What happens when drivers of automated vehicles take over control in critical brake situations?

Even with automated vehicles, driving situations with short time headways and extreme vehicle dynamics may arise when unpredictable events occur. If drivers take back control under such conditions, it is uncertain how they behave and how well they can cope with the situation. This issue has not been investigated yet and is subject to our study. In a driving simulator, non-distracted participants (N = 42) experienced nine critical situations caused by a braking vehicle in front of them. Time headway and longitudinal vehicle dynamics were varied to create different degrees of objective criticality. Participants' criticality ratings, take-over behavior, and driving performance were recorded and analyzed. The results indicate that participants were sensitive to changes in objective criticality and adapted their behavior. Take-over times were very fast under all conditions and participants showed higher criticality ratings, more intense decelerations, and more lane changes with increasing objective criticality. To avoid a collision, participants decelerated much more than the automation and changed lanes, even though this was not necessary. Thereby, they raised the risk of vehicle instability, rear-end collisions, and collisions with overtaking vehicles. To conclude, take-overs in critical brake situations may be a threat to the safety of drivers and other road users because drivers' reactions are more pronounced than necessary. These results suggest that assistive functions are required to support drivers in critical take-over situations.