An Evaluation of External Human-Machine Interfaces and Compliance with Federal Motor Vehicle Safety Standard 108

For Automated Vehicles (AVs) to be successful, they must integrate into society in a way that makes everyone confident in how AVs work to serve people and their communities. This integration requires that AVs communicate effectively, not only with other vehicles, but with all road users, including pedestrians and cyclists. One proposed method of AV communication is through an external human-machine interface (eHMI). While many studies have evaluated eHMI solutions, few have considered their compliance with relevant Federal Motor Vehicle Safety Standards (FMVSS) and their scalability. This study evaluated the effectiveness of a lightbar eHMI to communicate AV intent by measuring user comprehension of the eHMI and its impact on pedestrians’ trust and acceptance of AVs. In a virtual reality scene, 33 participants experienced one of three eHMI conditions (no lightbar, FMVSS-compliant lightbar, non-compliant lightbar) of an AV that communicated its intent when navigating a busy intersection. On average, it required two exposures for participants to correctly describe the lightbar’s purpose. Overall, participants found the lightbar useful and satisfying, and the majority would expect and prefer that AVs be equipped with a lightbar. The FMVSS-compliant lightbar had a positive effect on people's trust in self-driving vehicles, with participants reporting the lightbar provided clearer actions of what an AV would do and was more predictable compared to the non-compliant lightbar. This study demonstrates that an FMVSS-compliant lightbar is an effective and scalable solution to the issue of AV-pedestrian communication.