Centrality and Intermediacy

The concept of centrality is relatively straightforward as the vicinity of the terminal is either the origin or the destination of the movement, which in turn is linked with the level of economic activity. The extent of this vicinity remains to be fully assessed, but a range of about 100 km appears suitable. Intermediacy, however, is a multifaceted concept that relates to a number of issues:

Range. This is more a conventional aspect of intermediacy that tends to be of lesser importance in the contemporary situation. Due to technical limitations of the modes, such as the range of an aircraft or the need to refuel a coal powered ship, intermediate locations were used as stages to overcome the range gap as two locations could not be reached in a single trip. Intermediate airports such as Anchorage, Alaska (transpacific flights) or Gander, Newfoundland (transatlantic flights), are good examples of locations that were used to overcome the technical limitations of aircrafts before the introduction of long range couriers. Although range plays a more limited role for air passengers transportation, particularly with long range aircrafts, it is still prevalent for air freight transportation as the range of freight aircrafts is more limited due to heavier loads. Therefore, transpacific air cargo routes commonly involve a stop at Anchorage while Asia-Europe routes will have a stop at a Middle Eastern airport such as Dubai. To a lesser extent, intermediate locations are used for long distance trucking since rest periods are required for drivers, but the required facilities are very basic.
Gateway. It connects two systems of circulation and thus represents an intermediate location imposed by geographical constraints. Thus, in order to reach its final destination, a movement must use an intermediate location often implying a transfer from one mode to the other. Many gateways have also a significant centrality component as they represent industrial zones and large urban agglomerations.
Hub (Interception). A location nearby, or at, the convergence of several long distance routes can develop an intermediacy by "intercepting" some of the traffic. This is notably the case for intermediate hub terminals located along major long distance maritime corridors, such as Algeciras (Spain), San Juan (Puerto Rico) or Singapore.
Hub (Transcalar). A location is specifically used to serve as a connection between different scales of a transport system. Air transportation is a notable example with the emergence of hub-and-spoke network structures where the hub is an intermediate location between locations within a region, a continent or at the international level. Freight distribution, particularly cross-docking distribution centers, also rely on the usage of intermediate location to service specific market segments often supplied by distant sources.