Transparent internal human-machine interfaces in highly automated shuttles to support the communication of minimal risk maneuvers to the passengers.

• Transparency information can be used to improve passengers' experience when facing minimal risk maneuvers in highly automated vehicles (HAV). • Transparent design of an internal Human-Machine Interface (iHMI) has positive influence on passengers' subjective understanding of the Automated Driving System (ADS). • Usability is not negatively affected by the additional information hinting that it is perceived as relevant by passengers. • When time pressure is applied to passengers, there is specific informational need, like delay and arrival times. • Results point towards different informational needs that are specific to context and situation indicating potential benefits of adaptable interfaces. In Highly Automated Vehicles (HAVs) without operators on-board, user interaction with the vehicle automation plays an important role for a safe and inclusive use of these services. Especially when Minimal Risk Maneuvers (MRM) are performed by the system, passengers are faced with uncertain situations. A possibility to deepen passenger's understanding and predictability of these systeḿs and reduce their uncertainties is to enhance automation transparency. However, literature shows a lack regarding enhancing system transparency of HAVs during MRMs. Therefore, we investigated the impact of "observability" and "reasoning" as transparency influencing factors. In an online study, participants evaluated multiple internal Human-Machine Interfaces (iHMI) as shuttle passengers. The presented iHMIs varied regarding their level of transparency by giving different information about what the vehicle's "perception" and its "reasoning" is. Results show significant differences in the passengers' understanding between different iHMI variants providing evidence that information regarding the "perception" and "reasoning" of HAVs enhance system transparency. Results of the study may provide first insights into passengers' informational needs when using HAV. They highlight the potential benefits of system transparency when designing interfaces for HMIs of automated vehicles. [ABSTRACT FROM AUTHOR]