User-Centered Design of eHMI: Evaluating Message Types and Positions for Motorcyclists and Pedestrians

External human–machine interfaces (eHMIs) are crucial for facilitating effective communication between autonomous vehicles (AVs) and vulnerable road users (VRUs). To design a user-centered eHMI, we identified pedestrians and motorcyclists as target users and specified eHMI usage scenarios accordingly. We aimed to investigate effective eHMI designs (position and message type) for the users in different usage scenarios through interviews and user testing. In the interview phase, 30 participants discussed their preferences for eHMI message types and positions for various scenarios. The user-testing phase involved 63 participants evaluating the visibility, interaction comfort, perceived safety, and workload of different eHMI positions (front windshield, door panel, grille, and rear windshield) and message types (guidance and informational) through virtual reality. Motorcyclists generally preferred informational messages and the rear windshield position, whereas pedestrians preferred the guidance messages and the front windshield and grille positions. These findings concurred with the interview results. Additionally, significant interaction effects were observed between the eHMI position and the usage scenario. In conclusion, motorcyclists and pedestrians exhibited different preferences for eHMI message types+ and positions. These position preferences depend on the usage scenario. This research highlights the importance of considering the target users for eHMIs, their needs, and their usage scenarios, offering valuable insights for the development of advanced eHMI systems.