## 5 Discussion and Conclusions  
We propose a strategic framework for vertiport location allocation from a network structural perspective, offering a distinct alternative to conventional location-allocation approaches that are primarily based on localized optimization. While prior research has attempted to determine specific site locations despite the uncertainty surrounding UAM (e.g., aircraft types, flight operations, takeoff and landing requirements), we emphasize a methodology grounded in network analysis that accommodates such uncertainty during the early stages of commercialization. Evaluating the potential structural transformation of urban transportation networks resulting from vertiport placement is essential for maximizing synergies with existing modes and alleviating congestion. Accordingly, we construct and compare vertiport allocation scenarios based on key network criteria (degree centrality, betweenness centrality, vulnerability) using the Seoul Metropolitan Subway as a reference network. Statistical analyses confirm that performance indicators vary significantly across strategies, thereby supporting the relevance of a network-oriented approach for early-stage vertiport planning within multimodal urban mobility systems. We have determined that a network-oriented approach must be prioritized for vertiport location allocation. The placement of vertiports, considering centrality and vulnerability, significantly influences both the clustering and scalability of the network. As the proportion of vertiports in the network increases, the small-world property of the network strengthens rapidly, confirming that the introduction of a vertiport network can fundamentally alter the structure of Seoul Metropolitan railway transportation system. Additionally, we estimate that the network undergoes significant structural transformations as vertiport placement increase. Furthermore, the arbitrary allocation of vertiports introduces substantial uncertainty in the changes to clustering and scalability, reaffirming the necessity of a network-aware vertiport siting strategy. At this stage, before the construction of the first commercial vertiports, strategic decision-making regarding future network transformations is of paramount importance. We argue that both centrality and vulnerability must be considered in vertiport allocation. Using the transportation network of the Seoul Metropolitan Area, we assessed  
Figure 4: Comparison of key network performance indicators. strategies prioritizing either centrality or vulnerability by evaluating network efficiency, connectivity, and average shortest path length. The strategy of selecting nodes with high degree centrality outperformed the vulnerability-based approach in terms of network connectivity. From the perspective of network efficiency and average shortest path length, prioritizing nodes with high vulnerability indicators was found to be more advantageous. These findings suggest that strategies prioritizing nodes with high centrality may enhance network connectivity but could also lead to structural imbalances by overburdening critical nodes. In contrast, we propose that allocating vertiports to structurally vulnerable nodes offers a viable alternative, promoting greater resilience and enabling more balanced network expansion in the context of urban transportation systems. Since network efficiency is a crucial metric for identifying robust alternatives under future uncertainties [58], and since our analysis showed that vulnerability-based network scenarios resulted in higher efficiency scores, we verified that considering vulnerability is essential in vertiport location-allocation strategies to minimize potential losses and balance trade-offs among alternatives. Within the scope of our review, we found that previous studies have primarily focused on connectivity in vertiport site selection [77; 78; 79; 80].