## 5 Current Trends
### Descriptive Analysis
On the other hand, the maximisation of QoS and SW, as well as the minimisation of SD, and NMC, are the least researched in SMP with only one paper each. Figure 12: Distribution of Papers based on the Optimisation Objectives. Based on Figure 13, the minimisation of TC covered the largest percentage of papers (33 papers) that considered the ridesharing problem. This is followed by works on the minimisation of TM, the maximisation of the NMR, and the minimisation of TD which cover 16, 16, and 15, respectively. Referring to Figure 14, the minimisation of TC covers the largest portion of works in the taxisharing problem (nine papers). This is followed by TM minimisation which covers four papers. The maximisation of the NMR and minimisation of TD are ranked third among the other objectives in the selected papers. In Figure 15, the minimisation of TC covers 14 of the papers, which is the largest portion covered in the optimisation objective of carpooling problem. The minimisation of the TD and maximisation of the NMR are the second (7 papers) and third (5 papers) largest, respectively. The minimisation of NV, PC, and CE have the coverage of one paper each. Figure 14: Distribution of Papers based on the Optimisation Objectives: Taxisharing. Figure 13: Distribution of Papers based on the Optimisation Objectives: Ridesharing. For the multi-modal problem, the largest percentage of paper coverage in Figure 16 is the minimisation of TC (six papers). The minimisation of TM and maximisation of the NMR are ranked second highest among the other objectives which is covered in four papers. There is one vanpooling-related paper among the selected literature. The only objective that is considered is the minimisation of the TC. On the other hand, there are two selected papers that fall under the busyooling problem. The considered objectives in the busyooling problem are the TD minimisation and the NMR maximisation which are covered in two papers and one paper in the review, respectively. Table S5 (see Supplementary File) gives a list of the detailed approaches used in the selected literature to solve the SMP. On the other hand, Table S6 (see Supplementary file) presents a summary of the types of approaches used in the papers for solving the SMP. The approaches are categorised into three different types, which are exact approach, heuristic and metaheuristic approaches and other approaches. The distribution of the 103 selected papers based on the solution approaches used for the SMP over the last decade are tabulated in Table 7 below. Figure 16: Distribution of Papers based on the Optimisation Objectives: Multi-Modal. Figure 15: Distribution of Papers based on the Optimisation Objectives: Carpooling. The solution approaches are grouped into four different categories, which are: (i) exact only; (ii) heuristic and metaheuristic; (iii) exact, heuristic, and metaheuristic; and (iv) other approaches. The research papers under the first category considered only the exact algorithm as the solution approach of the problem. The second category covered the selected papers which used heuristics and/or metaheuristics to solve the problem. The third category are the papers which proposed both new exact algorithm and heuristic/metaheuristic algorithms in solving the problem. For those research papers that make use of CPLEX or other exact approaches to compare the solutions (as the benchmark for small instances) of another newly proposed heuristic/metaheuristic are grouped under the second category instead of the third category. Lastly, the papers which used other methods such as simulation method are categorised under the fourth category. As shown in Table 8, the heuristic and metaheuristic approaches are the most popular approaches considered in solving the SMP, especially for ridesharing, carpooling, and kaisharing problems. This is followed by the exact algorithm. Note that there no research has been conducted in taxisharing and carpooling problems using other approaches.