MPGPP - self-indulgent number play or realistic rethinking of actual energy use?

Member biodiversivist has a post over on Grist where s/he talks about MPGPP and how it’s more accurate than the standard MPG. MPGPP (miles per gallon per person) takes not only the car’s fuel use into account, but also the number of passengers riding in the car at that time. It’s the calculation that makes busses a green mode of transportation, even though anyone who’s ridden a bike behind one knows that they sure do spew out some nasty exhaust.
In some ways, biodiversivist’s MPGPP entry is a bit self-exculpating. But it also makes sense and is a good idea. If we all calculated a monthly MPGPP average, it would help us realize the total impact our transportation choices have on the environment. Each time I ride my bike to the store instead of drive, my MPGPP would increase, and that may encourage me to ride next time, to see how high I can get my MPGPP. [Source: Grist, Tip from the inexhaustible Joel A]