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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:

Typically, moving a passenger a mile by bus requires roughly 20 percent more energy than moving a passenger around by car. So, just in terms of energy expenditure, bus actually fares worse than car.

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?

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    This might be on topic to debunk notable claims related to sustainable living (I'm not sure)... but honestly, the folks over at Skeptics SE are better suited to dig into the meat behind these claims. They'll want some cited sources though. They wont engage in the wild conjecture this type of hearsay this is likely to attract. – Robert Cartaino Jan 30 '13 at 16:56
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    @RobertCartaino: Indeed; the conjucture without references makes this a difficult question to answer; it would be better if it focused on how the relative impacts are measured. – Martijn Pieters Jan 30 '13 at 17:38
  • I've revised the question, see if you feel it is better now? – WilliamKF Jan 30 '13 at 18:10
  • It's to be on Skeptics.SE. However, it doesn't seem reliable, since 10 persons may sit in bus maybe in night hours, but in peek hours it's from 50 to 150 – Danubian Sailor Jan 30 '13 at 18:49
  • @lechlukasz: it depends on area. In US, where most people prefer cars, the average could be ten people. In India, average is probably bigger than the capacity of standing people for which the bus was designed! – Pavel Jan 30 '13 at 23:04