# Physiological Evaluation of User Experience in Unstable Automated Driving: A Comparative Study
## 4 Discussion  
Automated-driving technology is rapidly becoming mainstream through initiatives like self-driving taxis and pilot public transportation programs. Beyond technical sophistication, securing user trust and acceptance has emerged as a critical challenge. Essential functions such as motion planning and decision-making for safe automated driving must be implemented in ways that users can trust and accept without anxiety across various traffic environments [51, 70]. Research indicates that individuals who experience dangerous situations during automated driving show decreased trust, suggesting that high acceptance cannot be expected until safe automated driving is guaranteed [71]. Additionally, studies show that higher perceived risk by users can diminish trust in the technology regardless of system performance [72], highlighting the importance of eliminating unstable elements in automated-driving technology. The main implications of this study are as follows. First, this study identified the relationship between the emotional and physiological responses of users in an unstable automated-driving situation. Human-centered automated-driving research has focused on evaluating the reliability of automated-driving behavior in various traffic situations [70, 71, 72]. In addition, studies have been conducted to evaluate how much anxiety or risk users feel during automated driving [28, 29, 30, 49, 50]. Previous studies evaluated users' responses to dangerous behaviors such as sudden braking or sharp turns of automated-driving vehicles, and physiological changes, such as increased heart rate and dilated pupils, were observed in unrecognized dangerous situations. This study is significant in that it analyzed the moments when frustration and startle response felt by users in automated-driving situations are maximized. Specifically, the frustration reported by participants was correlated with increased EMG (leg) activity, and the startle response in sudden braking situations was related to pupil dilation. The physiological responses of the participants showed greater changes in sudden vehicle-braking situations than in continuous frustration. This is consistent with previous research findings [29] showing a decrease in heart rate and dilated pupils in sudden situations, suggesting that the frustration and surprise reactions experienced in uncontrollable situations can cause physiological changes. However, there are limitations in interpreting these physiological responses as simple cause-and-effect relationships. The highway merge-section case well illustrates this complexity. While participants reported high levels of frustration in this section, their physiological responses showed unexpected patterns. Specifically, EMG measurements showed a greater increase in arm activity rather than leg activity when driving in the acceleration lane, similar to startle responses at crosswalks. This suggests that the emotions experienced by participants in merge sections may have been more complex than simple frustration, emphasizing the need for in-depth research on users' physical and psychological responses in various automated-driving situations. Second, individual preferences should be considered when evaluating automated-driving behaviors in different traffic situations. Just as preferences for automated-driving vehicles differ by gender, age, education, and income [73, 74], preferences for automated-driving styles also show significant individual differences [51]. Although this study failed to present significant statistical analysis results due to the small sample size, some differences according to gender and age group could be identified in the analyzed trends. In terms of frustration evaluation, the electromyography (EMG) changes were greater in women (difference between Seg. U-1 and U-3: 0.164) than in men (difference between Seg. U-1 and U-3: 0.065) at the uncontrolled intersection, and people in their 40s and 50s (difference between Seg.