## 3 Review of Studies Related to MaaS in the Context of Multi-
### Potential Future Research Directions  
The above review of the recent papers related to MaaS in the context of multi-modal mobility shows that there are various facets in terms of research issues or questions. These research issues or questions include (i) enabling factors/barriers of modal shift to sustainable transport, (ii) analysis of the potential impact caused by MaaS, (iii) strategies to facilitate the implementation of multi-modal shared mobility, (iv) advanced frameworks/models to simulate/optimize the performance of MaaS and (v) effective approaches to dealing with threats while protecting privacy and allowing data sharing in MaaS. Several potential future research directions based on the review of the papers in the previous subsection are summarized in this subsection. * Enabling factors/barriers of modal shift to sustainable transport The study [46] identified the challenges and opportunities of sustainable mobility in post-COVID-19 period and projected that MaaS might be created due to the widespread available real-time crowding information in mass transit systems and apps. To enable a shift away from cars, the study [46] suggests several future research directions, including multi-modal coordination, flexible choice of a variety of transport modes, congestion pricing and market-rate parking policies. To benefit from MaaS, the study of [47] mentioned that the network of different transport modes (e.g., the public transportation network, the cycling network and the pedestrian network), must be complete in itself. The study [50] attempted to identify principles for sustainable MaaS in the context of unsustainable car dependency. The study also pointed out several application scenarios not explored in the literature: (1) How to deal with the situation of not having access to a car? (2) Is it possible for a multi-modal journey with luggage? (3) Does the service support families with children? (4) Can it provide access for those with mobility impairments? (5) Does it provide service for people without a smartphone? The work [48] attempted to find the answer to the questions: (1) who and why users took part in the trial of a MaaS service in Sydney and (2) whether the trial experience satisfied the users' motives. As the analysis of the study [48] was based on the trial service prototype of a MaaS, further studies considering changes to design of services, target group, or relevant context that may significantly influence the users' experiences are required to generalize the findings to all MaaS operations. (ii) Analysis of the potential impact caused by MaaS  
The study [84] probed into the question: how MaaS could impact GHG emissions in an urban setting? As the study [84] was based on Amsterdam, the Netherlands, a future research direction is to investigate to what extent the conclusions made still hold. The emerging MaaS relies on integration of different transport modes as well as cooperation of transport service providers to improve efficiency, performance and sustainability. Therefore, MaaS would have impact on existing transport modes. The work [85] investigated whether the way bus services might be offered will be changed under MaaS. The study [85] suggested several future research directions: (1) investigating the extent to which individuals would be willing to not own a car, (2) under what conditions individuals owning cars would be willing to offer their cars for use by others, and (3) how much travelers are willing to pay for improving the journey experience. The study [86] focused on the competitiveness analysis of MaaS travel options with respect to private car usage. in a one-origin-one-destination network based on consideration of fixed cost for travel and the inconvenience cost due to multi-modal trips. Incorporation of the factors of transit frequency, capacity of vehicles and hidden costs associated with the comfort level during trips in the analysis requires a further study. Development of a management scheme to optimize the system is a challenging issue.