> when this was a primarily agrarian country
it sounds ironic, but much of the shift to suburban development patterns in the USA was indeed driven by the Great Migration into cities – specifically, the migration of formerly-enslaved Black Americans out of the rural south and into cities. US public policy was very explicit about disinvesting in cities and destroying vibrant urban neighborhoods, replacing them with freeways and parking lots.
today, large portions of US cities are zoned for exclusively single-family homes, and other zoning requirements like parking minimums, and minimum setback and lot sizes continue to slow urban, transit-oriented redevelopment.
