## 4 Results and Discussion
### Lateral Distance Between Parallel Vehicles
#### 4.4.2 Factors Affecting Lateral Distance  
With a constant lane width, an increase in the width of the vehicle body compresses the available lateral spacing, further affecting the vehicle's control of lateral distance. To grasp the relationship between lateral distance and vehicle body widths, the sum of the widths of parallel vehicles is grouped at a distance of 0.2 m, and the distribution of lateral distance in each group is counted separately, as shown in Figure 13 (where the boxed portion indicates the 5 th-95 th percentile and the dotted line is the mean value of the different groups). It can be seen that, with the vehicle width sum increasing, the growth trend of the geometric center distance is not obvious, while the wheel distance has a significant downward trend. For two cars with a width sum of about 1.6 m, the average wheel distance is about 2.5 m. For two trucks with a width of about 2.5 m, the average wheel distance is less than 1.8 m. Therefore, the effect of the body width on the geometric center distance is relatively weak. However, since the geometric center distance does not change significantly, an increase in the width will lead to a significant decrease in the wheel distance.  
In the existing studies, speed is considered to be the main factor affecting the lateral distance between parallel vehicles and a higher speed requires a greater lateral distance to ensure driving safety. To verify the above viewpoints, parameters such as speed sum, speed difference and speed squared difference are considered as factors affecting the wheel distance. According to the survey results of the best-selling models in China's passenger car market, the body width of a car is generally around 1.9 m. To exclude the influence of body width, only vehicles with a width of 1.9 \(\pm\) 0.1 m are considered. Figure 14 shows the metrological heat map of the relationship between different speed indicators and wheel distance for typical parallel vehicles. It can be seen that the effects  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Combination**} & \multirow{2}{*}{**Lateral Distance**} & \multirow{2}{*}{**Mean/m**} & \multirow{2}{*}{**S.D./m**} & \multicolumn{3}{c}{**Typical Percentile Values/m**} & \multirow{2}{*}{**Kurtosis**} & \multirow{2}{*}{**Skewness**} & \multicolumn{2}{c}{**Normal Distribution**} \\  & & & & **15 th** & **25 th** & **50 th** & **75 th** & **85 th** & & **Test Results** \\ \hline \multirow{2}{*}{CC} & Geometric center distance & 4.09 & 0.46 & 3.65 & 3.80 & 4.07 & 4.35 & 4.52 & 1.70 & 0.57 & \textless{}0.001 \\  & wheel distance & 2.16 & 0.44 & 1.71 & 1.86 & 2.15 & 2.44 & 2.60 & 1.44 & 0.39 & \textless{}0.001 \\ \hline \multirow{2}{*}{CT} & Geometric center distance & 4.21 & 0.49 & 3.74 & 3.89 & 4.17 & 4.46 & 4.63 & 2.13 & 0.82 & \textless{}0.001 \\  & wheel distance & 1.99 & 0.50 & 1.52 & 1.67 & 1.95 & 2.25 & 2.43 & 2.03 & 0.79 & \textless{}0.001 \\ \hline \multirow{2}{*}{TT} & Geometric center distance & 4.27 & 0.33 & 3.94 & 4.06 & 4.26 & 4.47 & 4.59 & 2.62 & 0.46 & 0.034 \\  & wheel distance & 1.77 & 0.33 & 1.45 & 1.56 & 1.76 & 1.97 & 2.09 & 2.65 & 0.47 & 0.024 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Descriptive statistics of the lateral distance of different vehicle combinations.  
Figure 13: The variation law of the lateral distance of vehicles along with the width sum of vehicles.  
of the three indicators on wheel distance have the same pattern. The wheel distance is mainly distributed around 2.1 \(\pm\) 0.4 m; for different groups of a single indicator, the frequency number decreases step by step with group 2.1 m as the center to both ends. The Pearson correlation coefficients between the three indicators and wheel distance were \(-\)0.032, 0.049 and 0.028. Thus, it can be concluded that speed does not significantly affect wheel distance in the freeway driving environment.