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Environmental engineers Mikhail Chester and Arpad Horvath at the University of California at Davis (woot for UCD) recently did a study on environmental impacts of various modes of transport.



 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.




The pair give an example of how the use of oil, gas or coal to generate electricity to power trains can skew the picture.


Boston has a metro system with high energy efficiency. The trouble is, 82 percent of the energy to drive it comes from dirty fossil fuels.


By comparison, San Francisco's local railway is less energy-efficient than Boston's. But it turns out to be rather greener, as only 49 percent of the electricity is derived from fossils.


The paper points out that the "tailpipe" quotient does not include emissions that come from building transport infrastructure -- railways, airport terminals, roads and so on -- nor the emissions that come from maintaining this infrastructure over its operational lifetime.


These often-unacknowledged factors add substantially to the global-warming burden.


In fact, they add 63 percent to the "tailpipe" emissions of a car, 31 percent to those of a plane, and 55 percent to those of a train.


And another big variable that may be overlooked in green thinking is seat occupancy.


A saloon (sedan) car or even an 4x4 that is fully occupied may be responsible for less greenhouse gas per kilometer travelled per person than a suburban train that is a quarter full, the researchers calculate.


Interesting stuff.  The trouble is when you book a seat on a train, you don't know how full it will be beforehand!  Personally when I've taken the train, it's always been pretty full.