## 7 Review of Studies Related to Integration in the Context of Multi-Modal Mobility
### Potential Future Research Directions  
The above review of the recent papers related to integration and pricing issues in the context of multi-modal mobility indicates that there are several research issues or questions to be studied. These research issues or questions include (i) methods to orchestrate collaboration for alliance formation of MaaS, (ii) generic simulation methods for MaaS, (iii) systematic approaches to achieve different level of integration and (iv) effective approaches to bundling and pricing transportation services for MaaS. Several potential future research directions based on the review of the papers in the previous subsection are summarized in this subsection. (i) Methods to orchestrate collaboration for alliance formation of MaaS  
Although the alliance formation model and the propositions proposed in the study [106] have illustrated the necessary preconditions for designing a governance structure to provide MaaS services, it was only applied in a MaaS pilot in the Netherlands. The proposed conceptual model and resulting propositions should be refined and tested in other MaaS-applications to confirm the validity of the model. Different business models need to be examined rigorously based on game theoretical approaches to identify which actors takes, or should take, the lead in alliance formation of MaaS. The study [105] shows that consumers are willing to pay for collaboration activities as long as it can deliver them better door-to-door travel options across the whole journey. Questions such as (1) How to orchestrate such collaboration? (2) How are the institutional rules created to make consumers understand? (3) How are the institutions themselves to be formed? (4) What competing goals and pressures are internal to the different operators and operator types, and how these can be resolved through network governance and management to allow greater participation? and (5) How to bridge between operators with inconsistent goals? need to be studied further. For example, the requirement for profitability creates tension between publicly owned and privately owned operators and this affects their collaborative activities. Additional collaboration mechanisms such as frequent rider incentives and consideration of group travel rather than individually ticketed travel need to be studied. (ii) Generic simulation methods for MaaS  
Although some of the issues have been addressed in the studies [80; 81; 82; 83] based on simulation approach, there are limitations in these studies. For example, the study [80] lacks technical support if additional components need to be implemented. The methodology was applied to Bristol only. It needs to be validated for large complex scenarios to assess competition mechanism for providing seamless integrated door-to-door journeys. For the paper [81], further studies are required to evaluate more complex scenarios, verify the influence of differences in simulators' resolutions on performance, and introduce shared infrastructures. Although the proposed co-simulation method in [82] enables the assessment of a wide variety of the effects for multiple stakeholders of autonomous mobility-on-demand services such as optimum vehicle fleet size, the vehicles' occupancy, waiting time, detour time for each passenger, congestion, and the environment, transferability of the co-simulation method to other cities need to be validated. There are several limitations in the results of the presented scenario in [83], which serves as a technical demonstrator. The results of the study [83] were based on the same decision parameters for the mode choice of public transport and ridesourcing services. To assess the realistic situation, these parameters must be replaced by data collected from surveys covering the usage of ride-hailing and ride-pooling vehicles to obtain more realistic results concerning demand changes and mode shifts.