## 8 Discussion
### Opportunities and Challenges
Although the research issue \"performance evaluation in multi-modal transport systems\" was less explored, this was due to the lack of theory and advanced tools to support performance evaluation for real multi-modal transport systems. With more MaaS pilot projects being executed, advances in theory and the development of advanced tools for modeling, analyzing, and simulating multi-modal transport systems, the research issue 'performance evaluation in multi-modal transport systems' is likely to be addressed more comprehensively in the future. According to the review in this paper, most studies were conducted for a city or a specific area covering a few cities. Whether the approaches are applicable and the results are transferable to other cities or areas needs to be verified. Effectiveness of an approach to address a specific issue requires a comparative study of applying the approach to different cities or areas. However, a comparative study of applying the same approach to address a specific issue in different cities or areas was less explored in [52; 70; 82; 84; 88; 107]. For example, in [84], the authors studied how MaaS could impact GHG emissions in an urban setting based on activity-based model for Amsterdam, the Netherlands. Another example, paper [70] addressed the issue about how to develop a framework for modelling various facets of AMoD to demonstrate the benefits of the proposed framework based on a case study in Singapore. The paper [52] explored mobility motives of travelers to find the determinants for travelers to adopt sustainable mobility behaviors based on a sample of the Swiss population. The study [107] probed into the problem about how to estimate long-term travel delay in multi-modal systems with different transport modes based on the Shenzhen city, China. The paper [88] aimed to study the impact of strategies on sustainable mobility for a metropolitan region in the Netherlands including Rotterdam and The Hague by employing activity based travel demand models. The proposed co-simulation method in [82] was applied to assess a wide variety of the effects for multiple stakeholders of autonomous mobility-on-demand services for the Royal Borough of Greenwich (London, UK). The gaps regarding the applicability of existing approaches and the transferability of existing results in [52; 70; 82; 84; 88; 107] to other cities or areas need to be filled in the future. Apart from the research issues and research gaps mentioned above, the implementation of multi-modal shared mobility calls for a collaborative partnership between service providers and the government to jointly establish a merit-based business model, improve infrastructure provision, and create smart applications. The requirements of designing environmentally sustainable, economically viable, and socially acceptable urban mobility systems pose challenges in the decision-making processes of cities. The issue is to develop an innovative and collaborative decision framework to support integrated sustainable urban policy design for sustainable urban mobility. Many papers included in this review suggest government intervention or policy implementation to support the development of multi-modal MaaS services. For example, the study [49] indicates that without proper government intervention, the efficiency and equity promises of MaaS are not achievable. The study [98] suggests that government policy makers should innovatively promote shared MaaS through policy implementation. The study [64] provides insight into how governance stakeholders shape sustainability transitions and technology uptake by exploring their perspectives in depth. The study [53] explores the connection between life course and sustainability mobility behaviors of millennials. The study suggests policy interventions to support a sustainable lifestyle. The study [60] recommends the formulation of policies to increase bicycle ownership and improve cycling infrastructure.