## 8 Discussion
### Opportunities and Challenges  
Several opportunities, challenges, and research gaps identified in this review are discussed below. In Table 15, the total number of papers related to the research topic \"modelling, simulation and planning\" is greatest. Transport system models play a pivotal role in the development of MaaS with multi-modal transport as they provide the capabilities to study and optimize MaaS in real urban areas. In the literature, several modelling frame \begin{table}
\begin{tabular}{c c} \hline \hline
**Topic** & **No. of Publications** \\ \hline Modeling, simulation and planning & 19 \\ \hline Adoption factors & 19 \\ \hline Integration & 15 \\ \hline Impact & 13 \\ \hline Implementation & 10 \\ \hline Performance & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 16: Number of papers by research areas. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Issue** & **No. of Papers** & **Topic** \\ \hline \multirow{6}{*}{2} & (i) Factors influencing adoption of sustainable & 4 & Adoption factors \\  & mobility behaviors & \\ \cline{2-3}  & (ii) Approaches to promoting sustainable mobility & 3 & Adoption factors \\ \cline{2-3}  & (iii) Factors with impact on transport modes & 3 & Impact \\  & and sustainability & 5 & Modeling, simulation \\ \cline{2-3}  & (iv) Modeling, simulation and planning in multi-modal & 5 & and planning \\ \cline{2-3}  & (v) Urban policy design and implementation methods & 2 & Implementation \\  & for sustainable urban mobility & 2 & Performance \\ \cline{2-3}  & (vi) Performance evaluation in multi-modal & 2 & Performance \\  & transport systems & & \\ \hline \multirow{6}{*}{3} & (i) Modeling, simulation and planning in multi-modal & 6 & Modeling, simulation \\  & transport networks & & and planning \\ \cline{2-3}  & (ii) Leveraging the benefits of shared mobility to & 2 & Implementation \\ \cline{2-3}  & enhance multi-modal transport & 2 & Impact \\ \cline{2-3}  & (iii) Studying the relation between different transport & & \\ \cline{2-3}  & modes and the impacts of transport modes & 2 & Impact \\ \hline \multirow{6}{*}{4} & (i) Impacts and substitution effects of micromobility & 5 & Impact \\ \cline{2-3}  & (ii) Characteristics and adoption factors of & 6 & Adoption factors \\ \cline{2-3}  & micromobility in multi-modal transport networks & & \\ \hline \multirow{6}{*}{5} & (i) Methods to orchestrate collaboration for alliance & 2 & Implementation \\  & formation of MaaS & 4 & Modeling, simulation \\ \cline{2-3}  & (ii) Generic simulation methods for MaaS & 4 & and planning \\ \cline{1-1} \cline{2-3}  & (iii) Systematic approaches to achieving different levels & 6 & Integration \\ \cline{1-1} \cline{2-3}  & (iv) Effective approaches to bundling and pricing & 9 & Integration \\ \cline{1-1} \cline{2-3}  & transportation services for MaaS & & \\ \hline \hline \end{tabular}
\end{table} 
Table 15: _Cont._works for the design of sustainable transport services have been proposed. For example, a modelling framework called Sustainable MaaS for achieving sustainability objectives and goals has been proposed in [134]. A hypothesis on the evolution of MaaS divides the evolution of MaaS into several stages, N-MaaS (No MaaS), MaaS 1.0, MaaS 2.0 and MaaS 3.0 [134]. N-MaaS refers to separate services without integration. MaaS 1.0 refers to integrated transport services with ICT based platform for MaaS and user acceptance. MaaS 2.0 includes the features of MaaS 1.0 and transport system models to design and manage the system with the DSS platform. MaaS 3.0 includes the features of MaaS 2.0 and takes into account the sustainability objectives, targets and goals with Space Economic Transport Interaction (SETI) models [135] and Environmental Impact Functions (EIFs) [136], where the SETI model consists of a Spatial economic macro-model and a transport macro-model to capture the interaction between spatial economic and transport.