Vehicles also need a system of converting the energy into movement; this is most commonly done through wheels, propellers and pressure. Vehicles are most commonly staffed by a driver. However, some systems, such as people movers and some rapid transits, are fully automated. For passenger transport, the vehicle must have a compartment for the passengers. Simple vehicles, such as automobiles, bicycles or simple aircraft, may have one of the passengers as a driver. Public land transport refers to carriage of people and goods by government or commercial entities which is made available to the public at large for the purpose of facilitating the economy and society they serve. Most transport infrastructure and large transport vehicles are operated in this manner. Funds to pay for such transport may come from taxes, subscriptions, direct user fees, or some combination. The vast majority of public transport is land-based, with commuting and postal delivery being the primary purposes. Commercial land transport refers to carriage of people and goods by commercial entities made available at cost to individuals, businesses, and the government for the purpose of profiting the entities providing the travel. Most infrastructure used is publicly owned, and vehicles tend to be large and efficient to maximize capacity and profit margins. Freight shipping and long-distance travel are common uses served by commercial land transport. Military land transport refers to carriage of people and goods by the military or other operators for the purpose of supporting military operations, both in peacetime as well as in combat areas. Such activity may use a combination of public infrastructure as well as military-specific infrastructure and in many cases is designed to operate with little or no infrastructure when necessary. Vehicles can range from basic commercial or even private vehicles to those specifically designed for military use. Private land transport refers to individuals and organizations transporting themselves and their own people, animals, and goods at their own discretion. Vehicles used are typically smaller, though publicly owned infrastructure is often used for travel. Relocation of travelers and cargo are the most common uses of transport. However, other uses exist, such as the strategic and tactical relocation of armed forces during warfare, or the civilian mobility construction or emergency equipment. Passenger transport, or travel, is divided into public and private transport. Public transport is scheduled services on fixed routes, while private is vehicles that provide ad hoc services at the riders desire. The latter offers better flexibility, but has lower capacity, and a higher environmental impact. Travel may be as part of daily commuting, for business, leisure or migration. Short-haul transport is dominated by the automobile and mass transit.