## 5 Discussion and Conclusions  
The fundamental purpose of HMI is to facilitate seamless communication between humans and vehicles, particularly in scenarios where vehicles operate at higher levels of AD technology [75]. Future research on SAV design is expected to focus on understanding users' perceptions, optimizing information organization, and advancing HMI development. Enhancing user acceptance and fostering receptiveness to new technologies is crucial for businesses and researchers seeking to improve HMI design and predict users' reactions [76].  
In this study, we introduce a methodological framework to standardize the assessment of user acceptance of HMIs in SAV. The test protocol encompasses deriving relevant use cases, defining parameters associated with user acceptance, and specifying the influencing factors for each parameter. By implementing this framework, the test protocol can effectively evaluate whether these requirements are met, providing a reliable basis for HMI evaluations and enabling meaningful comparisons among different HMI variants [77]. The standardized test protocol offers a valuable framework for researchers and practitioners and benefits the field. This contribution is poised to lay the groundwork for future interaction design and testing standards within autonomous driving.  
However, as this article remains theoretical, the proposed test protocol has yet to be applied. Additionally, mapping the data collected during experiments to the six criteria influencing user acceptance is challenging. Therefore, the following research phase will involve the practical application of the test protocol with diverse HMI design variations and specifications of autonomous driving systems. The method can be further enhanced through iterative refinement and detailed adjustments based on accumulated experience.  
Furthermore, due to its focus on public transportation, this study suggests that special groups in society, such as pregnant women, elderly individuals, persons with disabilities,  
Figure 14: Comprehensive data collection approach: quantitative and qualitative data perspectives through diverse methodologies [47; 74].  
and other mobility-challenged populations, should receive more attention. Future research efforts should compare the acceptance levels of different user groups towards various HMIs in different circumstances, such as extreme weather conditions, hazardous road sections, and traffic congestion, to evaluate the effectiveness of different HMIs.  
Conceptualization, M.Y. and G.C.; methodology, M.Y. and G.C.; formal analysis, M.Y.; investigation, M.Y.; resources, M.Y.; data curation, M.Y.; writing--original draft preparation, M.Y.; writing--review and editing, L.R. and G.C.; visualization, M.Y.; supervision, L.R. and G.C.; project administration, M.Y. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The current study was conducted at Politecnico di Milano's i.Drive (Interaction of Driver, Road, Infrastructure, Vehicle, and Environment) Laboratory ([[http://www.idrive.polimi.it/](http://www.idrive.polimi.it/)]([http://www.idrive.polimi.it/](http://www.idrive.polimi.it/)), accessed on 1 May 2024).  
The authors declare no conflicts of interest.