## 4 Review of Studies Related to Sustainability in the Context of Multi-Modal Mobility
### Review of Studies
Results: Solving the problem to transport a set of travelers is an essential issue in ridesharing systems. The problem to transport a set of travelers in a travel group is called a joint travel problem. This problem was addressed in [74]. The issue to solve the joint travel problem is to find the optimal joint path for a travel group, where a joint path involves multiple paths of individuals associated with multiple origins and destinations. A space-time multi-state super-network framework was proposed to address the joint travel problem. The optimal joint paths are found by developing stage-wise recursive formulations. Analysis indicates that the run-time complexity for the problem does not depends on the number of meeting/departing points. The study indicates that the joint travel problem is a variant of the Steiner tree problem and is NP-hard. 2. Question: How to develop methods or perform experiments to evaluate the operations of multi-modal multi-provider transport systems with different pricing schemes? Results: In cities with multi-modal transport networks, there are multiple transport modes. Different transport modes may be offered by different service providers with different pricing schemes. Such a multi-modal multi-provider market relies on developing methods or performing experiments to evaluate the operation of transport systems. A generalized market equilibrium model based on Karush-Kuhn-Tucker (KKT) optimality conditions of the optimization problem formulation was proposed in the study [75] to assess the operation of transport systems with private vehicles, walking, public transport, ridesourcing and ridesharing. Experiments were performed by applying the proposed model to an abstract network in Sydney to demonstrate its effectiveness in handling responses of market players to electric vehicles, changes in travel demand, greenhouse gas emissions limit, and barriers of ridesharing. 3. Question: What are the requirements, challenges and solutions for the whole system design of intelligent shared mobility systems? Results: Although the increasing number of papers on Shared Mobility Systems leads to several survey papers that summarize how a particular challenge is addressed individually, there lacks a survey from the \"whole system design\" point of view. A survey of flexible shared mobility systems supporting decision making for inter-operable multi-modal transport services is needed. The paper [103] categorizes the inter-connected challenges in different transportation modes of shared mobility systems and reviews how the resulting solutions address these challenges in the system. 4. Question: Is there a connection between ride-hailing/ridesharing adoption and transit service adoption? Results: Clarifying whether there is a connection between ride-hailing and transit service adoption is helpful for understanding the inter-modal complementarities and planning of multi-modal transport systems. Therefore, studying the connection between ride-hailing and transit service adoption is an important issue. The study [90] empirically identified the relationship between transit service and ride-hailing adoption in a fast-growing region in the United States. The study found that the surge periods for these two transport modes were not simultaneous temporally. The study also found that ridesharing complements transit adoption. 5. Question: How to develop a simulation framework for transportation planners to assess the impact of the emerging mobility modes with shared electric autonomous vehicles (AVs) on a city?Results: Studying the potential impact of the emerging mobility modes with shared electric autonomous vehicles (AVs) on a city daily life is an important issue for transportation planners. In [91], an agent-based simulation framework was proposed traffic planners to explore the cooperative integration of AVs using a decentralized control approach and assess the impact of the emerging mobility modes through simulation. A prototype was implemented and validated based on the data of Trento.