Implementing perimeter controls for the Perth CBD

The iMOVE project Modelling perimeter controls: Detailed simulation, undertaken with Main Roads Western Australia the University of Western Australia, and PATREC (Planning and Transport Research Centre) has been completed, and its final report, Implementing Perimeter Controls for the Perth CBD, is available for download.

Perimeter control (also known as gating) works by dividing the network into multiple zones and regulating their flow exchange at the boundaries. It aims at load-balancing between zones across the network to achieve a stable and optimum operation at the global level.

Earlier work

Effective implementation of perimeter control requires a good understanding of the behaviour of each zone. That understanding was investigated in an earlier iMOVE project, Improved network performance prediction through data-driven analytics and simulation, which investigated the feasibility of applying perimeter controls in Perth metro area using a mathematical model and some emerging datasets

Objectives

With the promise of that earlier work, the objectives of this particular project were:

• Segment the simulated Perth CBD network into multiple zones with well-defined macroscopic fundamental diagrams (MFD);
• Develop perimeter control strategies for these zones; and
• Demonstrate that MFD-based perimeter control can improve the overall network performance and driver experience in the Perth CBD area.

In order to meet these objectives work was divided into four tasks:

1. Validation of the simulated Perth CBD network as a realistic model;
2. Zoning of the simulated Perth CBD network;
3. Development of perimeter control strategies; and
4. Evaluation of the performance of MFD-based perimeter control for the Perth CBD network

The model

The Perth CBD model was built by Aurecon in the Aimsun Next simulation program, chosen as instead of the previously used abstract mathematical model because of its higher fidelity to the real-world traffic system, including travel demand, driver behaviour, and signal timing, etc.

The Perth model is a mesoscopic model, which means Individual vehicles are simulated as agents but their interaction with each other and the environment is largely simplified, and time is skipped as they pass between queues.

Two categories of gating methods have been tested in this project, ‘hard gating’ and ‘soft gating’. Hard gating involves mid-block signals acting as ‘boom gates’ to completely stop the traffic from entering a zone. They were inspired by ramp meters used in Smart Freeways. Although the practicality is questionable, they serve as a benchmark to gauge the performance of ‘soft gating’ because of the high controllability.

Soft Gating, on the other hand, adjusts the operation of existing traffic lights at zone boundaries to reduce the flow of vehicles entering congested areas. For example, it might give less green light time to incoming traffic while giving more to outgoing traffic. This approach is more flexible and practical because it works with existing infrastructure, but it requires careful tuning to balance flows effectively without causing frustration or inefficiency.

Conclusions and further research

This work produced some significant findings and recommendations:

• The simulation data suggest that the Perth CBD can be divided into zones with well-behaved MFDs;
• Perimeter control can be applied to reduce traffic congestion in a protected zone with the cost of increased delays at the gates;
• The type and strength of perimeter control can be modified to alter the balance between reduced congestion inside and increased delays to enter;
• If the increased flow inside the system outweighs the delay imposed at the perimeter, the overall performance can be improved using perimeter control;
• Results show that existing traffic lights can be used as gates if the controller can dynamically reallocate green times;
• Recommend reconfiguration of the perimeter lights to split-phases during the control period (AM peak) but the system could also work using existing phases although it is more difficult to set up and ongoing monitoring might be required; and
• Network flow can be potentially increased by 39% and trip completions can be increased by 17%.

This research points to a number of possibilities for further work, including:

• Results suggest that it is feasible to implement perimeter control around the Perth CBD but a detailed design stage would need to be undertaken, and controller software modification would be necessary. Therefore, a feasibility study into the implementation of soft gating control via existing SCATS infrastructure is recommended.
• Implementing queue detectors to assist the perimeter control strategies with handling excessive spillback of vehicles outside of the perimeter. It could also be further investigated using a model with larger coverage.
• Application to a larger model to examine the effects of the controllers on vehicles outside the protected zone.
• Application using a smaller number of zones with boundary intersections for soft gating to explore the concept of MFD-based perimeter control more completely.
• The selection of the density set point for zones could be more thoroughly investigated based on their controllability.
• The scope in this work was limited to the AM peak but theoretically the same principle could apply to the PM peak, although further research is required because it is dominated by flows out of the city.

Expected project impacts

This iMOVE project demonstrated an innovative theoretical proof of concept approach to improve overall road traffic network performance. At this stage there are still significant unknowns around the localised traffic impacts outside the study area and the additional cost/effort/infrastructure needed to integrate the MFD concept into MRWA’s existing systems. The outcomes of this initial work are promising and further study to advance the research should be considered.”

Graham Jacoby, Network Operations Analysis Manager, Traffic and Road Network Performance, Networks Operations Directorate, Main Roads Western Australia

Final report

Download your copy of the final report, Implementing Perimeter Controls for the Perth CBD, by clicking the button below.