I ran across this blog posting at the Monitor:
How green are trains, public transportation, and hybrid cars? It depends.
The blog post references this research paper:
Environmental assessment of passenger transportation should include infrastructure and supply chains
While the results didn't really surprise me (such as the fact that nearly empty buses pollute a lot more per passenger than an SUV and that all environmental impacts of transportation options should be measured, not just operational ones), they did make me ponder the implications of the findings.
To me, the take-away is that public transit systems that don't implement serious TOD planning and zoning changes that allow high density and resulting high levels of use aren't really doing the environment any good. Also, we need to power our buses with something other than diesel engines (or feed those engines biodiesel).
Any other thoughts?