| Summary: | Bottleneck roads with narrowed width often only allow one vehicle to pass at once. In this situation, human drivers need to negotiate their right-of-way via, e.g., hand gestures and eye contact. However, when a human-driven vehicle (MV) confronts a driver-less automated vehicle (AV), explicit communication between drivers is no longer possible. External human–machine interfaces (eHMIs) on AVs may facilitate communication in unobscured situations, but MV-drivers can fail to perceive the eHMI information on the AV with other vehicles in front of the AV, blocking the MV’s view. Even if the visibility is not impaired, AV broadcast communications do not target on specific receivers, it is not unlikely that other vehicles may wrongly perceive this information. Instead, an internal human–machine interface (iHMI) can uni-cast the AV intention to MVs since the information on iHMIs is direct to MV-drivers and visible in visibility-blocked situations. However, iHMIs require vehicle-to-vehicle communication technology, and the conveyed information might not be highly trusted as the information is transmitted to MVs rather than being seen directly from AVs. Therefore, this paper proposes a synchronous iHMI+eHMI method for a more unambiguous communication in this multi-vehicle bottleneck road situation. The designed iHMI+eHMI is compared with the baseline i.e., without HMI, iHMI, and eHMI in a video-based driving simulation by subjective evaluations from structured questionnaires. The results (N=24) indicate that HMIs (iHMI, eHMI, and iHMI+eHMI) are more helpful than vehicles without any HMI for the AV-MV communication, and iHMI+eHMI achieves the best performance when the views of MV-drivers are obscured.
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