Megaregions of the United States are clustered networks of American cities, which are currently estimated to contain a total population exceeding 237 million. America 2050, a project of the Regional Plan Association, lists 11 megaregions in the United States, Canada and Mexico. Megapolitan areas were explored in a July 2005 report by Robert E. Lang and Dawn Dhavale of the Metropolitan Institute at Virginia Tech. A later 2007 article by Lang and Arthur C. Nelson uses 20 megapolitan areas grouped into 10 megaregions. The concept is based on the original megalopolis model.