How much cleaner is public transportation versus a car?

• 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".[1]
• 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 trains.[2] 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 and Pendolino trains weigh more per seat than the Airbus A380 airliner.
• 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.[3]

Sources:

• [1] Lyndsey Layton, "Study Lists Mass Transit Benefits", The Washington Post, July 17, 2002, Page B05
• [2] Paul Marston (2004-06-21). "Cars are more fuel-efficient than trains, claims study". The Daily Telegraph (Telegraph Media Group). Retrieved on 2009-03-18.
• [3] Newman, 1999