Variable speed limit modelling to improve traffic safety and efficiency of mixed traffic flow by a two-stage framework

A two-stage variable speed limits (VSL) modelling framework is proposed to enhance traffic safety and efficiency at freeway bottlenecks with mixed traffic flow comprising both human-driven and autonomous vehicles. The first-stage macroscopic VSL control framework is based on an extended cell transmission model (ECTM) and a VSL optimal control model. The ECTM serves as a traffic state predictor and characterises mixed traffic flow by modelling various characteristics such as driving behaviours and flow performance. The ECTM-based optimal control model aims to improve safety and efficiency. The second-stage microscopic validation framework is designed using the VISSIM_COM to validate the effectiveness and sophistication of the proposed VSL strategy. The proposed strategy is compared with a fixed speed limit strategy (FSL) and a VSL strategy considering only efficiency improvement (VSL_E). The results demonstrate that the proposed strategy improves traffic efficiency compared to the FSL and significantly outperforms VSL_E in traffic safety.