Types of Transportation Networks and Vulnerabilities

Types of Transportation Networks and Vulnerabilities

Depending on the mode they represent, transportation networks have different configurations:

  • Air networks. Such networks are commonly a nodal hierarchy often organized around a hub-and-spoke structure, underlining that nodes (airports) are the core elements of air networks. The importance of a node is usually related to the traffic (passengers and freight) it handles and the level of connectivity (links to other nodes). There is a hierarchy of flows ranging from regional (short distance feeders) to international (inter-hub). Due to its high degree of hubbing, air transportation networks are particularly vulnerable to disruptions at major hubs, while disruptions at smaller hubs will have limited consequences.
  • Maritime networks. Such networks are a circuitous nodal hierarchy, implying that services are commonly arranged along a sequence of nodes (ports) with inter-range services that loop back to the port of origin. While point-to-point services are reflective of bulk shipping, container shipping is organized between deep-sea and feeder services with transshipment hubs acting as the interface. The vulnerability of maritime networks has different considerations depending on if the node is a hub or a gateway. Disruptions at a hub will mostly impact maritime shipping networks while disruptions at a gateway will mostly impact the hinterland.
  • Logistical networks. Such networks are a sequential multi-nodal hierarchy, implying that there are separate networks within networks. A typical logistics sequence is organized along three stages; raw materials and parts, manufacturing and distribution, each supported by a specific network (manufacturing network, distribution network). They represent sourcing relationships between actors and such networks are vulnerable to disruptions impacting one actor (e.g. a manufacturing plant, a distribution center) and the connected activities (upstream and downstream). This is commonly known as the cumulative effect where a small disruption could result in significant impacts along a supply chain since a product is often made of numerous components and if a part is missing, a supply chain could come temporarily to a halt.
  • Road networks. Such networks are hierarchical meshes, each servicing a different scale. They have no tangible nodes but fixed paths with known capacity. While an interstate highway system is designed to connect a nation or a large region, local streets are only connecting adjacent activities to a wider framework. Because of their mesh structure, road networks are not highly vulnerable to disruptions, unless this disruption is at a wide-scale (e.g. a major snowstorm or a hurricane) or impacting a strategic connector such as bridges or tunnels. High connectivity road networks can be disrupted if a high-level connection is closed, which forces traffic on lower level connections that may not have the capacity to handle the load.
  • Rail networks. Such networks are a linear nodal hierarchy with nodes related to intermodal yards, train and transit stations. Because of the fixed character of their paths and capacity, they are allocated usage windows during which grouped units circulate. While linear rail networks are vulnerable to disruptions, complex rail and transit networks have a mesh-like structure, making them more resilient.
  • Power grids. Such networks have a sequential linear hierarchy where the main nodes are power generation facilities from which electricity is distributed across high voltage transmission lines to stations for regional distribution. These substations transform electricity from high to low voltage which is distributed to facilities for final use. Very close to the final consumer, transformers may further reduce the voltage to safer levels. Power grid networks are usually highly redundant but subject to a hierarchical vulnerability where the higher up in the hierarchy, the more extensive the disruption.