Visual or Auditory First? The Modality Shift Effect in Two-Stage Takeover Warning

Two-stage warning systems play a crucial role in ensuring safe and stable takeovers in conditional automated driving when drivers are engaged in non-driving-related tasks (NDRT). This study aims to investigate the effectiveness of visual and auditory warning modalities, particularly when NDRT have overlapping modality resources with the first-stage takeover warning. Two simulated driving experiments were conducted in a driving simulator, and we discovered that visual warnings were significantly more affected by the NDRT modality overlap than auditory warnings. The findings highlighted that modality overlap between takeover warnings and NDRT can negatively affect takeover stability. These results also indicate that it is imperative to take into account the modality shift effect and its first-stage modality while designing two-stage warning systems. Our results suggest that using auditory warnings in the first stage leads to a more stable takeover performance than visual warnings. We found that the modality of the initial takeover warning significantly influences drivers’ attention allocation and alertness. These insights provide empirical support for the industrial design of two-stage takeover warning systems, paving the way for more diversified and effective automobile intelligent cockpit designs.