Metropolitan retail nucleation.

We argue that U.S. metropolitan retail nucleation, as represented by sales in the Central Business District and in retail centers, is a consequence of population growth in at least three transportation epochs, the walking and horse era, the electric streetcar era, and the automobile period. Population growth in each epoch has had a different effect on retail nucleation, with the most recent growth having had a pronounced positive effect on development of retail centers and a slight negative effect on the sales of the Central Business District.