## 4 Review of Studies Related to Sustainability in the Context of Multi-Modal Mobility
### Potential Future Research Directions  
The above review of the recent papers related to sustainability in the context of multi-modal mobility shows that the research issues or questions include: (i) factors influencing adoption of sustainable mobility behaviors, (ii) approaches to promoting sustainable mobility, (iii) factors with impact on transport modes and sustainability, (iv) modeling, simulation and planning in multi-modal transport networks, (v) urban policy design and implementation methods for sustainable urban mobility and (vi) performance evaluation in multi-modal transport systems. Several potential future research directions based on the review of the papers in the previous subsection are summarized in this subsection. (i) Factors influencing adoption of sustainable mobility behaviors  
The paper [52] explored mobility motives of travelers to find the determinants for travelers to adopt sustainable mobility behaviors. The study [52] was conducted based on a sample of the Swiss population. Transferability of the results requires a further study. Consideration of implicit motives is an interesting extension to the study [52]. The study [53] explored the connection between life course and sustainability mobility behaviors of millennials in Sydney, Melbourne and Canberra. The study suggested policy interventions to support a sustainable lifestyle. However, the study did not cover rural Australia. Whether the results of this study hold true in other countries with different environments and constraints is an interesting research question. Further study may include the life course stages and typologies of mobility in survey research to determine their prevalence in the population. Although the factors that drive the ride-hailing adoption and the associated mode substitution effects of ride-hailing were explored in [56], the study was limited. Use patterns of other household members such as children could be worth exploring further. In addition, safety perceptions and experiences of ride-hailing require further studies. The model of human needs and travel satisfaction have been incorporated in the framework of the paper [55] to study the connection between mode choice and travel satisfaction. There are several future research directions related to the paper [55]: (1) studying the consistency of results across different groups in population with various travel purposes; (2) extending the study for leisure travel with joint travel; (3) studying the connection between mode choice and travel satisfaction for younger generations; (4) further study on per trip satisfaction of commuters (as the study [55] focuses on the overall satisfaction of commuters with their commuting choices overall rather than per trip). (ii) Approaches to promoting sustainable mobility  
The study [54] explored the possibility to develop a smartphone app to enable and encourage users to make sustainable mobility choices, reduce car dependency/emissions energy consumption and conduct a field test based on the app. There are several future research directions stemming from the study [54] for further improvement: (1) extend the application with automatic travel routes and transport modes detection and a multi-modal travel planning system, to support users' daily travel needs; (2) provide goal setting function, feedback, rewards at the individual level and at the community level or punishment and (3) exploit the power of social interactions. Understanding the characteristics and motives of different types of carsharing adopters can help promote sharing based sustainable mobility. The differences between business-to-consumer carsharing adopters and peer-to-peer carsharing adopters in terms of characteristics and motives were studied in [57].