A major rationale — perhaps the major rationale — touted by supporters of mass transit is that by reducing our output of greenhouse gases and other pollutants, transit can help save the environment. The proposition seems intuitive and even obvious: by no longer encasing each traveler in thousands of pounds of difficult-to-move metal, surely transit is more energy-efficient. Plenty of analyses prove this. But then again, Aristotle, who was revered as the infallible font of truth for more than 1,000 years, proved that heavier objects fall faster than lighter ones and that women have fewer teeth than men. Might studies that demonstrate transit is greener be similarly wrong?
They might. The reason is that many studies of energy efficiency by mode often make questionable and -- depending on the author's point of view -- self-serving assumptions. The main trick is to look at autos with but one passenger and compare them to transit vehicles in which every seat is full. (For example, see this.)
But in the real world, this is emphatically not the case.