Engel's law, Petty's law, and agglomeration

This paper presents a model of structural change and agglomeration. A decline in transportation costs, by enhancing consumers' purchasing power, leads to Engel's law of the demand shift from agricultural to non-agricultural goods. At the same time, the decline in transportation costs, by enlarging the extent of the market for non-agricultural goods, induces Petty's law of the labor reallocation from agriculture to non-agricultural activities. These structural transformations weaken dispersion forces given by farmers tied to the land, whereas they strengthen agglomeration forces generated by footloose non-agricultural workers. Thus, a substantial decline in transportation costs gives rise to agglomeration of non-agricultural activities.