Conditions for the Realization of a Spatial Interaction

The seminal work of Ullman on spatial interaction states that a spatial interaction between location B and A can occur only if the three following conditions are met:

• Complementarity. If location B produces / generates something that location A requires, then an interaction is possible because a supply / demand relationship has been established between those two locations; they have become complementary to one another. The same applies in the other direction (A to B), which creates a situation of reciprocity common in commuting or international trade.
• Intervening opportunity. If location C offers the same characteristics (namely complementarity) than location A and is also closer to location B, an interaction between B and A will not occur and will be replaced by an interaction between B and C.
• Transferability. Transport infrastructures (modes and terminals) must be present to support an interaction between B and A. Also, these infrastructures must have a capacity and availability which are compatible with the requirements of such an interaction.