The pollution problem was never about clean cars...

"Just 15 of the world's biggest ships may now emit as much pollution as all the world's 760m cars"

http://www.treehugger.com/files/2009/04/cargo-ship-emissions-more-than-760-million-cars.php

It's always been about large scale transportation and package movement. Even in city, large transport trucks burn massive amounts of fuel as compared to even a Hummer. I'm sure there's a good chart for this somewhere... I'll look around and post when available...