The Alward Institute provides access to county-to-county commodity trade flow estimates annually (2004 - present) produced by two (2) types of spatial interaction models. These flow are estimated for commodities between every US county pair.  Regional social accounts are used to calibrate each model.

The first category of trade flows are for manufactured commodities and are estimated using a double constrained gravity model. Imports and exports between regions are modeled as flows between county nodes. In general terms, the import and export flows between regions are thought to be proportional to the "mass", "attractiveness" or "size" of an economy and inversely proportional to the "distance" or cost of moving goods and services between them. Mass variables often are interpreted as gross supply and demand while distance is frequently equated with the cost of moving goods and services from one location to another.

Our second category of trade flow estimates are produced by a linear programming model that estimates county-to-county trade for services. This model seeks to minimize the total distance of trade for all counties in the United States, subject to satisfying the demand and supply constraints for a given region. It assumes that consumers and firms seek to minimize the distance traveled to acquire day-to-day services such as day care, dry cleaning, or IT services.

Both trade modeling systems adapt to and incorporate extant trade flow and freight flow data sets.

The regional social accounting system offers a unique opportunity for modeling domestic trade flows between regions using spatial interaction models. Regional social accounting system is complete and consistent. Gross supply and demand for all regions add up to supply and demand on a (domestic) national basis.