Environmental engineers Mikhail Chester and Arpad Horvath at the University of California at Davis say that when these costs are included, a more complex and challenging picture emerges.
In some circumstances, for instance, it could be more eco-friendly to drive into a city -- even in an SUV, the bete noire of green groups -- rather than take a suburban train. It depends on seat occupancy and the underlying carbon cost of the mode of transport.
"We are encouraging people to look at not the average ranking of modes, because there is a different basket of configurations that determine the outcome," Chester told AFP in a phone interview.
"There's no overall solution that's the same all the time."