Countdown featured multi-modal take-over request signal for drivers’ strategic preparation in automated driving mode change

In conditional automated driving, drivers may be required to start manual driving from automated driving mode after a take-over request (TOR), which is similar to the concept of adaptive automation in general human-in-loop automation. The mode change may cause human operator’s performance deficits after the transition. The objective of the study was to investigate different TOR designs to ameliorate the deficits by forcing drivers to engage in manual driving more effectively. An experiment using a driving simulator was conducted to assess five prototypes of TOR features, including four features using countdowns consisting of combinations of different modalities and codes. Fifteen drivers participated in the experiment, and reaction time for mode transition, preference on the TOR features, and situation awareness (SA) during the transition was collected for performance measures. Results revealed that the non-verbal sound cue (beep) yielded a shorter reaction time, while participants subjectively preferred verbal sound cues (speech). Drivers’ SA was not different for TOR features, but the level of SA was affected by different aspects of SA during the adaptive automation mode changes (take over). The results may provide insights into designing multimodal TOR signals and drivers’ behavior during take-over tasks.