NCPA - National Center for Policy Analysis


June 10, 2009

Public transport may not be as green as you think, say Mikhail Chester and Arpad Horvath, environmental engineers at the University of California at Davis.  There are an array of factors often unknown to the public, such as hidden or displaced emissions that ramp up the simple "tailpipe" tally, which is based on how much carbon is spewed out by the fossil fuels used to make a trip.

The researchers 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 but more green, as only 49 percent of the electricity is derived from fossils.
  • Yet, the "tailpipe" quotient does not include emissions that come from building transport infrastructure or 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.

Another big variable that may be overlooked in green thinking is seat occupancy.  A sedan car or even a 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.

Getting a complete view of the ultimate environmental cost of the type of transport, over its entire lifespan, should help decision-makers to make smarter investments, they conclude.

Source: Editorial, "Think twice about 'green' transport, say scientists,", June 7, 2009; based upon: Mikhail Chester and Arpad Horvath, "Environmental assessment of passenger transportation should include infrastructure and supply chains," Environmental Research Letters, June 2009.

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