## 4 Review of Studies Related to Sustainability in the Context of Multi-Modal Mobility
### Review of Studies  
The increasing uptake of shared mobility/ridesharing in recent years is reflected in the strong growth of shared mobility market. According to the reports of [126], the global shared mobility market size is expected to grow at a compound annual growth rate (CAGR) of 15.8% from 2023 to 2030. According to the reports of [127], the global shared mobility market size exhibits a compound annual growth rate (CAGR) of 12.00% during the forecast period (2024-2032). Despite the differences between the forecast of the compound annual growth rates, both reports project the strong growth of the global shared mobility in the upcoming years. The growing trend of the global shared mobility market in recent years has attracted a lot of attention from key stakeholders, including transportation planners, service providers, researchers, practitioners and travelers. The emerging shared mobility and ridesharing transport modes have raised several interesting research questions for relevant stakeholders and posed challenges in their application. These research issues include requirements of shared mobility systems, travel planning, clarification of the connection between shared mobility and other transport modes, investigation of the impacts brought by shared mobility, decision models to determine the routes and travel modes to optimize performance or minimize travelers' travel cost in multi-modal transport system with shared mobility/ridesharing and allocation of resources to meet the travel demand. Some of the research questions related to the research issues mentioned above along with the studies that respond to these questions are as follows. \begin{table}
\begin{tabular}{c p{284.5 pt}} \hline \hline
**Paper** & **Research Questions** \\ \hline
[74] & How to solve the joint travel problem to find the optimal joint path to transport a set of travelers, where a joint path involves multiple paths of individuals associated with multiple origins and destinations? \\ \hline
[75] & How to develop methods or perform experiments to evaluate the operations of multi-modal multi-provider transport systems with different pricing schemes? \\ \hline
[103] & What are the requirements, challenges and future direction for the whole system design of intelligent shared mobility systems? \\ \hline
[90] & Is there a connection between ride-hailing/ridesharing adoption and transit service adoption? \\ \hline
[91] & 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? \\ \hline
[104] & What are the types, variants, characteristics of shared mobility problems, the related solution approaches, the current trends and future research directions? \\ \hline
[76] & How to develop a decision model for travelers to choose routes and decide travel modes to minimize travelers’ generalized travel cost in a multi-modal transportation system with ridesharing and public transit? \\ \hline
[77] & How to simulate and optimize multi-modal transport systems with P2P ridesharing, transit, city bike-sharing and walking? \\ \hline
[78] & Can ridesharing help reduce traffic congestion in a multi-modal transport system? If ridesharing cannot necessarily reduce traffic congestion, is it possible to reduce traffic congestion by implementing pricing strategies along with ridesharing? \\ \hline
[79] & How to optimally allocate and deploy designated autonomous vehicle (AV) lanes for shared autonomous vehicles (SAVs) considering flows of regular vehicles? \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of research questions addressed in the papers published between 2019 and 2022 in the Web of Science Core Collection with keywords “shared mobility/ridesharing” and “multi-modal” in the abstract. 1. Question: How to solve the joint travel problem to find the optimal joint path to transport a set of travelers, where a joint path involves multiple paths of individuals associated with multiple origins and destinations?