EvolveLight: A Self-Evolving Agentic Framework for Adaptive Traffic Signal Control

Traffic signal control (TSC) is fundamental to urban mobility, yet traffic demand is spatially heterogeneous and continuously evolving, making it difficult for a single control algorithm to sustain stable performance across regions and control periods. Existing approaches primarily improve adaptability through predefined control principles, specialized policy architectures, or task-specific training, limiting their adaptability in evolving traffic dynamics. We present EvolveLight, an LLM-based agentic framework that enables continual adaptive TSC through a closed loop of traffic diagnosis, controller construction, feedback, and skill development. EvolveLight coordinates three specialized agents: a planning agent that analyzes traffic dynamics, partitions the road network into spatiotemporally coupled regions, and assigns suitable control strategies; a modeling agent that constructs region-specific control policies; and a skill agent that develops reusable traffic analysis and control policy development skills. These skills are maintained in a persistent versioned skill library. Their utility is evaluated through downstream traffic improvements and fed back into future retrieval and refinement. To mutually improve the reasoning capabilities of the three agents, we introduce a decoupled cooperative multi-agent RL (DCMA-RL) mechanism. It starts from supervised cold-start training with rejection-sampled high-quality demonstrations. Afterwards, DCMA-RL alternates role-wise RL optimization with independently normalized cooperative and role-specific rewards, enabling stable specialization while incorporating delayed cooperative downstream TSC performance feedback into agent learning. Extensive experiments across five real-world traffic datasets demonstrate better or competitive improvements over existing baselines, with superior performance during 24-hour adaptation as experience and reusable skills accumulate. Our project is available at https://anonymous.4open.science/r/EvolveLight-8341.