The American Public Transportation Association is a neat resource. Their statistics page is one of the best single clearinghouses for transit ridership data in North America. They also have a handy illustration of why transportation is so important to environmentalism. Being a transit advocacy agency, APTA focuses on carbon emissions reductions that can be had by commuting via transit in place of driving, but it doesn’t take much mental extrapolation to imagine what additional gains could be had by living in a walkable neighborhood.

A couple of their graphics:

Thanks to CommuterPageBlog for the heads up.