## 5 Current Trends
This is shown in Figure 7:7 Finally, the current trends of modelling and solution approaches applied in the scope of this survey based on year are presented below in Figures 36-39 for clearer depiction. Figure 7:8 Disaster Types Addressed by Literature According to Year of Publication. Figure 7:9 Modelling Description Applied in Literature According to Year of Publication. Figure 10.0 Disaster Phases Addressed by Literature According to Year of Publication. In general, the modelling of classified VRPs in this survey can be divided by those adopting graph theory (such as the minimum cost flow model and spanning tree model), agent based model, such as in [113, 119, 124], among others, the set partitioning model, as can be seen in [51], and the MDP model. There is also a trend of the combined OR problem that also addressed the VRP to fully capture the important mechanism of humanitarian operations, such as the LRP, which can be seen in [42, 44, 48, 73, 74, 77, 79, 89, 138, 139], followed by the LIRP in [80, 81], scheduling and routing in [85, 92], ARP in [96], as well as ALRP in [57]. From these models, various interesting and complex problems are addressed. As a general trend, it could be seen that most work addressed split delivery, such as in [42, 46, 47, 49, 50, 51, 61, 71, 73, 75, 76, 78, 81, 83, 85, 87, 94, 96, 102, 116, 123, 128, 158] and multi-trip VRP that can be seen in [31, 47, 50, 61, 76, 88, 90, 94, 97, 98, 124, 126, 129], ref. [134, 149, 159] normally stressed on the problem of vehicle limitations such as the numbers of vehicle or the fuel limitation. However some works addressed the VRP in this scope by assuming flexible fleet size such as [30, 87]. Additionally, heterogeneous fleet as can be seen in [75] is usually assumed for works that deal with complex requirements for the selected routing operations. It is also not uncommon to see multi-depots, such as in [70, 75, 80, 87, 94, 98], and ref. [82] being applied to address a more realistic scenario rather than depicting only one depot. Understandably most literature involving ambulance routing are often addressing VRP for rescue operation, which could be seen in [150, 151, 155]. These compliment the works that address urgent medical needs of victim that can be seen in [113, 150, 151, 155, 156, 157]. In particular, road condition is one of the most important key factors when addressing routing in humanitarian operation. Various conditions of road can best be modelled through a flow model that details the flow of the road, capacity, as well as impedance. Such detail can be seen in [121, 125, 136, 153], to cite a few. Some works make use of the dispersion of disaster effects, as can be seen in [157] that link the road's condition based on circled areas of the affected region. Some works assume binary capacity of road, such as [114, 146]. On the other hand, uncertain road capacity could be observed in [67, 75]. Meanwhile, ref. [76] addressed the problem of repairing some roads while executing delivery operation. Furthermore, various applications of the fuzzy method are observed, such as in [42, 44, 68, 77] and [88], which address uncertainty on the parameters into the fuzzy parameter. The fuzzy method is also applied to address multi-objectives such as in [34]. Meanwhile, a multi-criteria fuzzy clustering is applied to estimate certain parameter as can be seen in [102]. What are also worth noting are works that address inter-modal network, such as in [33, 148], involving maritime and land transport as well as air transport, respectively. Furthermore, ref. [53] addressed the cooperation between air and land transport. More depictions of inter-modal networks are seen in [33, 59, 104, 118, 90]. Additionally, the routing problem is not unique to just land vehicles. Some works, such as [60, 64, 74, 152], only address drone routing, while [63, 124] addresses the helicopter routing. To that note, the trend of cooperation between drones and vehicles in enabling more sophisticated  
Figure 10.1 Solution Approaches Applied in Literature According to Year of Publication.