# Microscopic Simulation of Heterogeneous Traffic Flow on Multi-Lane Ring Roads and Highways
## 6 Conclusions  
To assess the impact of mixed traffic scenarios involving CAVs, AVs, and HDVs on traffic efficiency, dynamics, safety, and environment, with vehicle types including both cars and trucks, all experiments were conducted on the multi-lane circular road and highway. Numerical analysis revealed that different experimental scenarios exhibited different traffic performance on different road networks. The experimental results demonstrated that on different road networks, traffic performance improves as the proportion of HDVs decreases and the percentage of CAVs and AVs increases. Specifically, lower average speed and longer travel time for scenarios with a low proportion of CAVs and AVs (a high proportion of HDVs) resulted in lower overall traffic efficiency compared to the baseline scenario. Similarly, in the scenario with the high proportion of HDVs, both lane changes and traffic emissions were lower compared to the baseline scenario. Moreover, when compared to other scenarios, the higher number of lane changes and a smaller reduction in traffic emissions suggested that the improvement in traffic stability and environment is minimal in mixed traffic. In the case of the ring road, this could be attributed to the fact that the high proportion of HDVs results in frequent speed adjustments, turning maneuvers, and unpredictable traffic congestion and fluctuations. The integration of CAVs and AVs largely offsets the negative effect of HDVs on traffic flow. Figure 14: The reduction in traffic emissions in mixed traffic on the highway. (**a**) CO emissions; (**b**) HC emissions; (**c**) NO\({}_{\times}\) emissions. For the highway, this may be related to the interaction between different vehicle types and the complexity of the on-ramp. Conversely, compared to the baseline scenario, traffic safety had a negative impact in the scenario with a high proportion of HDVs. This may be because frequent acceleration and deceleration of vehicles will increase traffic risk. Moreover, as the proportion of CAVs and AVs increased, the negative impact on traffic safety decreased gradually. This indicated that the benefits of CAVs in mixed traffic flow are diminished. In scenarios with a high proportion of CAVs and AVs, the smoother operation of vehicles led to optimal traffic efficiency and stability. In addition, the greatest reduction in traffic emissions of CO, HC, and NOx resulted in minimal environmental impact. This is due to the fact that vehicle-to-vehicle communication and coordination enhance driving routes and speed, reducing traffic congestion and unnecessary stops. In scenarios with a moderate proportion of CAVs and AVs, traffic performance fell between that of low and high proportions of CAVs and AVs. For the ring road, in terms of traffic safety, in scenarios with a moderate proportion of CAVs and AVs, traffic risk gradually decreased and approached the number of traffic conflicts seen in the baseline scenario when the proportion of CAVs was below 40%. This indicated that the adverse effect on traffic safety in mixed traffic decreased. However, traffic safety improved noticeably when the proportion of CAVs was 40% or higher. In scenarios with a high proportion of CAVs and AVs, traffic risk steadily declined and neared the number of traffic conflicts observed in the baseline scenario when the percentage of CAVs was less than 45%. This suggested that the negative impact on traffic safety in mixed traffic experienced a gradual fall. However, traffic safety improves when the proportion of CAVs is not less than 45%. Notably, when the proportion of CAVs was 90%, traffic risk was nearly eliminated, resulting in optimal traffic safety. For the highway, regarding traffic safety, traffic safety progressively improved as the proportion of CAVs rose. In scenarios with a moderate proportion of CAVs and AVs, the adverse effect on traffic safety steadily declined until the ratio of CAVs reached 50%, after which traffic safety significantly improved.