The intensity of possible contacts with markets. The so-called Harris market potential uses bilateral distances to weight surrounding locations’ GDP:
A more sophisticated version of Harris’s market potential uses road travel times for lorries as weights, instead of great circle distances:
See Holl (2011)
J. Econ. Geog. DOI 0.1093/jeg/lbr030 on the productivity impacts of market potential and transport infrastructure investment in the manufacturing sector in Spain.