HALO: Hierarchical Reinforcement Learning for Large-Scale Adaptive Traffic Signal Control

TL;DR

HALO introduces a hierarchical reinforcement learning framework for large-scale adaptive traffic signal control, effectively balancing global coordination and local decision-making to improve traffic flow in complex urban networks.

Contribution

The paper presents a novel hierarchical RL approach with Transformer-LSTM encoders and an adversarial goal-setting mechanism for scalable, coordinated traffic signal control.

Findings

HALO outperforms state-of-the-art methods in large-scale traffic networks.

HALO reduces average travel time by up to 6.8%.

HALO maintains robust performance across diverse traffic scenarios.

Abstract

Adaptive traffic signal control (ATSC) is essential for mitigating urban congestion in modern smart cities, where traffic infrastructure is evolving into interconnected Web-of-Things (WoT) environments with thousands of sensing-and-control nodes. However, existing methods face a critical scalability-coordination tradeoff: centralized approaches optimize global objectives but become computationally intractable at city scale, while decentralized multi-agent methods scale efficiently yet lack network-level coherence, resulting in suboptimal performance. In this paper, we present HALO, a hierarchical reinforcement learning framework that addresses this tradeoff for large-scale ATSC. HALO decouples decision-making into two levels: a high-level global guidance policy employs Transformer-LSTM encoders to model spatio-temporal dependencies across the entire network and broadcast compact guidance signals, while low-level local intersection policies execute decentralized control conditioned on both local observations and global context. To ensure better alignment of global-local objectives, we introduce an adversarial goal-setting mechanism where the global policy proposes challenging-yet-feasible network-level targets that local policies are trained to surpass, fostering robust coordination. We evaluate HALO extensively on multiple standard benchmarks, and a newly constructed large-scale Manhattan-like network with 2,668 intersections under real-world traffic patterns, including peak transitions, adverse weather and holiday surges. Results demonstrate HALO shows competitive performance and becomes increasingly dominant as network complexity grows across small-scale benchmarks, while delivering the strongest performance in all large-scale regimes, offering up to 6.8% lower average travel time and 5.0% lower average delay than the best state-of-the-art.