How Pedestrians like Automated Vehicles to Communicate via Light-Based eHMIs

Automated vehicles (AV) will increasingly find their way into road traffic. To ensure the trust of vulnerable road users in particular, external Human-Machine-Interfaces (eHMI) solutions are recommended. A focus group study was conducted to better understand the needs of vulnerable road users regarding communication by fully AVs. A VW T6 Multivan with a Communication Matrix (CMX) unit integrated into the front of car as an eHMI was used as a test vehicle. The communication behaviour of the AV was manipulated when a pedestrian indicated an intention to cross. Based on the traffic volume (high or low), the vehicle decided to allow the pedestrian to cross the road or not and showed a corresponding message on the eHMI. Four visualizations were compared: no visualization, a horizontal line represented an automation status bar, an animation with deceleration, stopping, and acceleration, and the combination of automation status bar and animation. 37 participants observed each combination of traffic volume and eHMI visualization in groups. The participants rated the visualizations on a questionnaire, assessing user experience and comprehensibility. After all trials had been completed, semi-structured group discussions were held. It was discussed whether there were differences in the preferred eHMI communication between the scenarios and whether the eHMI messages were perceived as I-versus-you messages. The results show a difference in eHMI preference between the two scenarios: while there is no clear preference in the case of high traffic volume (vehicle does not stop), there is a preference for the animation presentation in the case of low traffic volume (vehicle stops). The participants justified this with the request to be actively noticed by the vehicle when the vehicle granted them permission to cross the road. The study thus provided valuable information on the design of eHMI systems for automated vehicles.