Trade area analysis with Reilly’s Law and the Huff model

For centres A and B separated by D, the distance from A is D ÷ (1 + (mass B ÷ mass A)^(1/n)). With n = 2, A = 400,000, B = 100,000 and D = 60 km, the point is 40 km from A and 20 km from B.

The 50/50 boundary is theoretical and only reflects the declared inputs.

For origin i and centre j, utility is mass_j ÷ distance_ij^n. Probability is that utility divided by the sum across accessible centres.

With A of mass 10,000 at 5 km and B of mass 20,000 at 10 km, n = 2 gives utilities 400 and 200, or about 66.7% for A and 33.3% for B.

Huff probability depends on attractiveness, distance, included competitors and exponent. It is neither observed visits nor a market-share forecast.

Travel time, road networks, barriers, income, assortments and habits require separate data and calibration.

Plot implements Retail Gravitation with Reilly, Huff, 41 positions along a corridor and a configurable exponent. The result remains an estimate for that corridor and the selected parameters.

It is not a GIS: no polygons, isochrones or automatic site optimisation. Map handles operational flows and risks and does not own this method.