## 3 Materials and Methods
### Definition of Relevant Use Cases  
HMI [54] encompasses the devices that facilitate interaction between individuals and vehicles to accomplish seamless human-computer collaboration in AD scenarios [55]. As [PERSON] (2009) has noted, HMI is a communication channel that enables interaction between human users and AVs [53], divided into internal HMI (iHMI) and external HMI (eHMI). Previous studies have predominantly concentrated on the interactions between vehicles and pedestrians at crosswalks in urban settings, particularly at low speeds, examining eHMI aspects and the transition between AD systems and human drivers, focusing on iHMI perspectives. Nonetheless, this research scope is limited and does not encompass the full range of potential HMI applications. To assess user acceptance of an HMI in a standardized manner, study participants must encounter the HMI within a diverse set of relevant use cases. Consequently, defining pertinent use cases is pivotal in the evaluation process, ensuring that the testing protocol yields comparable and meaningful outcomes. This research employed a systematic process to address a comprehensive range of use cases for an HMI. Figure 4 provides a visual representation of it. Specifically, the procedure includes collecting and consolidating use cases and their specifications, refining them iteratively by eliminating redundancies, and considering various factors. Figure 4: Overview of the different approaches to select relevant use cases of a human–machine interface (HMI) and their categorization. 3.2.1. Defining the Use Case of an eHMI and iHMI  
The foundation of this methodology was the articulation of an HMI use case. According to the different use scenarios, HMI in SAV can be divided into external HMI (eHMI) and internal HMI (iHMI) [56]. A scenario involving an SAV and at least one human road user sharing the same space and time is considered a use case for an eHMI [57]. In this situation, eHMI plays a crucial role in helping human road users comprehend and anticipate the actions of AVs. This assistance allows them to promptly modify their intended maneuvers, for instance, by altering speed or direction. Traffic conflicts arise when the movements of two or more road users intersect, underscoring the importance of eHMI as a necessary interactive medium between road users and AVs to mitigate potential traffic conflicts [58]. In addition, an iHMI use case is defined as a situation in which at least one passenger interacts within the SAV [56]. In this case, the HMI is dedicated to enhancing passenger engagement and usability through demand-driven interactive services. The interactive behavior here means that passengers can understand the intention of the SAV through the interface and acquire as much information as possible about the SAV's driving situation and other information during the ride. Using iHMI as a means of in-vehicle information transmission can enhance the transparency of communication and help users understand the necessity and realizability of remote operation and control transfer [59]. This allows users to draw conclusions about their own interactive behavior from both an eHMI perspective and an iHMI perspective. 3.2.2. Display-Based Approach and Layout-Based Approach  
Those two approaches gather all the possible information that an HMI of an SAV can present and may appear in all the different locations and carriers. An examination of 15 prevalent concept ASBs (refer to Figure 5) was carried out via content analysis of a diverse range of media, showcasing the interactive display capabilities of the HMI. The existing HMI solutions from these case studies are summarized in two aspects:  
* **Information exchange of external HMI:**  
Figure 5: Fifteen selected examples of Autonomous Shuttle Buses (ASBs) for public transportation to deliver humans.