## 2 The Method and Materials
### The Refined Search  
As we are interested in the recent research development, the date range for the search is from January 2017 to December 2024. The search resulted in over 189 papers. To identify the emerging research directions in the past 8 years, we refined the search by changing the query string in several ways to obtain published papers related to different issues or sub-categories.  
The items detected in the network visualization by VOSviewer were used as keywords for refining the search. In addition, as integration and pricing are two important issues in MaaS, \"MaaS\" is also combined with \"integration\" or \"pricing\" in the refined search. To categorize the works, we refined the search by the following combinations of keywords:  
1. (\"MaaS\" AND \"multi-modal\");
2. (\"sustainable\" AND \"mobility\" AND \"multi-modal\") OR (\"sustainability\" AND \"mobility\" AND \"multi-modal\");
3. (\"shared mobility\" AND \"multi-modal\") OR (\"ridesharing\" AND \"multi-modal\");
4. (\"bike\" AND \"transport\" AND \"multi-modal\") OR (\"micromobility\" AND \"multi-modal\") OR (\"scooter\" AND \"transport\" AND \"multi-modal\");
5. (\"MaaS\" AND \"integration\") OR (\"MaaS\" AND \"pricing\").  
As the goal of this paper is to find the potential research directions, we narrowed the search scope by setting the publication date range from 1 January 2017 to 31 December 2024. By searching the Web of Science Core Collection with the query string AB = (\"MaaS\" AND \"multi-modal\") for the papers published between 2017 and 2024, we found 18 papers. By searching the Web of Science Core Collection with the query string AB = (\"sustainable\" AND \"mobility\" AND \"multi-modal\") OR AB = (\"sustainability\" AND \"mobility\" AND \"multi-modal\"), we found 31 papers. By searching the Web of Science Core Collection with the query string AB = (\"shared mobility\" AND \"multi-modal\") OR AB = (\"ridesharing\" AND \"multi-modal\"), we found 13 papers. By searching the Web of Science Core Collection with the query string AB = ((\"bike\" AND \"transport\" AND \"multi-modal\") OR (\"micromobility\" AND \"multi-modal\") OR (\"scooter\" AND \"transport\" AND \"multi-modal\")), we found 17 papers. By searching the Web of Science Core Collection with the query string AB = (\"MaaS\" AND \"integration\") OR AB = (\"MaaS\" AND \"pricing\"), we found 96 papers.  
As the papers obtained from the research may not be of interest to multi-modal, content analysis was performed to identify the topics covered in each paper. Through content analysis, we identified several topics, including adoption factors, modeling, simulation and planning, impact, implementation, integration and performance. Table 3 summarizes the topic of each paper in this review. Note that Table 3 only summarizes the topic for each paper. The details regarding the research questions, results/contributions and gaps of the reviewed papers will be presented in the tables of Sections 3-7. In-depth discussions related to the emerging research directions are included in Section 8.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Topic** & **Papers** \\ \hline \multirow{4}{*}{Adoption factors} & [46] (2021), [47] (2022), [48] (2023), [49] (2020), [50] (2022), [51] (2021), [52] \\  & (2020), [53] (2017), [54] (2019), [55] (2020), [56] (2020), [57] (2019), [58] (2023), [59] (2024), [60] (2017), [61] (201