# Microscopic Simulation of Heterogeneous Traffic Flow on Multi-Lane Ring Roads and Highways
## 5 Numerical Simulation
### Results and Discussion
#### 5.2.3 Impacts on Traffic Safety  
In numerical simulation experiments, the number of vehicle conflicts was used to evaluate the impact of heterogeneous traffic on traffic safety on the ring road and highway. Figure 11 showed the average number of potential traffic conflicts for different proportions of mixed traffic flow on the ring road. The number of potential conflicts in mixed traffic decreased as the proportion of CAVs increased, with the reduction being most significant at a higher proportion of CAVs.  
Compared to the baseline scenario, Scenario 1 exhibited a higher number of traffic conflicts, and as the proportion of CAVs and AVs increased, the number of conflicts showed a general decline. This suggested that a higher proportion of HDVs might have a negative impact on traffic safety, but this adverse effect gradually diminishes as the number of CAVs increases. In Scenario 2, when the percentage of CAVs rose to 30%, the number of traffic conflicts steadily decreased, approaching the level seen in the baseline scenario. This suggested that the negative impact on traffic safety in mixed traffic gradually diminishes. However, once the proportion of CAVs reaches or exceeds 40%, the impact on traffic safety becomes positive, indicating a gradual improvement overall in traffic safety.  
In Scenario 3, when the percentage of CAVs was less than 45%, the number of traffic conflicts gradually declined, approaching the level seen in the baseline scenario. This indicated that the negative impact on traffic safety in mixed flow progressively decreases. However, traffic safety improves when the proportion of CAVs reaches or surpasses 45%. Particularly, when the proportion of CAVs reaches 90%, traffic conflicts are nearly eliminated, indicating optimal traffic safety.  
The distribution of traffic conflicts in mixed traffic on the highway is shown in Figure 12. It is obvious that the number of conflicts gradually decreased as the proportion of CAVs and AVs increased. In Scenario 1, when the proportion of CAVs reached 20%, the number of traffic conflicts approached that of the baseline scenario. This suggested that a low proportion of CAVs in a mixed traffic environment may not be sufficient to control traffic flow effectively, leading to more potential conflicts. However, as the proportion of CAVs increased, the traffic flow became more stable and coordinated, resulting in a sharp decrease in conflicts. The number of conflicts in Scenario 2 was slightly lower than that in Scenario 1, and when the ratio of CAVs was not less than 50%, the number of traffic conflicts  
Figure 11: The average number of potential conflicts for different scenarios on the ring road.  
fell below those in the baseline scenario. In Scenario 3, the number of traffic conflicts was significantly lower than in other scenarios, and when the proportion of CAVs was over 30%, the number of conflicts gradually approached zero. This indicated that a higher proportion of CAVs or a more balanced vehicle distribution in Scenario 3 can significantly improve traffic safety. This is likely because the interactions between CAVs and HDVs are smoother, thereby reducing the potential risk of conflicts.  
Overall, the introduction and increase in the proportion of autonomous vehicles have a positive impact on traffic safety. However, the effect on traffic safety is different in different scenarios, which may be related to the composition of mixed traffic flow in each situation.