Road User Response to Automated Driving Mode External Human-Machine Interfaces: Driver, Cyclist, and Pedestrian Perspectives

External human-machine interfaces (eHMIs) allow automated driving systems (ADS) to communicate important information to other road users. Research on the recommended eHMI design for indicating ADS mode via cyan-colored lamps (SAE-J3134) is limited. Furthermore, drivers and cyclists have generally been underrepresented in eHMI research. We assessed the recommended eHMI for comprehensibility and impact on intended forward-walking/cycling/driving behavior. An online, simulation-based experiment was conducted with 48 participants in Canada. The participants watched conflicting-path scenarios selected from a list of ISO-developed use cases. Three experimental conditions simulated interacting with an approaching vehicle driven by a human driver (no eHMI), ADS with a human driver (eHMI-D), and ADS without a human driver (eHMI-ND). The actions were recorded via keyboard inputs. Surveys and interviews measured perceptions of the eHMI design and traffic scenarios, and general opinions on ADS. The decision to proceed along the conflicting path was influenced by right-of-way, regardless of the ADS status. Higher comfort and trust, along with a lower risk, were associated with pedestrians’ and cyclists’ having the right-of-way, but not with the ADS status. As drivers, participants trusted a human-driven vehicle (no eHMI) more than eHMI-ND. Participants generally supported the use of eHMIs and held favorable opinions toward the proposed design and ADS in general. Concerns were raised about confusion with existing vehicle lights, visibility in mixed environmental conditions, and initial comprehensibility and intuitiveness of eHMIs. Assessing the recommended eHMI design in higher-realism environments and for cross-cultural effectiveness warrants further research. Public campaigns might improve eHMI comprehension.