The Greater Miami area, composed of the three counties of Miami-Dade, Broward, and Palm Beach, also known collectively as South Florida, is home to a wide variety of public and private transportation systems. These include heavy rail mass transit (Metrorail), commuter rail (Tri-Rail), automated guideway transit (Metromover), highways, two major airports (Miami International Airport (MIA) and Fort Lauderdale – Hollywood International Airport (FLL)) and seaports (Port of Miami and Port Everglades), as well as three county-wide bus networks (Miami-Dade Metrobus, Broward County Transit (BCT), and Palm Tran), which cover the entire urbanized area of South Florida. Census and ridership data show that Miami has the highest public transportation usage of any city in Florida, as about 17% of Miamians use public transportation on a regular basis, compared to about 4% of commuters in the South Florida metropolitan area. The majority of public transportation in Miami is operated by Miami-Dade Transit (MDT), which is currently the largest transit system in Florida and was the 14th largest transit system in the United States in 2011.
South Florida is one of the most densely populated urbanized areas in the United States overall, being bound by the Atlantic Ocean to the east and the South Florida Water Management District and the Everglades to the west, with a fairly strict Urban Development Boundary (UDB). As of the 2010 US Census, South Florida is both the eighth most populous and eighth most densely populated metropolitan area in the United States. Now, with a population of over five and a half million people living in an urbanized area of only 1,116.1 sq mi (2,891 km2), it has an average population density of over 5,000 residents per square mile. According to the population as of the 2010 US Census, the 35.68 sq mi (92 km2) Miami city proper has an average population density of about 12,139 residents per square mile, with Downtown area, particularly Brickell, being the fastest growing and most dense neighborhoods.