Public transportation is often touted as a significant part of battling global climate change ("global warming" is generally out of fashion these days). The idea is that transporting large numbers of people using one vehicle (such as buses and trains) is more environmentally friendly than people traveling in single cars. The output of greenhouse gasses is dependent on how many vehicles are on the road, all burning gasoline; by reducing these numbers, the output of these gasses on the environment is lessened. This is the most important argument for public transportation: reducing greenhouse gas emissions from transportation will lower environmental impact, especially in climate change involving warming.
One important aspect of this issue is that many environmental standards, such as limitations in gas emissions mandated by the government, reduce overall fuel efficiency. This causes a paradox wherein reduction in per-mile gas emission actually raises the number of gallons of fuel burned; research into the best gas emission versus best mileage numbers is ongoing. At the moment, though, public transportation is more efficient and environmentally friendly than private transportation; however, many areas, especially rural areas, have few, if any, public transportation venues in place. It is, however, hard to argue against the overall reduction in toxic gas emissions from transportation, whether for climate change or simply for the public health.