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	Reilly's Law of Retail Gravitation	Accessibility Analysis
Field	Human Geography	Human Geography
Family	Regression model	Process / pipeline
Year of origin	1931	1959
Originator	William J. Reilly	Walter G. Hansen
Type	Deterministic gravity model of retail trade-area delineation	Spatial index of the ease of reaching opportunities from a location
Seminal source	Reilly, W. J. (1931). The Law of Retail Gravitation. Knickerbocker Press, New York.	Hansen, W. G. (1959). How accessibility shapes land use. Journal of the American Institute of Planners, 25(2), 73–76.
Aliases	Law of Retail Gravitation, Reilly's Retail Gravitation Model, Retail Breaking-Point Model, Reilly Gravity Model	Hansen Accessibility, Gravity Accessibility Measure, Potential Accessibility, Spatial Accessibility Index
Related	4	4
Summary	Reilly's law of retail gravitation is a deterministic model that predicts how an intermediate town's retail trade divides between two larger competing cities. Formulated by William J. Reilly in 1931 by analogy with Newtonian gravity, it states that each city attracts trade in direct proportion to its population and in inverse proportion to the square of the distance to it. Solving for the point of equal attraction yields the famous breaking point — the boundary along the route between two cities where their trade areas meet.	Accessibility analysis measures how easily opportunities — jobs, shops, clinics, parks — can be reached from a given location, combining the attractiveness (size) of destinations with the cost of travelling to them. The gravity-based formulation introduced by Walter Hansen in 1959 sums the opportunities at all destinations, each discounted by a distance-decay function of travel cost, producing a single accessibility score per origin that has become a foundational concept in transport geography and urban planning.