## 7 Review of Studies Related to Integration in the Context of Multi-Modal Mobility
### Potential Future Research Directions
Although the papers [110; 113] have characterized different degrees of integration by defining five levels of MaaS integration, how to achieve each level of MaaS integration is not fully addressed in existing studies. For example, the study [109] addressed the issue to integrate freight transport into a Mobility-as-a-Service (MaaS) environment, the paper [111] addressed the issue to fully integrate public transportation, the study [112] presented how a public transport authority developed a MaaS for rural areas by integrating a public transport service with carpooling and the study [114] illustrated how to integrate one-to-many peer-to-peer ridesharing and public transit for morning commute for MaaS. There still lack systematic approaches to achieving different levels of integration for MaaS. Obviously, this calls for more studies on the development of methodologies to effectively achieve each level of integration for MaaS. Effective approaches to bundling and pricing transportation services for MaaS  
The results obtained in the study [115] were based on a survey and analysis with a mixed logit model. Although experiments were performed in the study [115] to estimate and compare consumers' willingness to pay for stand-alone transportation services with service bundles to assess the benefits for consumers, the study was limited. Investigation of the sensitivity of the willingness to pay of consumers with respect to different app features/presentation requires a further study. Conduction of similar experiments in different geographical areas need to be performed to assess the robustness and validity of the results. The study of the effects of flat-rate on bundling transportation services is an interesting issue. The effects of other factors (such as mobility behavior, socio-demographic and household characteristics) on bundling transportation services are interesting research questions to be studied. Although a method to estimate the latent demand for MaaS based on a stated preference survey and a choice model was proposed in [116], how to combine the proposed model with a simulation technique to capture the dynamics affecting the potential demand for MaaS is an interesting issue. The factor of subsides and the pricing schemes based on the choice of transportation modes to include in the bundle require further studies. Although an integrated choice and latent variable model has been developed in [117] to capture the factors that influence user subscription to MaaS bundles and estimate the users' subscription willingness, to improve the estimation of the added value of bundle, conduction of surveys with more detailed local population data is required. The study [118] assessed the influence of applying different bundling and pricing schemes in MaaS on the sustainable transportation. An extended study is required to analyze differences between areas, considering factors such as the degree of congestion, ease of finding a parking lot,parking price and public transportation. The factors due to changes in the use of transportation modes and activity-travel patterns and the associated changes in emissions were not taken into consideration in the study [118]. Incorporation of behavioral data is required to elaborate the analysis of the effects of MaaS on sustainable transportation. An algorithm was developed in [119] for a model proposed to allow travelers to make multi-modal trips offered by multiple operators to efficiently generate stability conditions. However, the issue addressed in the study was based on a static problem formulation. An interesting issue is to extend the study to address day-to-day operations for a dynamic problem setting. Calibration of the model based on real data is also an important issue in applying the proposed method. The proposed assignment game and algorithm has the potential to be applied to other areas such as freight, airlines, other two-sided markets. Applying the proposed algorithms in these areas are interesting but non-trivial research issues.