John Tierney suggests that a car trip may be more environmentally sound than walking, or taking a pedicab; after all, human beings have to consume fuel, too. I'm inherently sceptical of these sorts of claims; my impression is that they usually count all of the greenhouse gases emitted in growing and transporting the food, but calculate only the actual carbon emissions from burning the gasoline. Gasoline, however, has to be extracted and transported, too. Obviously, it's more energy dense than food--otherwise, we'd burn apples in our engines. But for all of the moralizing about Guatamalan raspberries we don't ship all or even most of the food we eat from across an ocean. Also, they seem to be very dependent on how many passengers you carry--full cars with high mileage are relatively energy efficient, but they're also relatively rare. Most of the cars I see on the road have a single occupant.
That said, what his numbers do point to is that the net benefit is probably much closer to zero than most of us would suspect.