Distance decay is a geographical term which describes the effect of distance on cultural or spatial interactions. The distance decay effect states that the interaction between two locales declines as the distance between them increases. Once the distance is outside of the two locales' activity space, their interactions begin to decrease.
With the advent of faster travel, distance has less effect than it did in the past, except where places previously well-connected by railroads, for example, have fallen off the beaten path. Advances in communications technology, such as telegraphs, telephones, broadcasting, and internet, have further decreased the effects of distance.
Distance decay is graphically represented by a curving line that swoops concavely downward as distance along the x-axis increases. Distance decay can be mathematically represented as an Inverse-square law by the expression
- density of pedestrian traffic
- street quality
- quality of shops (this may be an artifact of the definitions of 'quality' and 'center')
- height of buildings
- price of land
Distance decay weighs into the decision to migrate, leading many migrants to move less far than they originally contemplated.
Related terms include "friction of distance," which describes the force that creates distance decay and Waldo R. Tobler's "First law of geography", an informal statement that "All things are related, but near things are more related than far things." "Loss of Strength Gradient" holds that the amount of a nation’s military power that could be brought to bear in any part of the world depends on geographic distance.
- Kristin Taverna, Weimin Xi. "Working Group on Distance Decay". University of North Carolina at Chapel Hill.
- Fellman, Getis, and Getis. Human Geography, eighth edition, pp. 68–69.
- Petr Matous, Yasuyuki Todo, Dagne Mojo (2013) "Boots are made for walking: interactions across physical and social space in infrastructure-poor regions" Journal of Transport Geography
- Nekola, J. C., and P. S. White. 1999. The distance decay of similarity in biogeography and ecology. Journal of Biogeography 26: 867-878. 
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