Wednesday, November 6, 2013

No, driving is not better for the environment than walking

Timothy Taylor, citing Richard B. McKenzie, says you can make “a plausible argument” that driving is environmentally friendlier than going by foot.

The argument hinges on two premises. The first is that the human body is less efficient at converting fuel to energy than an internal combustion engine. That sounds plausible; I don’t doubt it. The second is that our food production system is a major source of global warming. Think tractors, cow farts, fertilizer, 18-wheelers, packaging and distribution, the whole shebang from soil to plate. Also, the complexity of the food system means it is incredibly lossy: According to McKenzie, only 1.3 percent of the energy used to produce food is output as human activity.

You can see where this is going: The inefficiencies in food production are so massive, and human energy output so low, that gasoline-based transport starts to look good by comparison. McKenzie outsources his ultimate conclusion to Derek Dunn-Rankin, described as an engineering professor “and an avid environmentalist": 

“[A] 180-pound person walking one mile to and from work at a pace of two miles per hour will burn 200 calories above the 2,000 calories burned each day to maintain the body's basic metabolism. However, the production of those 200 calories in food takes fifteen to twenty times as much energy in the form of fossil fuels. This means that driving a high fuel economy car (40 miles per gallon) will use, in fossil fuel energy, only about two-thirds to one half the energy that the person uses in replacing the calories expended on walks. … Energy use and pollution do not have a one-to-one correspondence, which causes Dunn-Rankin to conclude, "My bottom line would be that walking can be 1.5 to 2 times more polluting than driving (if you use a high mileage car). If you use a monster car, you are better off walking always.”

The 19 footnotes in McKenzie’s post give it an air of rigor, and his conclusion is just the sort of thing that could go viral, given its counterintuitive frisson. “Stupid environmentalists,” I can hear conservatives saying, “always complaining about cars and suburbs. Don’t they know it’s been proven that driving is better for the environment than walking?”

And that would be a shame, because it’s nonsense. Here are three reasons why. 

First, if you’re going to look at the entire food chain to determine the energy cost of human walking, it’s only fair to do the same thing for the car. That means you have to factor in the energy costs of producing it. It’s not clear how much that adds, but it’s significant: Estimates range from 10 percent to 100 percent and everywhere in between. I’m not going to speculate on the exact number, but if it’s 50 percent to 100 percent, then we’ve just put walking right back on par with driving, mile for mile.

Second, Dunn-Rankin’s result depends on the use of a high-mpg car, around 40 miles per gallon. If you drive a pickup truck or an SUV, your mileage is worse, and you have to adjust the figures accordingly. 

Third, doing this calculation on a per-mile basis ignores the obvious and important point that people typically drive much farther than they walk. No one buys a car to go half a mile here, a quarter-mile there. Moreover, land use patterns change as cars become more prevalent in society – you get less density and more suburban sprawl. To see the real impact of driving vs. walking, you have to take that into account.

Suppose someone walks a mile to work each way – a 20 to 30 minute journey, not unreasonable. Compare that with the average U.S. commute, which was 16 miles one way as of 2005. Each day the walker travels 2 miles, the driver 32. Thus, even if you accept the notion that walking is twice as bad for the environment as driving, the average driving commuter still does eight times the daily damage of a walker.

And again, that's for someone driving a car that gets 40 miles per gallon. If the driver is in a pickup getting half that mileage, the commute is 16 times worse for the environment than walking.

It’s boring and obvious to suggest that walking is gentler on the Earth than driving. Unfortunately, it also seems to be true.


  1. A big problem that leaps out to me is that if the average adult walked an extra five miles a day, he wouldn't need any extra calories. The average adult could walk five more miles, eat the same amount of food, and never "run out" of calories. In fact, I'm quite confident he would still be over his ideal weight.

    The human body just doesn't work like a car, where if you do more, you must have more fuel or you'll run out. Our metabolism is very adjustable, an advantage in historical times, a disadvantage today in modern countries.

    One of the top scientists in this area is T. Colin Campbell of Cornell. He headed the Cornell-Oxford China Study, the largest and best epidemiological study in history, and he has a long decorated career. Here's what he wrote in his 2006 book, The China Study:

    How can it be that even the least active Chinese [in the 1980s] consume even more calories [than Americans per pound of bodyweight], yet have no overweight problems? What is their secret?...

    ...I have a more comprehensive interpretation that is based on our own considerable research and on the studies of others. It goes like this. Provided we aren't restricting our calorie intake, those of us who consume a high fat, high protein diet simply retain more calories than we need...

    ...The body employs a delicate balancing act and some very intricate mechanisms in deciding how to use the calories being consumed. When we treat our body well by eating the right foods, it knows how to partition the calories away from body fat and into the more desirable functions...or just disposing of any excess.

    – Pages 99-101.

  2. To do such a comparison correctly you would also have to account for the public infrastructure which makes driving possible vs the infrastructure which makes walking possible. The driving infrastructure is far more expensive, particularly considering that much of the walking infrastructure is still needed to get to and from the car.

    A Prius gets only around 25mpg on short trips (eg 2 miles, 5 minutes). Hybrids are relatively good at stop & go, and warm up quicker, but their mileage is still strongly affected.

    To get 40mpg in a car you have to be going along non-stop fairly fast, at which point bicycling would become the only reasonable human-powered comparison

    Bicycling at 10-15mph takes about the same energy per minute as walking, for 3-4x the distance.

    Further walking is exercise. When I have a driving commute I tend to slowly get fatter. When I have a walking/transit commute I don't.

  3. Also, the estimates of the energy costs of producing a car only include the energy expended by the manufacturing plants and transporting the vehicles. They don't include the calories burned by the employees of the auto industry and dealerships.

  4. The blanket claim 'driving is better than going by foot' is clearly nonsense, but I think Timothy Taylor and Richard McKenzie are making a more intriguing claim - that for short journeys of a mile or less, driving is kinder to the environment than walking. I'm not sure their claim holds up, but I think your critique misses the mark. For example, you can see straight away that your 3rd point is completely irrelevant to this decision.

    A key point, which I think you have missed, is that this is a choice at the margin. It assumes you already own a car. So choosing to use it leads to no extra costs along the manufacturing chain, making your first point irrelevant.

    The pro-car argument does depend on the car, of course, as you say - Tim Taylor makes this point too. I think if the claim fails, it will be because it misrepresents how our metabolism works, as Richard says, or because cars are a lot less efficient on short journeys, as Jeffrey hints at. I agree with him that bicycles are probably the way to go here.

  5. I suspect their claim is BS. Pimentel and Pimentel have done a lot of work on this, and they give figures for the energy costs of food that are (usually) nowhere near as high as the 15-20x claimed. For calories of beef protein, yes, those factors apply, but the fat content alone of many meat cuts that ratio back a factor of two or more.

    Another quick sanity check is the price of the food; if the energy inputs are 15 times the energy content in the food, you should see that in the price of the food. For example, a gallon of vegetable oil should cost 15 times the cost of a gallon of gasoline, or a gallon of peanut butter should cost 15 times that cost, etc (the energy content of vegetable oil, peanut butter, and gasoline is roughly the same, around 30kCal/gallon). Nope, a gallon of vegetable oil is 7 bucks online at Walmart.

    4.5 kilos of sugar is $5.50, about 60% of the calorie content of a gallon of oil, or about $9.20.

    Let's try rice -- 3650 kCal per kilogram, 8.2kg to make 30kCal, or 18 pounds. 20 pound bag at Walmart is $10.

    Oats (notable for their high ERoEI of 5.1:1 in P&P), 3800 kCal/kilo. 7.9 kg, 17.5lbs, 10lbs is $8 at CostCo, so $14 for an equivalent amount.

    Okay, how about Costco Organic Extra Virgin Olive Oil? 2 liters, $17.43 for that, or $33 for the energy content of a gallon of gasoline. Knock the gasoline price down to $3 because of taxes, that's only 11x higher. For organic, extra virgin olive oil.

    Sorry, but this is a bunch of truthy contrarian BS. They're cherry-picking. I've also tested this anecdotally, I do about 2500 kCal worth of utility biking each week, and I eat somewhat more, but it also lowers the set point on my weight, and I don't fill in with meat (in particular, using only protein for those extra calories is not healthy). All else is NOT equal.

    There's a second reason this is truthy contrarian BS -- at 2500kCal of exercise per week, I get *enough* exercise. To be healthy, I need to burn those calories through exercise, one way or another (I could, instead, choose to be fatter and die sooner, but somehow I don't like that choice). If I don't walk, if I don't bike, I will do something else that will burn those same calories (and don't forget to add in the energy cost of the A/C at the gym).

    That is, just because walking requires food to fuel it, and that food has an upstream energy cost, you don't get to count that energy as "saved" just because someone drives instead of walks. If they have a rational desire to stay healthy, they will burn those calories some other way through exercise -- no svaings result.