Transportation in the United States is facilitated by road, air, rail, and waterways. The vast majority of passenger travel occurs by automobile for shorter distances, and airplane (or railroad, in some regions) for longer distances. In descending order, most cargoes travel by railroad, truck, pipeline, or boat; air shipping is typically used only for perishables and premium express shipments. Transportation is the largest source of greenhouse gas emissions by the United States. Ownership and jurisdiction The overwhelming majority of roads in the United States are owned and maintained by state and local governments. Federally maintained roads are generally found only on federal lands (such as national parks) and at federal facilities (like military bases). The Interstate Highway System is partly funded by the federal government but owned and maintained by individual state governments. There are a few private highways in the United States, which use tolls to pay for construction and maintenance. There are many local private roads, generally serving remote or insular residences. Passenger and freight rail systems, bus systems, water ferries, and dams may be under either public or private ownership and operation. Civilian airlines are all privately owned. Most airports are owned and operated by local government authorities, but there are also some private airports. The Transportation Security Administration has provided security at most major airports since 2001. The United States Department of Transportation and its divisions provide regulation, supervision, and funding for all aspects of transportation, except for customs, immigration, and security, which are the responsibility of the United States Department of Homeland Security. Each state has its own Department of Transportation, which builds and maintains state highways, and depending upon the state, may either directly operate or supervise other modes of transportation. Aviation law is almost entirely a federal matter, while automobile traffic laws are enacted and enforced by state and local authorities (except on federal property and in unorganized territory). Economic jurisdiction over tidelands is shared between the state and federal governments, while the United States Coast Guard is the primary enforcer of law and security on U.S. waterways. Mode share Passenger Passenger transportation is dominated by a network of over 3.9 million miles of highways which is pervasive and highly developed by global standards. Passenger transportation is dominated by passenger vehicles (including cars, trucks, vans, and motorcycles), which account for 86% of passenger-miles traveled. The remaining 14% was handled by planes, trains, and