### User-Centered Evaluation  
As considering the needs of human users is a crucial requirement in user-centered HMI design [11], this study aims to examine HMIs that are deployed in highly automated shuttle vehicles in light of these user needs. All of these vehicles have in common that they were operated in the context of living labs on public roads. The authors' goal is to understand how on-board operator HMIs are evaluated by their actual users, i.e., the operators that accompany the shuttle vehicles throughout the operational phase of the living labs. We conducted this study with an awareness that any evaluation heavily depends on the concrete tasks the on-board operators are required to execute, as well as the design of their respective HMIs. Aiming for high ecological validity, the interactions between operator and HMI were examined in the concrete context of use, i.e., on the road while transporting passengers, rather than a standardized lab study. As the interviewed operators typically interacted with HMIs on a frequent basis for an extended period of time (see the Methods section), flaws in the system that surface only after extensive system use were more likely to become evident in this analysis.  
This evaluation focuses on the following factors:  
1. _Work Context_: What are the circumstances that the on-board operators are exposed to during their shifts on the shuttles? The focus is on ergonomic aspects of the operators' workspaces, physical demands imposed upon them, the technical equipment they work with, and the way their work is organized.
2. _Acceptance_: Do the on-board operators approve of the workspaces, particularly the HMIs, they work with? Acceptance is closely tied to usability and user satisfaction [12].
3. _System Transparency_: Is the state of the system, in this case, the highly automated shuttle, immediately clear and accessible to the operators? As the automation is responsible for the execution of the driving task by default and the operator is asked to intervene occasionally only, it is crucial for the operator to be aware of the system state at any given time. When the operator is required to intervene, the reason for this need, as well as the way of interacting with the system, is supposed to be disclosed.
4. _Trust_: Do the operators trust the reliability and functionalities of their workspaces on board the shuttles, particularly the HMIs? Trust is closely tied to subjective safety and may originate in good usability, reliability, and user satisfaction [13; 14].
5. _Suitability of HMI Elements for Tasks_: Are the HMIs the on-board operators use for fulfilling their tasks on the shuttle actually suitable for these tasks? As tasks vary between living labs, so do HMI elements (for detailed descriptions of the respective HMI setups, see the Methods section).  
Across the heterogeneity of the examined living labs (shuttles and environments) and the related operator tasks, the aim is to distill \"best practices\" of on-board operator HMI design that should be continued in future HMI designs. In addition, issues regarding the currently used HMIs will be highlighted from the perspective of on-board operators that frequently interact with these technologies, ensuring ecological validity.