Variable Speed Limit Control for Freeways: A Multi-Objective Optimization Strategy for Balancing the Emission Reduction in Carbon Monoxide and Hydrocarbons with Traffic Operation Efficiency

As highway traffic demand continues to rise, research on balancing CO + HC emissions and traffic efficiency through variable speed limit (VSL) systems has become a critical topic. However, existing research has primarily focused on homogeneous road segments and connected autonomous driving scenarios, resulting in a gap in alignment with the operational requirements of actual road segments. To this end, this study focuses on heterogeneous highway sections as the core scenario. Based on the modified Greenshields model and the non-dominated sorting genetic algorithm (NSGA-II), it proposes a zoned VSL strategy optimized for dual objectives of traffic efficiency and CO + HC emissions. The case study results from the Qin-Nan section of the G75 Lanhai Expressway demonstrate that this strategy, through zonal differentiated speed limit setting, effectively enhances traffic flow stability and continuity. It achieves a synergistic increase in both traffic flow and vehicle speed while simultaneously curbing the progression of congestion during high-traffic scenarios. Additionally, this strategy achieves a cumulative reduction in CO + HC emissions of approximately 9.5% while maintaining traffic efficiency. It offers new insights for optimizing speed limit schemes on expressways under environmental considerations, demonstrating significant practical engineering value.