## 2 Review of Related Works and Technological Trends
### Contributions and Distinctions of This Study  
Most studies on truck platooning have focused on improving vehicle control performance or developing infrastructure to support platooning. Studies that considered the changes in driver stress or psychological conditions caused by the introduction of platooning have mostly remained at the level of simple monitoring and lacked a quantitative evaluation or analysis of drivers' psychological discomfort. In summary, these studies have not systematically addressed drivers' psychological stability during actual truck platooning experiences. This study analyzes the psychological stability of drivers expected to be the most affected by changes in the driving environment when truck platooning is introduced. Beyond simply monitoring drivers' conditions during truck platooning, this study develops a quantitative index to evaluate their psychological stability and uses it to derive an appropriate intervehicle time gap for each scenario. In addition, the effectiveness of the see-through functionality, which should enhance psychological stability, is quantitatively analyzed. Development of a Quantitative Evaluation Index--Stability Index' for Assessing Drivers' Psychological Stability  
In this section, a quantitative evaluation index is developed to assess the psychological stability of drivers within a truck platoon. A review of related studies confirmed that the physiological signals of the human body can be utilized to monitor its psychological states, such as stability and anxiety [22; 23]. Physiological signals commonly used to analyze human psychological states are the heart rate (HR), respiratory interval (RI), and galvanic skin response (GSR). In this study, RI and GSR were selected as indicators for developing the evaluation index owing to their sensitivity to stability and anxiety. HR, on the other hand, was deemed inappropriate as an indicator for assessing psychological stability,as it varies significantly depending on emotional states such as happiness-sadness or compassion-anger, rather than directly reflecting stability or anxiety [24; 25]. In general, the higher the psychological stability, the longer the RI and the lower the GSR [22]. The definitions of RI and GSR are as follows: The RI refers to the average respiratory interval over one minute and is used to assess the driver's psychological anxiety level based on the respiratory cycle. Mathematically, RI is defined as the sum of inhalation time gaps per minute divided by the total number of inhalations, where \(RPT(i)\) represents the respiration peak time of the \(i\)-th inhalation and \(n\) denotes the total number of inhalations per minute:  
\[\text{RI}(\text{t})=\frac{1}{n-1}{\sum_{i=1}^{n-1}}(RPT(i+1)-RPT(i)), \tag{1}\]  
where \(t\) denotes the time divided into one-minute intervals during the experiment and \(t_{0}\) denotes the starting time of the experiment. GSR quantifies changes in the driver's psychological anxiety using the electrical signals of the skin, which varies with sweat gland activity. Mathematically, it is defined as the inverse of skin resistance (R), measured in \(\Omega\). To unify the units with RI, this study adopted the GA, which represents the average GSR value per minute, and is defined as follows:  
\[\text{GA}(\text{t})=\frac{1}{n}{\sum_{i=1}^{n}}\text{GSR}(i). \tag{2}\]  
However, because RI and GSR represent physiological signals at a specific point in time, they are insufficient for evaluating drivers' psychological stability unless their variations under different conditions are analyzed. Therefore, this study defined two additional metrics: the respiration interval ratio (RIR) and the galvanic skin response average ratio (GAR). First, the RIR expresses the percentage change in the current RI relative to the RI measured one minute prior to the start of the experiment.