## 5 Discussion of Findings and Recommendations for Future AD Studies
### Constructs  
First, it turned out that some studies included parameters whose focus of research was not clearly defined. Despite setting up a large number of constructs during the expert workshop and adding further options during the review process, there still remained a considerable number of of parameters that could not be assigned free of doubt. This resulted in the two constructs _general attitude_ and _interaction behavior_ (see Table 1). In these instances, information was rather scarce and only high level indications about interface evaluation were provided by the authors. We therefore encourage researchers to first clearly state objectives and classify these within a specific body of research. This should allow both authors and readers to eventually compare the obtained results with existing findings, and derive implications of the reported work.  
\(\Rightarrow\) REC 6: Clearly communicate which constructs are addressed by an experiment to foster transparency  
The majority of parameters were well classifiable within our database. The safety construct constituted the largest part of all measures (see Table 1).This shows the importance of safety concerns when it comes to investigating driving automation technology. Investigations on TOR performance during system failures have been the first scenarios in human-automation interaction research [12; 13; 14; 145]. Here, many issues such as trust [18; 21; 97], controllability [12], fatigue [15], or mode awareness [17] have been discovered. Right now, it seems like a re-orientation of recent research is taking place which is also reflected in the remaining constructs of the database. Trust [146] and intention to use [35] constitute precursors of actual system use, and, due to the progress in time, technology, and scientific evidence, research has more closely investigated these constructs. Here, the aforementioned open research directions of long-term studies, absolute validity concerns in realistic driving and agreeableness of attitudinal and behavioral measures apply.  
Besides safety and acceptance, there remain other constructs that have rather been neglected until now. The reason why research has given less attention to usability, UX, or productivity until now might be that these are rather precursors of safety and acceptance. Another reason might be that the scenario where automation fails and humans need to step into action was an important issue to determine feasibility of driving automation in the first place. We argue that more emphasis should be paid to other types of interaction such as ongoing automation, user-initiated or planned transfers of control, as most likely, these use cases will occur more frequently than automation failures [59; 147]. From that, the need to investigate efficient and effective interaction arises [29]. Furthermore, it might not be sufficient to develop interfaces that users are satisfied with [34], but which they have fun and enjoy interacting with [32; 148]. Moreover, safety critical issues of AD, like TORs in SAE L3, also impact users overall driving experience [149]. Hence, an additional focus on UX and research on users' emotions and need fulfillment can pave the way to developing not only proper but great HMIs for AVs, which increase individual but also societal acceptance of this emerging technology.  
We also emphasize at this point that the respective research question largely impacts the investigated constructs. The existing large number of findings on safety might benefit researchers in the formulation of research questions since statements on future research and unanswered questions are an inherent part of a researcher's work. At the same time, this implies that there is a lot of room for research questions beyond established construct.  
\(\Rightarrow\) REC 7: Depending on the particular research question, we suggest going beyond established constructs like safety, trust, acceptance, or workload and regard topics from different perspectives.