Vulnerability through Vertical Collaboration in Transportation: A complex networks approach

This research addresses vulnerabilities that emerge from vertical collaboration in complex transportation networks with multiple carriers and transport modes and provides decision support tools for policy making in the field of collaborative transport. Vertical collaboration in our context describes the integration of service networks of different carriers and transport modes, and comes with many benefits such as efficiency, utilization of infrastructure & carbon footprint reduction. However, a higher level of collaboration creates new dependencies, for instance induced by the need for coordination, information exchange, and interconnected information systems, which can ultimately turn into disruption threats. While the benefits of vertical collaboration are extensively covered in existing research, knowledge on the concomitant vulnerabilities has been rather limited. We developed a novel multi-layer network model to capture both the physical and collaborative aspects of transportation networks with vertical collaboration in an integrated fashion and analysed it with a combination of well-known metrics from network science and new methods developed ourselves. We show that vulnerability is strongly influenced by the market structure of carriers. Systems with mainly similar-sized carriers are robust to targeted disruption, whereas the total magnitude of potential failure is very large. Systems with a few dominating carriers have a lower magnitude of failure but are highly susceptible to targeted disruption. Moreover, there is an optimal level of collaboration where the increasing risk of disruption cascades outweighs the decreasing marginal added benefits of additional collaboration.