If you walk 1.5 miles, Mr. Goodall calculates, and replace those calories by drinking about a cup of milk, the greenhouse emissions connected with that milk (like methane from the dairy farm and carbon dioxide from the delivery truck) are just about equal to the emissions from a typical car making the same trip. And if there were two of you making the trip, then the car would definitely be the more planet-friendly way to go.
These results would vary, of course, depending on exactly what kind of car you’re using and what kind of food you eat (or, if you’re going by pedicab, what kind of food your cabbie eats). Michael Bluejay, who’s done some number-crunching at BicycleUniverse.info, says that walking is actually worse than driving if you replace the calories with food in the standard American diet and if the car gets more than 24 miles per gallon. He calculates that bicycling is a win for the environment because it’s 117 percent more efficient (in calories expended per distance) than walking is, but he’s assuming one cyclist on a regular streamlined bike, not a cabbie pulling two other people in a pedicab dealing with lot more friction and air resistance.
Pointer via Freakonomics.
I know I for one never ever eat on days when I drive.