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32 FIGURE 9 Alarm manager in SCATS. traffic parameters, which help ATCS operators to monitor effectiveness of an ATCS before its field installation are very the quality of the executed signal timing plans and dynami- rare. The lack of pre-installation evaluations of ATCSs through cal changes in traffic conditions. All of the ATCSs provide microsimulation can be attributed to three major factors: tools and functionalities to monitor and track variations of operational traffic parameters. Figure 10 shows a Dynamic · A lack of confidence in microsimulation results, which is Map functionality supported by the LA DOT's ATCS, where still present among many traffic engineers and decision a set of traffic performance measures (such as volume, speed, makers. queue, stops, and delay) are reported dynamically in real · The complexity and costs of modeling field conditions time. These and similar traffic performance measures from in microsimulation and interfacing the microsimulation other ATCSs can be archived in system databases for future model to an ATCS software. use. ATCS users find this functionality, of archiving traffic · The costs and institutional issues (licensing) associated metrics, very useful. Only 17% of the interviewed ATCS users with acquiring ATCS software to be tested and/or eval- do not believe that reported traffic performance measures are uated in microsimulation. useful for other traffic engineering purposes. In spite of these limiting factors almost all ATCSs have been interfaced with certain microsimulation tools. Discussion ADAPTIVE TRAFFIC CONTROL SYSTEMS of these interfaces and relevant research studies is beyond AND MICROSIMULATION TOOLS the scope of this report. A reader is advised to review the A major disadvantage of field ATCS evaluations, reported bibliography section in Appendix C for further information through survey response and in the literature, is that these eval- on the most important studies regarding ATCS modeling in uations always require an ATCS to be installed and, as such, microsimulation. Table 10 shows microsimulation tools that they represent post-deployment justification studies. Also, as a have been coupled with the ATCSs described in this report. result of costly field data collections, these evaluations are not practical for the investigation of the long-term benefits COMMUNICATIONS of ATCS deployments. To address these issues traffic signal researchers and practitioners have interfaced traffic micro- The importance and costs of communications that are neces- simulation tools to ATCS software. Studies where micro- sary to provide reliable ATCS operations primarily depend on simulation, coupled with an ATCS, is used to evaluate the the way in which signals are interconnected in ATCS network
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33 FIGURE 10 Dynamic map in LA DOT ATCS. architecture. For distributed systems, in which the intersection system. Consequently, inexpensive communications alter- controller is responsible for control, communications between natives, including wireless alternatives, are viable options. The hardware elements at the intersections are the most impor- savings in communications infrastructure usually compensates tant. In distributed systems there is no need for a reliable for the potential higher cost of local controllers. Distributed communications network between intersections and a central systems typically cost between $10,000 and $30,000 per inter- section (Malek et al. 1997). TABLE 10 AVAILABLE INTERFACES BETWEEN ATCS Only 9% of interviewed ATCS users find peer-to-peer AND MICROSIMULATION TOOLS communications to be the most important type of communi- ATCS Microsimulation Tool cations for their systems. Another 9% put peer-to-peer com- munications as second in order of importance. Finally, 44% of ACS Lite CORSIM, VISSIM ATCS users do not believe that peer-to-peer communications BALANCE NONSTOP, VISSIM are important for their systems. InSync VISSIM LA ATCS CORSIM (offline post-processing interface) In centralized systems, a central computer makes control MOTION VISSIM decisions and directs the actions of individual controllers. These systems depend on reliable communications networks. OPAC CORSIM Because real-time control commands are transmitted from the RHODES CORSIM, Q-Paramics central computer to the local intersection, any interruption SCATS S-Paramics, VISSIM, AimSun in the communications network forces the local controller to SCOOT VISSIM, CORSIM, S-Paramics, AimSun operate without that real-time control and revert to its backup UTOPIA VISSIM, AimSun, S-Paramics plan, which usually is time-based coordination; however, this
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34 TABLE 11 COMMUNICATIONS BETWEEN CENTRAL AND LOCAL ENTITIES IN ATCS Criticality of Communications Between Central System and Local Controllers Percent of Agencies Critically important 62 Somewhat important 15 Not important 23 still requires a transition from central control to local control. those users who use centralized ATCSs, all were expected to During this transition, signal coordination is usually lost for give equal importance to communications between various a short period of time. For this reason, communications net- elements at the intersection. Results show that communi- works for centralized systems most often include some form cation between various elements at the intersection is con- of fixed communications, with most agencies preferring to sistently placed as second in importance, with 50% of users own their infrastructure. These communications media include selecting that choice. Eighteen percent of respondents give twisted-pair copper wire and fiber-optic cable. The physical the highest importance to this type of communication, media typically provide inherent reliability of 99.995% to whereas 32% of the respondents did not report this as being 99.99995%, with downtime ranging from a few seconds to a important. few minutes a year. In real systems, downtime is much higher because of physical intrusion on the infrastructure, though Figure 11 shows that approximately 80% of all ATCS some fiber network approaches even minimize the effects agencies use three major types of communication media of that danger. Communications networks for centralized (twisted pair, telephone lines, and fiber optic cables) to com- systems typically consume at least two-thirds of the cost of a municate between the central system and field controllers. system. Centrally controlled systems usually cost between These results can be explained by noting that ATCSs that need $40,000 and $80,000 per intersection (Malek et al. 1997). central-system-to-field-controller communication require very Table 11 shows how interviewed ATCS users perceive criti- reliable communication for their ATCS operations, which is cality of communications between their central systems (if ensured through the use of physical media between various any) and field local controllers. elements in their ATCS architecture. Although the users of distributed ATCSs value peer-to- According to the survey respondents, a similar share of var- peer and local-to-central communications differently from ious media types is observed for peer-to-peer communication Others; 7; 9% Wireless (application protocol or broadband systems); 7; Twisted Pair; 19; 25% 9% Microwave (terrestrial or satellite); 2; 3% Fiber Optic; 22; 28% Telephone Line; 20; 26% FIGURE 11 Communication media between central system and field controllers.