## 3 Proposed Methodology
### Establishment of Evaluation Indicator Model  
The optimization and evaluation of the interface design scheme is a complex, multi-indicator decision-making problem and the selection of the evaluation indicators has the characteristics of integrity and non-overlap. Interface design is an integrated system design, including function, layout and human factors as well as aesthetic aspects [33,34]. Taking the design of the in-vehicle HUD interface as an example, field research, online questionnaire surveys and user interviews are conducted to explore the factors affecting the design points of HUD interface [35,36]. In-vehicle HUD should first meet the basic functional requirements of information display and navigation and then increase the auxiliary functions of non-driving related tasks (NDRT) such as entertainment systems to improve the additional value of the product. When the driver interacts with HUD, the interface needs to be clearly visible and easy to read. Therefore, readability is also an evaluation indicator that cannot be ignored [37,38]. The ultimate goal of the design is to provide a better user experience. Aesthetic considerations in the design process can improve the user experience. Aesthetics can be analyzed by considering the color application, symbol design and interface layout.  
Through the multi-round Delphi method [39], the collected data and evaluation indicator factors are analyzed and discussed. Combined with the PSSUQ (Post-Study System Usability Questionnaire) [40], three evaluation indicators of the level 1 criteria layer and 15 evaluation indicators of the level 2 criteria layer for the evaluation of in-vehicle HUD interface design schemes are ultimately obtained. The level-1 layer has three criteria: usability, information quality and interface quality. It has the requirements of independence, non-overlap and integrity. At the same time, it has a certain degree of progression in the experience level. A\({}_{1}\)-A\({}_{4}\) represents the level-2 criterion layer of availability; B\({}_{1}\)-B\({}_{4}\) represents the level-2 criterion layer of information quality; C\({}_{1}\)-C\({}_{4}\) represents the level-2 criterion layer of interface quality, as shown in Table 1.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Layer 1** & **Layer 2** \\ \hline \multirow{8}{*}{Usability} & A\({}_{1}\): Generally, I think this interface is easy to operate. \\  & A\({}_{2}\): The interface is useful for completing my task. \\  & A\({}_{3}\): I can use this product to complete tasks quickly. \\  & A\({}_{4}\): Good display effect (appropriate brightness, no ghost shadow and no dizziness). \\  & A\({}_{5}\): This interface is easy to learn how to use. \\  & A\({}_{6}\): The error prompt on the interface can guide me on how to solve the problem. \\  & A\({}_{7}\): When the operation is wrong, I can quickly and simply start over. \\ \hline \multirow{4}{*}{Information quality} & B\({}_{1}\): The interface provides clear information (such as help and tips). \\  & B\({}_{2}\): The information provided is easy to understand. \\  & B\({}_{3}\): This information is useful to complete the task. \\  & B\({}_{4}\): The information organization structure on the interface is clear. \\ \hline \multirow{4}{*}{Interface quality} & C\({}_{1}\): The information of the interface will not cause \\  & visual obstruction. \\ \cline{1-1}  & C\({}_{2}\): The interface makes me feel happy and comfortable. \\ \cline{1-1}  & C\({}_{3}\): I like the interface of this product. \\ \cline{1-1}  & C\({}_{4}\): The product has all the functions expected. \\ \hline \hline \end{tabular}
\end{table}
Table 1: In-vehicle HUD interface design evaluation indicator system.