## 8 Discussion
### Analysis of Research Topics and Research Issues  
To review the papers efficiently and systematically, we identified the research topics and research issues. Figure 9 shows the research topics. There are connections between different topics. For example, studies of adoption factors might require modeling and/or simulation on one hand. Therefore, there is a connection between the topic \"adoption factors\" and the topic \"modeling, simulation and planning\". One the other hand, studies of modeling, simulation and planning typically relies on the knowledge of adoption factors to develop the simulation models or planning software. Another example, studies of integration might require modeling and/or simulation. Knowledge of integration is required to construct modeling, simulation and planning tools. Figure 9 shows the connections between the topics of the literature in Table 3. Tables 9-14 show the related research issues for each topic. In each of these figures, the research issues are listed in the left column while the related papers are listed in the right column. \begin{table}
\begin{tabular}{|l|l|} \hline \multicolumn{2}{|c|}{**Modeling, Simulation and Planning**} \\ \hline
**Advanced frameworks/models to simulate/optimize the performance of MaaS** & [65; 66; 67; 68] \\
**Modeling, simulation and planning in multi-modal transport networks** & [69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79] \\ \hline
**Generic simulation methods for MaaS** & [80; 81; 82; 83] \\ \hline \end{tabular}
\end{table} 
Table 10: Research issues related to the topic “adoption factors”. \begin{table}
\begin{tabular}{|l|l|} \hline \multicolumn{2}{|c|}{**Integration**} \\ \hline
**Systematic approaches to achieving different levels of integration** & [109; 110; 111; 112; 113; 114] \\ \hline
**Effective approaches to bundling and pricing transportation services for MaaS** & [115; 116; 117; 118; 119; 120; 121; 122; 123] \\ \hline \end{tabular}
\end{table} 
Table 11: Research issues related to the topic “integration”. \begin{table}
\begin{tabular}{|l|l|} \hline \multicolumn{2}{|c|}{**Impact**} \\ \hline
**Analysis of the potential impact caused by MaaS** & [84; 85; 86] \\ \hline
**Factors with impact on transport modes and sustainability** & [87; 88; 89] \\ \hline
**Studying the relation between different transport modes and the impacts of transport** & [90; 91] \\
**Impacts and substitution effects of micromobility** & [92; 93; 94; 95; 96] \\ \hline \end{tabular}
\end{table} 
Table 13: Research issues related to the topic “implementation”. Figure 9: The connections between topics. \begin{table}
\begin{tabular}{|l|l|} \hline \multicolumn{2}{|c|}{**Modeling, Simulation and Planning**} \\ \hline
**Aducanced frameworks/models to simulate/optimize the performance of MaaS** & [65; 66; 67; 68] \\ \hline
**Modeling, simulation and planning in multi-modal transport networks** & [69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79] \\ \hline
**Generic simulation methods for MaaS** & [80; 81; 82; 83] \\ \hline \end{tabular}
\end{table} 
Table 9: Research issues related to the topic “modeling, simulation and planning”. Through content analysis, several research issues and topics were identified in each category. These research issues and topics are summarized in Table 15. Note that there are 19 papers related to \"modeling, simulation and planning in multi-modal transport networks\" in research issue (iv) of Category 1, research issue (iv) of Category 2, research issue (i) of Category 3 and research issue (ii) of Category 5. Therefore, the research topic \"modeling, simulation and planning in multi-modal transport networks\" rank highest in terms of the numbers of papers. Note that there are also 19 papers related to \"adoption factors in research issue (i) of Category 1, research issue (i) and research issue (ii) of Category 2 and research issue (ii) of Category 4. Therefore, the research topic \"adoption factors\" also rank highest in terms of the numbers of papers. There are 15 papers related to \"integration\" in research issue (iii) and issue (iv) of Category 5.