The National Academies

NCHRP 03-38(3) [Completed]

Traffic Adaptive Control (Phase 1)---Critical Intersection Control Strategies; (Phase 2)---OPAC Control Strategies

  Project Data
Research Agency: Farradyne Systems, Inc.
Principal Investigator: R. David Henry and Christine M. Andrews
Effective Date: 9/1/1987
Completion Date: 3/31/1993

The inability of traditional fixed-time traffic signal control systems to automatically modify their timing plans, in response to both long-term and short-term changes in traffic demand, results in excessive delay and congestion. Of particular concern is the fact that correctable delay at poorly timed signals increases dramatically as demand approaches capacity.

At present, there is no consensus as to the best approach to providing traffic adaptive control in signalized networks. Although considerable research had been done on strategies that periodically recompute and change systemwide timing plans, very little research had been done with regard to the critical intersection control (CIC) strategy included in FHWA's Urban Traffic Control System (UTCS) software and other packages. Preliminary results from implementation of CIC, such as in the UTCS-enhanced-type system in Los Angeles, had suggested its potential as an effective adaptive control measure. However, further validation was needed. Recommendations and guidelines were needed regarding the proper application of CIC in different types of signalized network configurations and operating conditions.

The objectives of Phase 1 of this research were to (1) determine the effectiveness of a selected CIC strategy currently used in a first-generation computer-controlled signal system; (2) if the CIC strategy is shown to be effective, develop comprehensive guidelines and a user manual for its application; and (3) develop detailed recommendations for improvements to the CIC strategy for future implementation and evaluation.

All research has been completed, and the final report has been reviewed by the NCHRP panel. The CIC algorithm, in general, was found to perform no better than the fixed-time signal timing plan alternative. The researchers concluded that the CIC algorithm was not sufficiently responsive to react to the cycle-by-cycle variation in traffic demand, and hence, tended to make split changes in response to traffic demand that had long since passed through the intersection. The final report was not published in the regular NCHRP report series.

At a meeting in May 1990, the project panel reviewed the results of the research on the CIC strategies and planned a second phase of the research. Beginning early in 1991, the researchers initiated a study of a new traffic signal control approach known as Optimized Policies for Adaptive Control Strategy (OPAC).

The objectives of Phase 2 were to (1) enhance the isolated intersection control version of OPAC which, if tested successfully, was to be developed as an implementable product; (2) adapt and evaluate OPAC as a responsive traffic control strategy at selected intersections within a computer-controlled system, and (3) prepare documentation suitable for the field implementation of OPAC including software documentation, instructions for its calibration to field conditions, and operational procedures.

Extensive testing, using the TRAF-NETSIM model provided by FHWA, was aimed at adapting the OPAC algorithm for use within conventional traffic signal controllers. This did not prove feasible because conventional controllers did not have sufficient processing power. As a consequence, OPAC could not be evaluated at actual intersections within a computer-controlled system as originally intended.

Further research involved the development of a prototype "OPAC Coordinator" consisting of two processors: one for the execution of the optimization algorithm and the other for administrative duties including controller, detector, and operator I/O. The new coordinator employed the OS9 operating system, two processors (68040 and 68HC000), a VME BUS, and a standard NEMA interface to receive status information from and issue commands to conventional traffic signal controllers. The prototype was intended to be compatible with related efforts by California, Texas, and Carnegie Mellon University to develop general purpose Advanced Transportation Controllers and Controller Software.

The final report from the Phase 2 effort, which will not be published in the regular NCHRP report series, is in four parts: (1) Revised Interim Report, incorporating panel comments on an earlier draft version; (2) OPAC-RT Version 3.0 Software Documentation; (3) Using TRAFNETSIM to Simulate OPAC Control; and (4) Final Report.

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