Hazard warning lights or pictogram on a display? Investigating drivers' mental workload, behavior, and signal preference while overtaking a robotaxi equipped with rear communication systems during passenger drop-off

This study aimed to investigate the impact of different rear external human-machine interfaces (eHMI) signals sent by a robotaxi during passenger drop-off on a following human driver. Twenty-five participants completed four rides on a closed track, including the overtaking of a stopped robotaxi during passenger drop-off. When the participants' car was either 30 m or 60 m from the stationary robotaxi, a passenger exited it. The robotaxi communicated its intention to remain at stop using either a rear external display showing a pictogram (Display condition, 2 trials) or the conventional hazard warning lights (Hazard warning lights condition, 2 trials). After each ride, the participants rated the visibility and use of the eHMI signal, and their mental workload during the overtaking maneuver, using questionnaires. At the end of the experiment, the participants indicated their preferred eHMI signal and participated in semi-structured interviews aimed at assessing their understanding of the eHMI signals. Data were analyzed using linear mixed-effects models comparisons. The results indicated high levels of visibility and use of the eHMI signal in the Display condition, with most participants preferring this signal. Additionally, although the participants reported lower mental workload in the Hazard warning lights condition than in the Display condition, their mental workload remained at moderate intensity in the latter. Furthermore, the analysis of the participants' vehicle behavior showed that the participants had lower approach and overtaking speeds in the Display condition than in the Hazard warning lights condition, suggesting a more cautious driving behavior. These findings suggest that the eHMI signal in the Display condition may serve as a desirable and safety-enhancing means of communication for following human drivers.