## 8 Discussion
### Opportunities and Challenges
A comprehensive review of pricing in three emerging systems: ridesharing, carsharing, and shared autonomous vehicles (SAVs) can be found in the paper [142]. In [142], the authors categorize the pricing approaches in single modal mobility services into trip-based pricing, origin-based pricing, duration/distance-based pricing, and auction. However, an in-depth study is still lacking on how to apply or extend these single modal mobility service pricing approaches to address pricing issues in MaaS. A research issue closely related to pricing is cost allocation or allocation of benefits. The issue of cost allocation or allocation of benefits has been addressed in the context of single modal transport, but it has been less studied in the context of multi-modal transport. Cost-sharing mechanism design for ridesharing has been studied in [143]. The cost-sharing problem in the context of ridesharing refers to the problem to determine the allocation of the total ride cost between the driver and the passengers. The setting of the problem considered in [143] assumes that each driver has his/her own origin and destination. Several desirable properties of cost-sharing mechanisms have been identified and a framework to create specific cost-sharing mechanisms in the context of ridesharing has been proposed. However, whether the cost-sharing framework proposed in [143] can be applied to multi-modal transport systems requires further studies. In a single modal shared mobility system, how to allocate benefits to the drivers, passengers and service providers will influence the satisfaction of participants. Several schemes have been proposed to address this issue in [144; 145; 146]. In [145; 146], three schemes have been analyzed to compare their influence on satisfaction of participants. Another interesting issue that has been studied in single modal shared mobility system is to attract participants through the incentive of discount guarantee. For example, in [147; 148], different metaheuristic algorithms have been proposed to solve the ridesharing problem with discount guarantee. However, there lacks a study on how to provide an incentive with discount guarantee in MaaS systems. Cost allocation for ridesharing services considering fairness issue has been studied in [149]. However, whether the above studies can be extended to deal with MaaS requires further investigation to bridge the gaps identified above. The three research issues--\"enabling factors/barriers of modal shift to sustainable transport\", \"characteristics and adoption factors of micromobility in multi-modal transport networks\", and \"factors influencing adoption of sustainable mobility behaviors\"--account for a total of 16 papers. This indicates that adoption factors and barriers in the context of multi-modal transport were extensively studied in the past 8 years. The reason adoption factors and barriers were extensively studied is that they are closely related to critical success factors and critical failure factors, respectively. The former refers to essential elements that must be executed well to achieve strategic objectives, while the latter refers to elements that might prevent a project or organization from reaching its goals. Although the number of papers related to the research issue \"Approaches to promoting sustainable mobility\" is small, the development of innovative schemes to attract potential MaaS usersand enhance their participation through rewards and incentive programs remains an important topic for future research and a gap to be filled. When a new paradigm introduces a service that relies on existing ones, those existing services and their stakeholders may be affected by changes in market size and by the need to adapt how the old services operate in order to accommodate the new one.