Improving SCATS Operation during Congestion Periods Using Internal/External Traffic Metering Strategy

Afandizadeh Zargari, Shahriar; Dehghani, Nazli; Mirzahossein, Hamid; Hamedi, Masoud · 2016 · Crossref

DOI: 10.7307/ptt.v28i1.1718

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Summary

This paper addresses the challenge of preventing gridlock in urban signalized intersections during oversaturated conditions by improving the operation of the Sydney Coordinated Adaptive Traffic System (SCATS). The authors propose an internal/external traffic metering strategy that divides the urban network into a "protected" core and a "peripheral" upstream area. The primary motivation is to overcome limitations in existing SCATS methods, which often rely solely on saturation degree and fail to account for queue spillback or optimal queue lengths in upstream arteries. By restricting inbound flow to the capacity of critical intersections, the strategy aims to keep congestion in the protected network below a critical threshold while maximizing the utilization of upstream arterial capacity for buffering. The methodology involves a two-part mathematical model implemented in MATLAB and tested using the VISSIM traffic simulator. The first part of the model maximizes the offset between consecutive intersections to estimate optimal queue lengths and prevent spillback. The second part uses these outputs to adjust green time durations at entrance intersections, ensuring congestion remains under a critical level defined by fundamental diagrams. The study was applied to a real-world case in Tehran, Iran, involving eleven intelligent intersections connected to SCATS. Data inputs included physical network geometry, origin-destination matrices, and traffic volume statistics from various sources, including traffic counters and surveillance cameras. The model’s outputs were used to create conditional signal timing plans within SCATS, overriding standard plans when congestion was detected. The results demonstrate significant improvements in network performance when the internal/external traffic metering strategy is applied. Average travel times on two primary arteries, Beheshti and Motahari, decreased by 15.4% and 6.3%, respectively, during morning peak hours. Furthermore, the strategy successfully maintained average queue lengths near optimal levels. For instance, on Beheshti Avenue, the difference between the optimal queue length and the actual average queue length decreased from 5 vehicles to 1 vehicle after implementation. Similar improvements were observed on other entrance arteries, indicating that the model effectively balanced congestion management in the protected network with efficient flow buffering in the peripheral network. The significance of this research lies in its demonstration that integrating queue optimization and spillback prevention into SCATS operations can substantially enhance urban traffic performance during congestion. By actively managing inbound flows and utilizing upstream capacity, the proposed strategy prevents the deterioration of network stability associated with oversaturation. The findings suggest that such metering strategies are effective tools for travel demand management and signal timing optimization. The authors conclude that future research should focus on the online implementation of this model in networks fully equipped with queue detectors and SCATS connectivity to further refine real-time traffic control.

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• traffic density