But Arizona's Maricopa County, basically Phoenix and its suburbs, appears to have the most mobile residents, demographers said.
Maricopa residents reported moving there from 993 counties and moving out to 1,156 counties.
Curiously, the same counties were both the most common places of origin and destinations for Maricopa: Coconino, Pimal and Pina in Arizona and Los Angeles and San Diego in California.
The American Community Survey found that for 2005 to 2009 48.1 million U.S. residents moved, with 17.7 million changing counties.
Nearly 50,000 people moved from Los Angeles County to San Bernardino on its east, making that the most popular county-to-county move. It was followed by Los Angeles to Orange County on its south, southeast from Los Angeles to Riverside, from Orange to Los Angeles and from Miami-Dade to Broward, the Fort Lauderdale area.