Exploration of driver stress when resuming control from highly automated driving in an emergency situation

In a highly automated vehicle, the resumption of manual control often takes place in emergency situations automation cannot handle. Time pressure may increase driver stress and lead to risky behaviors. For safety purposes, this study examines the impacts of emergency takeovers on driver stress. The impacts of emergency takeovers on driver stress were explored through heart rate, pupil diameter and perceived stress, gathered from 36 drivers in a driving simulator. Three degrees of emergency were used: non–, moderate, and extreme emergency. The results indicate a pupil dilation, high levels of perceived stress and a long-lasting increase in heart rate when drivers had to resume control in moderate and extreme emergency situations. The results also show a lower perceived stress level and a bi-phasic cardiac pattern—composed of a short-lasting decrease in heart rate prior to long-lasting increase in heart rate—when drivers were warned to regain control (moderate emergency) compared to when they were not warned (extreme emergency). The results (1) suggest that time urgency is a key factor in the induction of driver stress during takeovers, (2) support the existence of a warning-induced preparatory response—characterized by a short-lasting decrease in heart rate—and previously observed in research on defensive behaviors, and (3) support the hypothesis that the preparatory response is a means of reducing driver stress. Overall, the results provide valuable information about cognitive preparation for emergency takeovers, and more generally about driver stress in emergency situations.