Variable Speed Limit on Motorway: A Feedback Control Implementation

Mannion, Rory; Ghosh, Bidisha · 2026 · Crossref

DOI: 10.1007/978-3-032-04774-8_95

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Summary

This paper addresses the inefficiency of manually managed Variable Speed Limits (VSL) on motorways, specifically targeting congestion on the M50 in Dublin, Ireland. While VSL is a known method for reducing traffic congestion and improving safety, current implementations often rely on human operators to adjust limits based on observed conditions. This manual approach is deemed inefficient and fails to maximize the potential of existing infrastructure. The authors propose an automated feedback control strategy using a Proportional Integral (PI) controller to dynamically adjust speed limits based on real-time traffic conditions, aiming to regulate vehicle entry into congested areas and accelerate congestion resolution. The methodology employs a Cell Transmission Model (CTM) to simulate a section of the M50 motorway. The simulation parameters were calibrated using observed traffic data from loop detectors supplied by Transport Infrastructure Ireland, including a critical density of 22 veh/km, a free-flow speed of 84 km/hr, and an assumed jam density of 100 veh/km. The proposed PI controller utilizes two macroscopic traffic variables as inputs: traffic density at the congestion site and traffic flow downstream of the VSL application area. The control system operates via two loops: an outer loop compares measured density against a critical setpoint to determine the desired downstream flow, and an inner loop adjusts the variable speed limit to achieve this desired flow. The study compares simulation results with and without the controller during morning rush hours (7–10 am). The results demonstrate that the PI-based VSL controller significantly improves traffic performance. The total Vehicle Hours Travelled (VHT) decreased from 843 in the no-control scenario to 620 with the controller, representing a 26.45% reduction in travel time. Density heatmaps revealed that the controller successfully maintained traffic density at the bottleneck near the critical level, ensuring maximum throughput, whereas the uncontrolled scenario saw densities fluctuating between 40–50 veh/km. Sensitivity analysis indicated that controller performance is dependent on gain tuning; increasing the integral gain of the secondary controller further improved travel time reductions, with gains ranging from 0.0003 to 0.05 yielding VHT reductions between 20.6% and 29.06%. The study concludes that automated PI control for VSL is an effective strategy for managing motorway congestion, offering substantial travel time savings compared to manual or uncontrolled methods. The controller proved robust in keeping bottleneck density near optimal levels, though the authors note that conventional tuning methods are necessary to achieve peak performance. The findings suggest that automating VSL implementation can maximize the benefits of existing infrastructure, addressing the long-standing congestion issues on the M50 and potentially other high-traffic motorways.

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-26; verification: verified.

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• speed management