly with field equipment and modify traffic signal programs in real time.
One of the largest and most advanced traffic signal system TMCs is the Automated Traffic Surveillance and Control System (ATSAC) in Los Angeles, California. ATSAC has been upgraded to use the Adaptive Traffic Control System (ATCS). ATCS can operate in the following modes (5):
A traffic signal system TMC is useful when helping to manage incidents. CCTV cameras, as well as other forms of surveillance, can be used to detect incidents. When an incident occurs, either an operator can manually select an incident traffic signal timing plan, or by the use of a traffic responsive expert system which automatically selects an incident timing plan. Changeable message signs, web sites and the media may be used to alert motorists.
The size and staffing requirement of a TMC can vary greatly, depending on the size of the urban area, and the functions that the TMC provides. Table 8-5, below, shows a summary of TMC size and staffing requirements for various TMC's in the US (6, 7, 8). Since TMC functions differ among agencies, the site and staff size may not be comparable among these agencies.
Table 8-5. Comparison of Various Traffic Signal System TMCs in the United States Location City / Area Approx. Population TMC Size Number of Traffic Signals on System Staff Los Angeles, CA (ATSAC) 3,700,000 sq ft 7 transportation engineers, including 1 supervisor. 2 systems analysts, 1 graphics designer, 1 traffic signal electrician, 1 secretary Miami-Dade County, FL 2,200,000 sq ft 13 employees San Antonio, TX 1,100,000 sq ft 765 1 engineer, 3 technicians Las Vegas, NV: Las Vegas Area Computer Traffic System (LVACTS) 1,500,000 (Covers Clark County) sq ft 700 4 administrative positions. 4 traffic operations positions. 4 maintenance positions Atlanta, GA 416,000 sq ft 650 Traffic signal operations: 1 engineer, 1 senior operator, 2 operators. CCTV: 1 engineer, 1 technician Albuquerque, NM 449,000 800 sq ft 450 4 employees (2 engineers) Denver, CO 555,000 sq ft 450 No dedicated staff for TMC. Approx 1.5 FTE, more during special events Seattle, WA 600,000 sq ft 432 One supervisor, two operators Phoenix, AZ 1,300,000 sq ft 400 1 supervisor, 4 technicians Boston, MA 590,000 sq ft 320 7-8 employees Renton, WA 53,000 700 sq ft 96 Initially, one part-time staff member. Can accommodate up to two full-time staff members Redmond, WA 48,000 800- sq ft (currently under construction) for traffic management area. - sq ft for signal shop area 25 (under construction)Control room: one supervisor, one operator
Signal shop: up to five maintenance staff
Sources: &#; Various TMCs &#; Freund, K. &#;A Comparison of Traffic Management Centers in the Puget Sound Region.&#; ITE Journal. Institute of Transportation Engineers, Washington, DC, July , pp. 46-50. &#; United States Census Bureau. United State Department of Commerce. . &#; Federal Highway Administration, USDOT, Oak Ridge National Laboratory, &#;Metropolitan Intelligent Transportation Systems Infrastructure Deployment Tracking Database &#; FY96.&#; January .
Because a signalized arterial may cross the boundaries of traffic systems or jurisdictions, it may be necessary to coordinate the operation of the systems so that traffic progression is maintained across a boundary. This requires the following:
This type of coordination is facilitated by the use of common time of day schedules for the traffic systems on opposite sides of the boundary.
Where the systems lie in different jurisdictions, a memorandum of understanding between the agencies may be appropriate to establish the basis for a joint planning effort. Inter-agency coordination is discussed in Reference 9.
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