Lowry Land-Use Transport Model

Basic employment is the export- or externally-oriented activity whose location is treated as given — manufacturing, ports, universities, government — and which drives the model. Non-basic, or service, employment is the retail and local-service activity that exists to serve the resident population and is located by the model itself. The split is central: basic jobs are the exogenous input, and population and service jobs are derived from them, so misclassifying employment corrupts the whole allocation.

Because the system is circular. Basic jobs generate resident workers, those residents demand services, and the resulting service jobs are themselves employment that generates still more residents, who demand still more services. A single pass cannot capture this feedback, so the residential and service allocations are repeated in alternation; each round adds a diminishing increment until population and employment stop changing, at which point the model has reached the equilibrium that is its solution.

The Lowry model is built on spatial-interaction (gravity) distribution as its allocation engine. Both the residential step (workers distributed around jobs) and the service step (service jobs distributed around population) use an impedance function of travel cost to share activity among competing zones — exactly the gravity logic of the spatial-interaction model. Lowry's contribution was to wire two such gravity allocations together in a feedback loop driven by basic employment, turning a flow model into a land-use allocation model.