Is Silent eHMI Enough? A Passenger-Centric Study on Effective eHMI for Autonomous Personal Mobility Vehicles in the Field

Autonomous personal mobility vehicle (APMV) is a miniaturized autonomous vehicle that is used in pedestrian-rich environments. In addition, the open bodywork design of APMVs exposes passengers to the communication between the eHMI deployed on APMVs and pedestrians. Therefore, to ensure an optimal passenger experience, eHMI designs for APMVs must consider the potential impact of APMV-pedestrian communications on passengers' psychological feelings. This study discussed three external human-machine interface (eHMI) designs, i.e., eHMI with text message (eHMI-T), eHMI with neutral voice (eHMI-NV) and eHMI with affective voice (eHMI-AV), from the perspective of APMV passengers in the communication between APMV and pedestrians. In the riding experiment, we found that eHMI-T may be less suitable for APMVs. This conclusion was drawn based on passengers' feedback, as they expressed feeling awkward during the ``silent period'' because the eHMI-T conveyed information only to pedestrians but not to passengers. Additionally, the affective voice cues on eHMI improved overall user experience of passengers, leading to higher ratings for both pragmatic and hedonic quality. The study also highlights the necessity of considering passengers' personalities when designing eHMI for APMVs to enhance their experience.