## 3 Review of Studies Related to MaaS in the Context of Multi-
### Review of Studies  
The transportation problems in cities with multi-transport modes can be modeled as \"System of Systems\" design problems [124]. A MaaS system is an example of a \"System of Systems\" in which different modes of transport systems work together with the goal to provide seamless end-to-end services to transport users. Due to the existence of multiple transport modes, there are many types of stakeholders involved in the management and operations of the MaaS system. These stakeholders include the transportation planners, service providers of different transport modes, MaaS providers and users or travelers of transport services. The transportation planners are responsible for suggesting the proper regulations to be made and adopted to enable efficient, safe and sustainable transport services and ensure that companies, organizations and individuals abide by them to promote transport market through healthy competition. In cities with multiple heterogeneous transport modes, there are typically multiple single transport mode service providers which provide single transport mode services to users or travelers within their service area. As the itineraries of users or travelers may not be covered in the service area of a single transport mode service provider, multiple heterogeneous transport modes are needed to fulfill the requirements of users or travelers. Without a MaaS system, users or travelers requiring multiple heterogeneous transport modes have to book and pay the tickets through multiple single transport mode service providers one by one. The role of MaaS providers is to improve the efficiency to plan, book and pay the tickets for the multiple heterogeneous transport modes needed according to the itineraries of users or travelers. The role of users or travelers is to plan, book, pay and use the tickets for the required multiple heterogeneous transport modes according to their itineraries by following the rules from the service providers of different transport modes and the transportation planners. Depending on the type of stakeholders mentioned above, the concerns or questions about taking part in the operations of MaaS vary. The concerns or questions of each type of stakeholders can be linked to related research issues or topics for further studies. For example, from the perspective of MaaS providers, capturing the heterogeneous travel needs of users' and the interactions between users' choices in planning and booking tickets for the multiple heterogeneous transport modes is an important issue. Another example, from the perspective of mothers with babies, as strollers are often accompanied with mothers, this may pose a challenge for mothers to access stations of multiple transport modes in MaaS and move from one transport mode to another. From the perspective of the government, transportation planners may want to understand how MaaS could impact GHG emissions in an urban setting and improve sustainability. For single mode transport service providers such as traditional bus operators, they may want to know whether MaaS will change how bus services are offered. This subsection focuses on the review of existing studies in Table 4 related to MaaS in the context of multi-modal mobility. The research questions along with the corresponding study results are summarized as follows. 1. Question: What are the challenges and opportunities of sustainable mobility in post-COVID-19 period? Results: The study [46] is about the challenges and opportunities of sustainable mobility in post-COVID-19 period. The study showed that one in five car-less households intended to purchase a car during COVID-19 period due to the need of physical distancing, convenience, safety, low traffic levels and low parking demand. The authors projected that a regional scale MaaS program may be created due to the widespread available real-time crowding information in mass transit systems and apps provided by transport service providers. 2. Question: Is MaaS able to address the challenges of mothers in multi-modal mobility?