Just a US-relevant study? or global? I guess it depends on local private provision. In some Asian countries we see the cheap and cheerful tuk-tuk making transport relatively cheap, but is it still too expensive? are there any good examples of a purely private transport system offering equal transportation access to all regardless of relative income?
Should Democracy 3 have bus usage and rail usage reduce inequality?
I heard that the Dubai transport system has a number of innovations and advantages, but it is more commercial and there is also more to work on, in terms of accessibility. I also did a little research about such system in the UK, which you can find on answershark.com/sociology.