Public transportation in the United States refers to publicly financed mass transit services across the nation. This includes various forms of bus, rail, ferry, and sometimes, airline services. Most established public transit systems are located in central, urban areas where there is enough density and public demand to require public transportation. In more auto-centric suburban localities, public transit is normally, but not always, less frequent and less common. Most public transit services in the United States are either national, regional/commuter, or local, depending on the type of service. Furthermore, sometimes "public transportation" in the United States is an umbrella term used synonymously with "alternative transportation", meaning any form of mobility that excludes driving alone