[T]he question of whether walking (or bicycling) is better or worse than driving has been discussed for more than two decades. The "drive vs. walk" question goes something like this: you're at home and need to go to a store three-quarters of a mile away (1.5 miles roundtrip). Concerned about your potential contribution to climate change and conscious that the production and transportation of the calories you'll burn in walking or bicycling can be very carbon-intensive, you wonder if driving to the store would release fewer greenhouse gases.
In his book, How to Live a Low-Carbon Life, Chris Goodall answers the question this way: "It makes more sense to drive than walk, if walking means you need to eat more to replace the energy lost." [...]
The media have already repeated Goodall's claims, with limited or no critical analysis. The Times (U.K.) stated: "Walking does more than driving to cause global warming, a leading environmentalist has calculated."
The Pacific Institute has just released their calculations for the walk vs. drive question, and the winner in terms of greenhouse gas emissions is: it depends. (That conclusion shouldn't surprise anyone who is familiar with comparative life-cycle analyses.)
According to the Pacific Institute, Goodall's calculations compared the greenhouse gas emissions associated with one of the most energy-intensive foods (top sirloin beef produced in Japan) with greenhouse gas emissions from a gasoline-fueled car that gets ~30.3 mpg.
In a more nuanced analysis, you could think of this more like a typical energy efficiency calculation, with the overall greenhouse gas emissions by mode a factor of both the carbon-intensity of the fuel/food and the fuel economy of the machine/person. We know that most humans (at least those who have not specialized in walking to the store and back) are not all that fuel-efficient themselves--we're constantly burning calories to run non-essential functions, like cognitive processes that allow us to invent cars and drive them. We also already know that fueling cars with corn ethanol is a losing proposition, so it's not shocking that feeding corn* to beef to fuel people to walk is no silver greenhouse gas bullet, either.
What about feeding a more average diet to an average person to walk, bike (said to be the "most efficient machine on Earth"), or drive an average car under average conditions? Pacific Institute calculates that "a typical person walking 1.5 miles in the U.S. would generate less than 25% of the GHG that would be generated if that person drove the same distance." Their calculation includes interesting assumptions such as beginning the automobile trip with a cold engine and the extra caloric demands on drivers having to navigate through congested traffic. The details are important, and in this case a good read. Unfortunately Pacific Institute doesn't appear to have run the specific calculations for pedicabs, which John Tierney requested on his blog earlier this year, but they do look at milk and other food sources.
The assumptions are also revealing. Goodall makes no secret of his agenda, which is to draw attention to the carbon-intensity of meat and dairy production. Livestock are thought to be comparable to transportation in greenhouse gas emissions at a global level. Car-lovers, car-haters, vegetarians, beef-eaters, environmental do-gooders, evil-dooers, and cynics will each have their own spin on these calculations. Mine is (still) that this is just yet another justification for making sure individuals are free to make their own decisions on how to personally economize on energy- and carbon-intensive activities--and possibly that my more physically-fit friends should run errands to the store for me in the future.
(*In the Japanese beef example, the animals were fed a mix of corn, wheat, soy and alfalfa.)