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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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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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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.
