# 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  
To measure the traffic efficiency of various road networks, the average speed and the distribution of travel time were explored for different mixed traffic scenarios with varying proportions of CAVs, AVs, and HDVs. Figure 5 illustrates the distribution of travel time in mixed traffic with different proportions of HDVs (80%, 40%, and 10%) on the ring road. The baseline scenario (100% HDVs) showed a broader range of travel time, suggesting greater variability and potential congestion or delay. It can be seen from Figure 5a that the travel time in mixed traffic was primarily clustered between 20 s and 25 s in Scenario 1 (80% HDVs), and the travel time decreased slightly as the proportion of CAVs increased (e.g., 15% and 20%). In addition, a higher percentage of CAVs seemed to slightly shift the travel time distribution towards shorter duration, suggesting minor improvements in traffic efficiency in mixed traffic. In Figure 5b, the peak number of vehicles was spread between 20 s and 25 s when the percentage of HDVs was reduced to 40%. Compared to Scenario 1 (80% HDVs), the introduction of CAVs and AVs can significantly shorten the duration. In Figure 5c, the travel time was generally \begin{table}
\begin{tabular}{c c} \hline \hline
**Assessment Criteria** & **KPIs** \\ \hline Traffic safety & Average number of potential conflicts \\ Traffic efficiency & Average speed; Travel time \\ Traffic dynamics & Number of lane changes \\ Traffic environment & CO emissions; HC emissions; NOX emissions \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Evaluation parameters for the experimental simulation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Simulation Scenarios** & **The Proportion of Mixed Traffic Flow (\%)** \\ \hline
**The Level of Automation and Connectivity** & **HDVs** & **CAVs** & **AVs** \\ \hline Baseline & 100 & 0 & 0 \\ \hline \multirow{4}{*}{Scenario 1} & & 0 & 20 \\  & & 5 & 15 \\  & & 10 & 10 \\  & & 15 & 5 \\  & & 20 & 0 \\ \hline \multirow{4}{*}{Scenario 2} & & 0 & 60 \\  & & 10 & 50 \\  & & 20 & 40 \\  & & 30 & 30 \\  & & 40 & 20 \\  & & 50 & 10 \\  & & 60 & 0 \\ \hline \multirow{4}{*}{Scenario 3} & & 0 & 90 \\  & & 15 & 75 \\  & & 30 & 60 \\ \cline{1-1}  & & 45 & 45 \\ \cline{1-1}  & & 60 & 30 \\ \cline{1-1}  & & 75 & 15 \\ \cline{1-1}  & & 90 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Experimental scenarios on the road networks. lower and more tightly clustered, with most durations falling between 15 s and 25 s. The combinations with higher percentages of CAVs and AVs showed the most significant improvement, with travel time shifting even further left, indicating faster and more consistent travel time. The line chart of the average speed of mixed traffic flow with different proportions on the ring road is shown in Figure 6.  
It can be observed that when the proportion of HDVs was high, the average speed of the mixed traffic flow was relatively low. This is because the introduction of CAVs and AVs largely offset the negative impact of HDVs on the traffic flow. As the percentage of HDVs dropped, the variation in average speed in mixed traffic became more noticeable. In the same situation (Scenario 1, 2, and 3), a higher number of CAVs or an appropriate combination of CAVs and AVs significantly improved traffic efficiency on the ring road. This is attributed to the fact that the collaborative abilities of CAVs help smooth traffic flow and reduce congestion, leading to increased average speed. These analyses demonstrate that as the proportion of HDVs decreases and the share of CAVs and AVs increases, traffic efficiency improves considerably. A higher proportion of CAVs and AVs contributed to shortening travel time and increasing average speed, highlighting their potential advantages in mixed traffic efficiency. Figure 5: The distribution of travel time for different proportions of mixed traffic on the ring road.