## 6 Conclusions and Future Work  
### Conclusions  
When developing a novel technology, it is essential to consider not only its technical aspects but also its user-centric factors to ensure successful deployment. In the case of truck platooning, a type of autonomous driving technology, although substantial technical progress has been made, studies focusing on user perspectives remain insufficient. In particular, as truck platooning is introduced, the intervehicle time gap is projected to decrease compared to that in manual trucking, which may pose a risk to drivers' psychological stability.  
Accordingly, this study developed a new quantitative evaluation index, namely, the stability index, to assess the psychological stability of following drivers during truck platooning with respect to the intervehicle time gap. The results of a quantitative analysis using the stability index showed that when the time gap was less than 0.6 s (15 m), drivers experienced a considerable decline in psychological stability. Meanwhile, increasing the time gap beyond 0.6 s did not have a significant impact on enhancing psychological stability.  
To verify whether this index appropriately reflected the drivers' actual psychological states, a post-experiment survey was conducted, and the results were compared with the quantitative evaluation results. The survey results were highly consistent with the quantitative results obtained using the stability index, confirming that the index effectively captured changes in the drivers' psychological stability during truck platooning.  
Additionally, this study analyzed whether the see-through functionality, which provides real-time forward traffic information from the leading vehicle to the following drivers, positively influences psychological stability. The drivers expressed more psychological stability even under shorter time gap conditions with the see-through functionality. The difference between the two experimental conditions was verified as statistically significant using the Wilcoxon signed-rank test, confirming that the see-through functionality has a statistically significant positive effect on improving drivers' psychological stability.  
The proposed stability index and see-through functionality can serve as practical tools for the development of truck platooning systems that consider drivers' psychological stability and can be used as a basis for determining the optimal time gap. Furthermore, the proposed stability index may be generalized to other autonomous driving applications where the user's psychological state is of critical importance. For instance, in adaptive cruise control or general vehicle platooning scenarios, drivers may experience psychological discomfort due to shortened headways or limited forward visibility. In such cases, the stability index could serve as a valuable tool for quantitatively evaluating driver comfort across various operational settings.  
Figure 4: Driver stability points from the post-survey.  
With the adoption of autonomous driving technology, driving environments are gradually shifting from being driver-centered to system-centered. However, the psychological stability of drivers and passengers, who remain inside the vehicles, must not be overlooked. Therefore, it is crucial to create an environment that ensures psychological comfort along with the technical development of autonomous driving. From this perspective, the proposed stability index is considered suitable as an evaluation metric for users' psychological stability and should contribute to the advancement of autonomous driving technology that incorporates user-centric considerations.