# Physiological Evaluation of User Experience in Unstable Automated Driving: A Comparative Study
###### Abstract  
While automated-driving technology is advancing rapidly, human-centered research is still in its early stages. Research on negative user responses to automated driving is particularly limited in complex roadway environments such as roundabouts, where driving decisions typically depend on driver judgment and traffic conditions. In these environments, automated-driving vehicles may exhibit unstable behaviors, such as sudden stops or forced intersection entries. Since successful interaction between users and automated systems is critical for widespread adoption, understanding when and how automated driving negatively affects users is essential. This study investigated user psychological responses and corresponding physiological changes during unstable automated-driving situations. Using a virtual environment driving simulator, we compared two scenarios: sensor-only automated driving (A.D(S)), which exhibited unstable driving patterns; and cooperative automated driving (A.D(C)), which achieved more stable performance through infrastructure communication. We analyzed the responses of 30 participants using electromyography (EMG) measurements and pupil diameter tracking, supplemented by qualitative evaluations. Results showed that A.D(S) participants experienced higher levels of frustration during prolonged waiting times compared to A.D(C) participants. In addition, sudden braking events elicited startle responses characterized by pupil dilation and elevated arm-muscle EMG readings. This research advances our understanding of how automated-driving behaviors affect user experience and emphasizes the importance of human factors in the development of automated-driving technologies. automated driving; user experience; physiological response; driving simulator 2020 ac  
side user experience (UX) research focused on user comfort [8; 9]. Currently, automated-driving technology has not yet reached a level where users feel consistently safe--a major barrier to widespread adoption of these services [5; 10; 11]. Automated-driving vehicles face limitations in maintaining stable driving performance through technology alone. While they can achieve smooth operation on continuous roads and signalized intersections using onboard sensors (LiDAR, radar, and cameras) and HD map technology [12; 13; 14], as these environments follow predictable, standardized traffic rules. However, they struggle to maintain stable performance in more complex environments, like roundabouts and unsignalized intersections, where navigation based solely on automated technology proves challenging [15; 16; 17]. In roundabouts, automated-driving vehicles may experience excessive latency when determining distances between entering vehicles or attempt dangerous entries that risk collisions with vehicles already in the circular lane [15]. This occurs because automated algorithms cannot fully replicate human drivers' intuitive judgment and communication skills. Similar unstable behavior emerges at unsignalized intersections and junctions, where vehicles must assess inter-vehicle distances and make entry decisions based on traffic flow. Further complications arise when objects suddenly emerge from sensor blind spots, triggering emergency braking or risking collisions. These situations can lead to unstable driving behaviors, such as missed entry opportunities or sudden braking, resulting in passenger frustration and startle responses [18; 19; 20; 21; 22; 23]. Unstable automated driving can trigger negative psychological changes, like frustration and shock, leading to measurable physiological responses [24; 25; 26; 27; 28; 29; 30]. During stable automated driving, users show decreased heart rate and reduced visual load [24]. However, stress and anxiety can manifest in increased electromyography (EMG) [25; 27], elevated heart rate [26], and pupil dilation [27]. These unpleasant experiences can erode confidence in and acceptance of automated driving [31].