Impact of various travelling options
Freakonomics makes the claim that for public transportation, provided ridership is sufficiently high, that the most energy efficient means is commuter trains and subways and that buses are the least efficient. Further, they claim that buses actually consume more energy than cars per rider due to average bus ridership being about ten passengers in that if each person riding the bus instead took a car and the bus did not run, there would be less energy consumed:
Is there any truth to this claim and how would one go about objectively measuring the complete impact (not just energy) of the various choices for moving about? Do the choices vary by city size and population density, for example New York City (cited as very efficient) versus Cincinnati? What are some of the non-obvious externalities of the various public transportation options? Finally, if the bus system is running whether I'm on it or not, surely it is more sustainable if I ride it instead of taking my car, but by riding it, I'm encouraging its existence, so should I ride the bus or drive to reach a more sustainable solution to transit?