Public transportation comprises passenger transportation services which are available for use by the general public, as opposed to modes for private use such as automobiles or vehicles for hire.
Public transport can consist of subways, trolleys and light rail, commuter trains, buses, van pool services, paratransit services for senior citizens and people with disabilities, ferries, water taxis, or monorails.
A 2002 study by the Brookings Institution and the American Enterprise Institute found that public transportation in the U.S uses approximately half the fuel required by cars, SUV's and light trucks. In addition, the study noted that "private vehicles emit about 95 percent more carbon monoxide, 92 percent more volatile organic compounds and about twice as much carbon dioxide and nitrogen oxide than public vehicles for every passenger mile traveled".
A 2004 study from Lancaster University concluded that there was no environmental benefit to be gained from persuading car or plane travellers to switch to diesel-powered UK trains. The study showed that trains had failed to keep up with the advances that the automotive and aviation industries had made in improved fuel efficiency. Express trains travelling from London to Edinburgh consumed 11.5 litres more fuel per seat than a modern diesel car.
Studies have shown that there is a strong inverse correlation between urban population density and energy consumption per capita, and that public transport could play a key role in increasing urban population densities, and thus reduce travel distances and fossil fuel consumption.