## 5 Driver Psychological Stability Evaluation Experiment Using Driving Simulator
### Evaluation of Drivers' Psychological Stability According to Presence of See-Through Functionality  
The second experiment was conducted to verify whether the provision of see-through functionality significantly enhanced psychological stability. The system instructed participants to select a comfortable time gap twice, once with the see-through functionality enabled and once without it. The distribution of the time gaps selected by the drivers for each condition is listed in Table 5.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multicolumn{5}{c}{**Intervehicle Time Gap**} \\ \cline{2-6}  & **0.2 s** & **0.4 s** & **0.6 s** & **0.8 s** & **1.0 s** \\  & **(5 m)** & **(10 m)** & **(15 m)** & **(20 m)** & **(25 m)** \\ \hline Stability Index & \(-\)0.0464 & \(-\)0.0215 & 0.0178 & 0.0153 & 0.0186 \\ \hline Change compared to 0.6 s & \(-\)0.0642 & \(-\)0.0393 & - & \(-\)0.0024 & \(+\)0.0008 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Stability index according to intervehicle time gap.  
Figure 3: Stability index variation according to intervehicle time gap.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multicolumn{5}{c}{**Intervehicle Time Gap**} \\ \cline{2-6}  & **0.2 s** & **0.4 s** & **0.6 s** & **0.8 s** & **1.0 s** \\  & **(5 m)** & **(10 m)** & **(15 m)** & **(20 m)** & **(25 m)** \\ \hline Number of drivers & 3 (15\%) & 0 (-) & 1 (5\%) & 3 (15\%) & 13 (65\%) \\ \cline{2-6}  & \multicolumn{5}{c}{Average time gap = 0.915 s (22.9 m)} \\ \hline Number of drivers & 3 (15\%) & 4 (20\%) & 1 (5\%) & 1 (5\%) & 11 (55\%) \\ \cline{2-6}   selecting time gap & \multicolumn{5}{c}{Average time gap = 0.865 s (21.6 m)} \\ \cline{2-6}   With see-through & \multicolumn{5}{c}{Average time gap = 0.865 s (21.6 m)} \\ \hline \hline \end{tabular}
\end{table}
Table 5: Changes in drivers’ psychological stability based on availability of see-through functionality.  
When the see-through functionality was not provided, the participants selected an average time gap of 0.915 s, corresponding to an intervehicle distance of approximately 22.9 m. When see-through functionality was provided, the selected average time gap was reduced to 0.865 s, which is equivalent to approximately 21.6 m.  
To verify whether the difference between the two mean time gaps was statistically significant, the Wilcoxon signed-rank test--a widely used nonparametric statistical method for analyzing paired sample data, such as pre/post comparisons--was conducted. The dataset used in this study consisted of 20 paired samples. Because the normality assumption was not satisfied, the use of this nonparametric test was deemed appropriate [26].  
The results of the Wilcoxon signed-rank test indicated that the difference between the two means was statistically significant at the 95% confidence interval. This confirmed that the see-through functionality, which provided real-time traffic information from the leading vehicle to the following vehicle within the platoon, had a statistically significant positive effect on the psychological stability of the following drivers. Table 6 presents the results of Wilcoxon signed-rank tests.