## 6 Conclusions  
In this study, we examined the usability of a domain-specific modeling language (DSML) called DSML4 PT, designed for model-driven engineering (MDE) of IoT-based public transportation systems. After introducing the syntax and semantics of the DSML4 PT modelling language, we demonstrated its application within an MDE methodology. The language and its integrated development environment (IDE) were assessed both quantitatively and qualitatively through a systematic evaluation. This evaluation involved the use of the language in the design and implementation of eight different real public transportation applications. The evaluation was carried out with the voluntary participation of ten software developers with varying knowledge and experience in the development of public transportation systems. A comparative analysis based on the multi-case studies revealed that around 80% of IoT-based public transportation systems could be automatically generated using DSML4 PT exclusively through modeling. Compared to the traditional software development methodologies, this novel approach with DSML4 PT also reduced the time required for the development of public transportation applications by nearly 50%. Moreover, the overall evaluation rating of the language, as assessed through a questionnaire-based assessment, was measured at 4.44 on a 5-point Likert scale. Developers who participated in the evaluation studies also provided valuable feedback on DSML4 PT. In particular, evaluators confirmed that the language's strengths lie in integrability, maintainability, and expressiveness. Many highlighted its practicality and its applicability across various perspectives. The DSML4 PT language, its IDE, and data and documents from the evaluation results discussed in this paper are all publicly available in [98]. Based on the current structure of DSML4 PT and its IDE, the technology readiness level (TRL) of these products is estimated to be between TRL 4 and TRL 5. This assessment is due to the validation of the usability through a systematic evaluation, where functionality was demonstrated using various applications from the public transportation industry.  
In our future work, we plan to improve the DSML4 PT language constructs and features by taking into account the suggestions of the evaluators. For instance, we will investigate alternatives for developing a web interface to utilize the modelling and code generation features of the language, as suggested by some of the participants. Another area of focus will be refining the graphical syntax currently provided for modelling both IoT and public transportation components within DSML4 PT IDE. An additional evaluation can be performed by the participation of DSML4 PT users to assess the adoption of the current visual representation of the modelling elements. If necessary, we can improve both the visual expressiveness and the correspondence between graphical symbols and the semantic constructs by revisiting the well-known Physics of Notations [104] principles.  
Conceptualization, S.A. and G.K.; methodology, S.A. and G.K.; software, S.A.; validation, S.A., G.K. and H.A.; investigation, S.A.; writing--original draft preparation, S.A.; writing--review and editing, G.K. and H.A.; visualization, S.A.; supervision, G.K.; funding acquisition, H.A. All authors have read and agreed to the published version of the manuscript.  
The research was partially funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R411), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.  
The study was conducted in accordance with the Declaration of Helsinki, and approved by the Scientific Research and Publication Ethics Committee (EGEBAYEK) of EGE UNIVERSITY (protocol code: 1604 approved on 31 August 2022).  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The authors declare no conflicts of interest.