Striving for a balance between safety performance and driver acceptance in auditory takeover requests

Designing an effective auditory takeover request (TOR) is critical for ensuring driving safety in conditionally automated vehicles. Our previous study revealed that earcon warnings as TORs are the most suitable for improving takeover safety performance among various types of auditory TORs, while drivers prefer speech TORs the most. This indicates that using either earcon or speech as TORs alone could not simultaneously ensure both takeover safety and driver acceptance. Since both aspects are important for the successful implementation of takeover warning systems, the present study further explored whether combining earcon and speech warnings as TORs could achieve a balance between the two aspects. We recruited 30 participants and conducted a simulated driving experiment to compare three TOR types (earcon, speech, and earcon + speech) in terms of takeover performance and driver acceptance (usefulness and satisfaction). The potential impact of takeover situation urgency was also considered. The results showed that regardless of the takeover situation urgency, the combined TOR was as effective as the earcon TOR in ensuring takeover safety. Additionally, the combined TOR received higher usefulness ratings than the speech TOR, especially in urgent situations, although it was associated with lower satisfaction ratings. Moreover, the combined TOR had no significant difference from the speech TOR in preference ranking. Overall, the combined TOR (earcon + speech) shows potential for improving the balance between takeover safety and driver acceptance in automated vehicles. The findings of this study have practical implications for researchers and designers when developing auditory takeover warning systems.