How to request drivers to prepare for takeovers during automated driving

Automated vehicles, with the exception of those featuring Level 5 full automation, necessitate drivers to take over control during transitions. By splitting a typical control transition into a preparation request and a takeover request, the two-stage takeover procedure aims to enhance driver performance during transitions. However, the effectiveness of this strategy depends on drivers' compliance with the preparation request, especially in the context of level 4 automated driving where drivers can completely disengage from the driving task and need to resume control on planned transitions. In this study, we conducted six simulator-based experiments to investigate the efficacy of six human machine interface (HMI) proposed to support driver monitoring behavior during the preparation period. A total of 120 participants (60 females and 60 males, aged 39.8 +/- 11.3 years old), with 20 participants in each of the six experiments, took part in the study. We measured drivers' takeover performance, monitoring behaviors, and subjective acceptance to assess the effectiveness of the HMI elements. The results indicate that the two-stage takeover procedure, which requests drivers' attention for a brief period, can enhance drivers' monitoring behaviors and their performance during the transition without negatively impacting their subjective acceptance of automated vehicles. Additionally, the findings underscore the importance of tailoring HMI design based on the system's functionality. Specifically, the handson-wheel requirement appears to be effective in promoting drivers' monitoring behavior and takeover performance without additional system requirements. Adapting the two-stage takeover to specific contexts and system capabilities can enhance drivers' performance and acceptance, crucial for the successful integration of automated vehicles onto public roads.