## 1 Introduction  
Although research on autonomous driving technology for passenger vehicles has been underway since the 1950s, studies on truck automation began only in the mid-1990s [1]. Among the various truck automation technologies, truck platooning is a technique in which multiple trucks form a convoy and drive together. The leading vehicle in a platoon is manually driven by a human driver, whereas the following vehicles use automated driving technology to follow the leader based on vehicle-to-everything (V2X) communication and onboard sensors.  
According to previous studies, autonomous driving and platooning technologies for trucks are projected to offer several benefits, such as increased traffic capacity, improved fuel efficiency through reduced air resistance from shorter intervehicle gaps, reduced emissions, and enhanced driver convenience [2; 3; 4; 5]. The effectiveness of truck platooning in terms of energy savings by reducing intervehicle distances has already been experimentally verified [1; 6]. Despite these advantages, several potential drawbacks have also been identified. First, as the intervehicle distance decreases, the risk to driving safety increases. Additionally, the formation and dissolution of a platoon may cause disturbances in the surrounding traffic flow and hinder prompt responses to unexpected situations [7].  
Despite various efforts to implement platooning for its benefits, most studies have focused solely on its technical aspects, with little consideration given to driver acceptance. Although drivers will be the primary users when platooning technology is commercialized, studies on their psychological stability during platooning are limited. If only technical aspects are considered, the platooning time gap will be minimized to meet the braking distance requirement, thereby maximizing the road capacity and fuel efficiency. However, compared with conventional human driving habits, a time gap may be perceived as excessively short. Therefore, if the time gap is determined solely based on technical considerations without considering human psychological stability, the following drivers may experience psychological discomfort due to the short distance to the preceding vehicle. Therefore, it is crucial to consider the psychological stability of the truck drivers in a platoon when determining the optimal intervehicle time gap.  
The objective of this study is to develop and validate a quantitative evaluation index for assessing the psychological stability of following drivers in a truck platoon. Driving simulation experiments were conducted to reproduce actual road environments. Professional truck drivers participated in these experiments under various driving scenarios and selected an appropriate intervehicle time gap. The psychological stability was quantitatively evaluated using the proposed index. In addition, a qualitative questionnaire survey was conducted to validate the index.  
Furthermore, this study examined the effect of the see-through functionality on enhancing the psychological stability of trailing drivers while reducing the intervehicle time gap by engaging them with real-time traffic conditions ahead of the leading vehicle within the platoon.  
The quantitative evaluation index developed in this study should be applicable to not only truck platooning but also platooning systems for general vehicles.