# Microscopic Simulation of Heterogeneous Traffic Flow on Multi-Lane Ring Roads and Highways
## 5 Numerical Simulation
### Results and Discussion
#### 5.2.1 Impacts on Traffic Efficiency
(**a**) The travel time for 80% of HDVs (Scenario 1); (**b**) The travel time for 40% of HDVs (Scenario 2); (**c**) The travel time for 10% of HDVs (Scenario 3). The distribution of travel time for different proportions of mixed traffic on the highway is depicted in Figure 7. In the baseline scenario, it is noticeable that the vehicle travel time was concentrated around 20 s, and some vehicles even exceeded 30 s. In Figure 7a, the travel time of most vehicles was clustered between 15 s and 25 s in Scenario 1, but the increase in the proportion of CAVs and AVs did not lead to a significant change in travel time. Figure 6: The average speed for different proportions of mixed traffic on the ring road. Figure 7: The distribution of travel time for different proportions of mixed traffic on the highway. (**a**) The travel time for 80% of HDVs (Scenario 1); (**b**) The travel time for 40% of HDVs (Scenario 2); (**c**) The travel time for 10% of HDVs (Scenario 3). Compared to Scenario 1, the travel time distribution in Scenario 2 was more concentrated, with a peak of around 20 s. In the case of a high proportion of CAVs and AVs (e.g., CAV-60%, AV-0%), the travel time distribution was very narrow, with almost all vehicles completing their trips in under 20 s. More CAVs resulted in faster, more consistent travel time, indicating that a higher presence of CAVs improves overall traffic flow. In Figure 7c, the distribution of travel time was more concentrated in shorter intervals (15-25 s), and a relatively short travel time was maintained in most scenarios with different proportions. As the proportion of HDVs decreased and the proportion of CAVs and AVs increased, the travel time became even shorter. When the proportion of HDVs was 10%, introducing CAVs and AVs greatly enhanced traffic efficiency, resulting in shorter travel times that were mostly concentrated in shorter intervals. From the above analysis, it is evident that in mixed traffic, as the proportion of CAVs and AVs increased, the distribution of travel time became more concentrated and shorter, suggesting an improvement in traffic efficiency. The higher the percentage of HDVs, the longer the travel time, especially in scenarios with fewer CAVs and AVs. The shift from 80% HDVs to 10% HDVs led to a significant reduction in travel time and a more efficient traffic flow. Figure 8 showed the average speed in the different scenarios on the highway. It is observed from Scenario 1 that the average speed remained at about 21 m/s as the proportion of CAVs increased. The slight increase in average speed compared to the baseline scenario suggested that with 80% HDVs, the introduction of CAVs and AVs has a limited effect on enhancing traffic efficiency. This could be due to the benefits of CAVs being neutralized in the mixed traffic flow. In Scenario 2, the average speed of traffic flow improved significantly as the proportion of CAVs in mixed traffic increased, especially when the percentage of CAVs exceeded 30%, with the average speed rising to over 22 m/s. Nevertheless, when the number of CAVs was small (around 10% or 20%), the improvement in average speed was fairly small, which may be due to the fact that bottlenecks and delay occur during merging and lane changes in mixed traffic flow. In Scenario 3, the average speed reached its peak, approaching 24.5 m/s. However, when the number of CAVs and AVs in the mixed traffic was lower, the average speed experienced a slight decline but was higher than the average speed in Scenarios 1 and 2. This could be related to the merging of the on-ramp and traffic density on the highway. From the analysis above, it is evident that the average speed tended to increase as the proportion of CAVs grew. This suggested that autonomous driving technology  
Figure 8: The average speed for different proportions of mixed traffic flow on the highway. contributes to enhancing traffic efficiency on the highway.