## 2 Materials and Methods
### System of Structuring Scenarios  
In order to highlight similarities among scenarios, a hierarchical structure with three levels is proposed. It consists, from top to bottom, of use case clusters (UCCs), use cases (UCs), and scenarios. The terminology for these terms is based on [PERSON] et al. [26] and [PERSON] et al. [27]. It is illustrated in Figure 5 and will be defined in the following paragraphs. A _scene_ describes a snapshot of the environment. It includes a scenery (e.g., lane networks, stationary elements, and environmental conditions), dynamic elements (e.g., dynamic objects' states and attributes), self-representations of actors, and observers (e.g., actors' and observers' states and attributes, skills, and abilities) as well as the relationships between those entities. A _scenario_ is defined as a temporal development of different scenes within a sequence of scenes. In order to characterize this temporal development, events and actions, as well as objectives, might be specified. Unlike a scene, a scenario describes a period of time. Scenarios start with an initial scene and can be visualized using interaction diagrams (cf. Figure 6). A _use case_ is a functional description of a technical system and its behavior for a specific use. Use cases can comprise numerous different scenarios, but a scenario can only contain a certain number of scenes arranged in a certain order. A _use case cluster_ comprises similar use cases. Figure 5: Relations between use case cluster (UCC), use case (UC), scenario, and scene. Adapted with permission from Ref. [27]. 2018, interACT. For research on human-machine interaction, the focus on singular static scenes does not suffice to describe processes of interaction. On the other side, the system-based level applied in use cases is too abstract to pay enough attention to these processes. Therefore, the focus of this paper will be on scenarios. Particularly, it will list and categorize various scenarios in a uniform structure to lay the groundwork for a scaffolding of scenarios pertaining to remotely operating AVs. The scenarios presented in this paper were compiled based on interviews and observations in control centers of public transport (see previous section). They are inspired by [PERSON] and [PERSON]'s notion of _Is Scenarios_[28]. These are events or chains of events occurring in a naturalistic setting. They are characterized by a \"narrative, textual description of actions that a certain user applies in order to attend to one or several tasks (_translated by the authors_)\" [28] (p. 71). They describe components of the context of use in interplay with the perspective of the interviewed or observed person. According to [PERSON] and [PERSON] [29], Is Scenarios are the central source for identifying demands and deriving user requirements. Even though tasks may be included in an Is Scenario, they are not in the focus--unlike in _Use Scenarios_, which describe the implementation of tasks by the user. Rather, as Is Scenarios investigate the interrelations between tasks, the relevance of specific resources is elucidated. This, in turn, may facilitate the identification of previously concealed demands. Furthermore, Is Scenarios may help surface the intertwining of various actors. As processes of interaction are vital to understand what is happening in urban mixed traffic settings including remote-controlled HAVs, they are used here to provide a scaffolding on the uppermost level, i.e., the level of use case clusters. On this top level, _actors_ play a significant role. This paper defines \"actor\" in accordance with the United Modeling Language. Thus, an actor \"specifies a role played by a user or any other system that interacts with the subject\" [27] (p. 586). It emphasizes that actors are not limited to human users  
Figure 6: Interaction diagram for the scenario with the cause “Vehicles parked in second row”. The lane is blocked, e.g., due to vehicles parked in second row. This causes delays or disturbs the onward journey of the shuttle.