Despite their significant potential to transform urban mobility, passengers still harbor safety concerns regarding robotaxis. Effective Human-Machine Interfaces (HMIs) may enhance users’ safety perceptions of these systems. Yet, strategies to improve such perceptions are not well-explored. To elucidate how system differences influence passenger perceptions, we conducted a field study involving a comparative analysis of two commercially operated robotaxi systems, System A and System B. Subsequently, we gathered data from passengers of both systems to assess their perceptions of safety. The results indicated that under normal operating conditions, passengers using both systems reported similar levels of safety. However, in the event of incidents, perceived safety significantly declined among passengers who used System B, whereas it remained stable for passengers using System A which provides more explanation on the nature of the incidents. These findings offer crucial insights for operators and provide a valuable resource for future transportation research.