Reilly’s Law of Retail Gravitation

We’re in a good location to observe Reilly’s law – Libby attracts very little commerce from North County, and we’ve had a great example of how a political decision that minimizes travel at Roosville changes the retail industry. 

Reilly’s law best applies to the midwest plains – an area where mountains and rivers have minimal effect.  On the other hand, where 37 is the route to Libby, and 93 the route to Whitefish and Kalispell, the limits created by mountains and rivers kind of cancel out.   

From the web-site article: “Reilly realized that the larger a city, the larger a trade area it would have and thus it would draw from a larger hinterland around the city. Two cities of equal size have a trade area boundary midway between the two cities. When cities are of unequal size, the boundary lies closer to the smaller city, giving the larger city a larger trade area.

Reilly called the boundary between two trade areas the breaking point (BP). On that line, exactly half the population shops at either of the two cities.

The formula is used between two cities to find the BP between the two. The distance between the two cities is divided by one plus the result of dividing the population of city B by the population of city A. The resulting BP is the distance from city A to the 50% boundary of the trade area.

One can determine the complete trade area of a city by determining the BP between multiple cities or centers.

Of course, Reilly’s law presumes that the cities are on a flat plain without any rivers, freeways, political boundaries, consumer preferences, or mountains to modify an individual’s progress toward a city.”

The populations can be accessed readily – the census count for Eureka is 1,380. The Census lists Libby at 2,775, Whitefish at 7,751 and Kalispell at 24,558.  With Libby and Kalispell essentially equal distance from Eureka, the retail gravitation of Kalispell greatly overpowers Libby – even if we ignore the population that is outside the city limits (and north-county has a higher percentage outside town limits).  In terms of county solidarity, Libby just doesn’t attract north-county commerce.

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