David R. Henderson  

McKenzie on Driving vs. Walking

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Which is more polluting--driving a mile to work or walking that mile? The easy answer is, of course, driving. Cars have tailpipes; people don't. Far more energy is needed to push a 3,000-pound car along the road than is needed to move a 150- to 250-pound body along a sidewalk. Walking seems like the green thing to do.

But appearances can be deceiving, making easy answers dead wrong. That's the case here when the calories expended in walking are replaced.


This is how UC Irvine economist Richard B. McKenzie begins his November Econlib Feature Article, "Why Walking to Work Can be More Polluting Than Driving to Work."

His reasoning is that a huge amount of energy is lost in the food supply chain. And, even when the food enters our bodies, "Only about 15 percent of the potential energy in food eaten goes into activities such as walking, as well as maintaining all bodily functions."

Put those two factors together and McKenzie concludes:

This means that the energy that the human body actually converts into work is meager percentage-wise--something on the order of 1.3 percent of the fossil fuel energy that is used along the entire length of the food-supply chain.

McKenzie's conclusion:
When it comes to energy use and greenhouse gases emitted, appearances can be grossly deceiving. Granted, people who drive everywhere are energy users and polluters. But walkers also use fossil fuels through the food they eat to replace the calories burned while walking. Of course, driving can be more polluting under some circumstances, such as when large SUVs are the preferred vehicles or when drivers insist on doing wheelies at every stoplight. Bicycling the distance can also be less polluting than driving. [Derek] Dunn-Rankin [who, McKenzie tells us, is "a professor of engineering at the University of California, Irvine and an avid environmentalist] sums up the central, largely counterintuitive, point of this commentary: "Driving a small [or moderate-size] car and not having to replace burned calories saves more energy (and greenhouse gases) than walking when the extra calories expended are replaced."


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COMMENTS (37 to date)
Taeyoung writes:

The worst greenhouse gas emitters of all, clearly, are people who exercise in gyms and accomplish no productive work at all for all their caloric expenditure. For shame!

Meets writes:

To people who walk really eat more?

Ben Kennedy writes:

The article isn't entirely clear on this - does it include the energy costs of both the fossil fuel supply chain and automobile supply chain when considering the energy cost of driving? Given that he is using these things as arguments against electric cars, the same cost should at least be added to the non-electric case

John Voorheis writes:

Some things:

a) Almost everyone in the US probably shouldn't be replacing the calories expended on that walk, right? In fact, if people are choosing to walk for health reasons, they probably aren't.

b) People who are more likely to choose walking over driving are also more likely to be vegetarian, and choose to eat organic produce (which has no conventional fertilizer inputs). I would be very surprised if the energy inputs in the food actually consumed by the average walker are the same as the unconditional average energy inputs of food production.

c) Then again, the average for-environmental-reasons-walker probably owns a more fuel efficient car.

d) This is conditional on already owning a car, though, right? If the choice is "buy a car and drive it" vs. "walk everywhere and maybe use transit" the calculus is different, no?

Arthur_500 writes:

With the text taken out of context I cannot say anything about the original purpose of the text. I can appreciate that there are other costs to an activity that may not be taken into account.

For example, everyone loves electric cars because they utilize no fossil fuels. But wait! That electricity came from somewhere. Those who endorse electric over the internal combustion engine have to recognize all the costs and that is often missing from the supportive arguments.

Without getting into the details of walking for chores versus driving, I think it is instructive that we look at all the costs when we want to make arguments. To me, that is the value of this text.

Finch writes:

> To people who walk really eat more?

Controlling for other things, I'd guess that they do. The control seems pretty important because the obese probably walk a lot less than the skinny for reasons that contribute to their being obese rather than skinny.

It's pretty well established that while exercise can change body composition and improve various health markers, it doesn't generally cause you to lose weight. That must be because it's causing you to eat more and/or rest more. Anecdotally, exercise certainly causes me to both eat more and rest more, but I'm generally doing things more intense than walking and one might worry about some threshold in response. You'll even hear advanced athletes claim that the kind of exercise they do influences the kind of food they want, shifting from carbs to proteins as the exercise shifts from light endurance work to heavy load-bearing work. That last bit might be bro science, but I heard an exercise physiology & nutrition professor claim it in a podcast yesterday.

Bob Knaus writes:

Even worse, all that walking makes you live longer! So there you are, burning up calories and emitting greenhouse gases until you are 100, when you would have croaked long ago if only you had driven everywhere.

From this "By way of contrast, although the gasoline-power engine is not a paragon of greenness, its energy efficiency is substantially higher in moving people and things from one point to another, with 14 to 30 percent of the potential energy in gasoline actually moving cars." it sounds like McKenzie is ignoring all the loses in the oil supply chain. From energy used to pump the oil out of the ground, to the tankers that float it around the world, to the refinery to the trucks that haul the finished gas to the gas station.

Talk about stacking the deck.

John Hall writes:

This is the sort of thing that made sense when I first read it, but after thinking about it more it doesn't make as much sense.

What if the person eats the same amount of food everyday (like a dieter who is calorie counting or just is habitual with food regardless of activity) or does not have control over meal portions (maybe someone else prepare their meals)? I don't see why you would count the impact of the energy cost of food on their walk vs. drive decision.

In addition, if you walk to the store instead of drive, then you might not do more exercise later that day. However, if you drive, then you might think to yourself that you still need to do some exercise and hit the gym on the what back (do you then count that energy?).

You would also need to account for the number of people in the car and the calories used while driving to get a proper accounting.

Jim writes:

Except that people do indeed have tailpipes. Sewage treatment can be a significant source of methane and nitrous oxide emissions. Not to mention the general health and environmental hazards presented by dealing with human waste in general. More calories used > more food through-put > more pollution.

Mike Rulle writes:

The professor is just pulling the leg of the climate change alarmists---right?

Do people still believe in the "danger" of "climate change"?

Our collective actions certainly suggest otherwise.

MingoV writes:

We burn approximately 1200 calories per day just to survive. A typical person's daily activity doubles the calories burned per day. A 180 pound person who walks a mile in 20 minutes burns 200 calories. That's the value to use when comparing walking to driving. A car that gets 20 mpg will use 1570 calories to travel a mile. McKenzie's comparisons are way off base.

Gee writes:

Shows why you should never trust an economist. They focus laser-like on first order effects and completely miss the rest of observable reality. Who eats more: the walker or the fatass who drives everywhere? While I'm in the car, why don't I go ahead and stop at Taco Bell for dinner

Jesse writes:

MingoV - That is exactly his point. You are comparing tank to wheels (or mouth to feet) Calories, not the overall supply chain. The Fossil fuel supply chain is very efficient at delivering Calories of oil to the tanks of cars - Probably over 80% efficient if figures regarding fossil fuel emissions of the oil industry (vs. the emissions from the product) are to be believed, whereas I have heard 10 or 20 times the fossil fuel input is typical for agriculture vs. the calories of the food actually produced.
Organic only helps so much, as organic tends to have larger land use and labour inputs. Labour produces emissions for the same reasons as walking does... and organic does not exclude things like tractors, or the fossil fuels used to supply irrigation water, combine that with lower yields and the net result is arguably not that different.

Ken P writes:

MingoV wrote- "A 180 pound person who walks a mile in 20 minutes burns 200 calories. That's the value to use when comparing walking to driving. A car that gets 20 mpg will use 1570 calories to travel a mile. McKenzie's comparisons are way off base."

IN addition to the supply chain aspect, you need to subtract all the calories that end up in waste to determine efficiency of the final fuel. Also, much of the unburned calories get turned into methane.

Andrew_FL writes:

@John Voorheis-"Almost everyone in the US probably shouldn't be replacing the calories expended on that walk, right? In fact, if people are choosing to walk for health reasons, they probably aren't."

They absolutely should replace most of the calories and not replace a vanishingly small portion of the calories they burn even if their goal is to lose weight. After all, at some point they wish to stabilize at a weight that is lower, not keep losing weight until they drop dead. So in the long run the successful walker must be replacing every calorie they burn walking. The unsuccessful walker will end up still gaining weight because he more than replaces his calories. But the walker who never reaches the point of exactly replacing his calories dies.

Now, if people are walking for health reasons, that is a good enough reason for them to choose to do it for themselves. But if one wishes to suggest that everyone-even people not aiming to lose weight-walk everywhere, one is suggesting total caloric intake ultimately should increase since most people will just replace the calories.

KLO writes:

The best available evidence is that people who walk a lot do not consume more energy per day than the sedentary. A study of hunter gatherers showed that, while they were much more active than office workers during peak periods of activity, they did not actually consume more energy per day. The proposed explanation was that the hunter gatherers compensated for higher peak energy usage by resting more than office workers.

What's more, energy consumption is highly correlated with weight, but not activity level. Unless you are talking about unusually high activity levels, there is very little relationship between how much a person walks and how much total energy that person consumes

Greg Jaxon writes:

Driving is better than walking.

The empirical evidence (that a reasonably free market has evolved to prefer the use of autos) would seem to settle the whole argument.

I personally found that walking a half mile worked better than driving the slightly greater distance to get to work by car: the car's battery died after a few weeks of too-short trips. At another employer, biking 5 miles to work under-performed driving there, since I had to shower before meeting people at work.

In aggregate, market participants have made all such experiments and (in aggregate) they spend more on motoring than on shoes etc.

This method of proof would be adequate if no prices are distorted by green taxes, farm subsidies, fuel taxes, foreign aid to oil producers, government-owned car companies, subsidized urban mass transit, etc...

Total cost analysis arguments are as futile as trying to explain how a pencil is made.

Pedro Albuquerque writes:

My personal experience and observation tells me that the correlation between the amount of calories ingested and walking to work is close to zero, and most probably negative. If you live a healthy, non-automobile-user lifestyle you are most surely lighter than the median consumer, and pure physics and chemistry dictates that you need lower levels of caloric ingestion because you don't need to support all the deadweight linked to obesity. At least this is exactly what happens to me when I keep my weight low.
Using automobiles, on the contrary, not only creates all kinds of negative externalities, but surely is a strong contributor to increased body mass and, consequently, caloric ingestion.
There's so much nonsense in this study that it deserves a nomination for the Ig Nobel Prize.

Pedro Albuquerque writes:

Oops, correction: "theory and observation tell us"...

Pedro Albuquerque writes:

Oops again: "experience and observation tell me"...

Finch writes:

Many of the commenters (MingoV, Gee, Pedro, KLO, etc) are saying that people who walk eat less than sedentary folk. Of course they do. Just as runners are skinny. But that doesn't mean running causes skinniness, or at least that the causality runs one way only, it just means it's hard to be a runner if you aren't skinny. In that sense, skinny causes running. Being light, fit, and healthy causes walking just as it works in the other direction.

If you take an individual and you increase the amount of exercise they get, they'll eat more. So that part of the original analysis is more or less correct. They probably won't eat enough to make up the burnt calories; they'll also rest more (defined broadly enough to include becoming a little more efficient). But they'll certainly eat more. It would be a dumb body design that didn't do this.

The criticism about not considering the energy used in the rest of the gasoline supply chain strikes me as more fruitful.

Finch writes:

When you talk about the effects of exercise you need to be very careful about controlling for the pre-existing characteristics of your subjects.

Yancey Ward writes:

Can't one roughly calculate this by comparing the cost of driving one mile vs walking one mile?

To a rough approximation, driving one mile for me costs the gasoline, or 15 cents; 1 cent for the oil change; 7 cents for the car; 2 cents for the insurance; 1 cent for the taxes and registration costs; and roughly 2 cents for other maintenance. or 28 cents/mile. The IRS allows for reimbursement of business miles at a rate of about 50-60 cents, so I am in the right ball park.

So, how much does the food cost to replace the calories expended?

Andrew_FL writes:

@Yancey Ward-The cost in dollars is not the same thing as the amount of emissions from each activity.

Okay looking at a lot of the comments I think a lot of people are missing the point:

For example, a common thread is:

People who *currently* walk rather than drive places are either health conscious or environment conscious individuals. So they eat less as well as drive less. Meanwhile people who drive everywhere are uniformly fatasses who eat too much. Therefore, if everyone walked instead of driving they would reduce the emissions from driving *and* the emissions from the supply chain of food.

What is wrong with this argument? Actually quite a bit! In the first place, it does not follow that people forced to walk instead of drive would somehow become the same as people currently walking in terms of their eating habits. This is a non sequitur. The fact that most people do not walk and instead drive indicates that those people are not the same as health conscious people, and we should expect that generally if forced to burn more calories, they will generally decide to replace those calories by eating more. They will eat when they get hungry and they'll get hungrier the more calories the burn. Second, it is absolutely not true that everyone who drives is unhealthy. Many people who drive are in fact perfectly healthy. When those people are forced to walk more, what do you think they will do, gradually waste away on an inadequate calorie intake, or increase their intake to restore balance?

Krishnan writes:

Overheard at a Global Warming Conference

"See? I knew it. I just knew it. The world at large is in danger of being overrun by all these people - who need food to survive - and worse, they convert this food into CO2. What we need is a world of robots who can convert energy directly from the sun (no conversion to CO2). The world will become safe for humans after we eliminate humans"

Pavel writes:

I am a bit confused here.
I don't own a car for purely practical reasons.
Roughly 7 months a year I take my bicycle to work (roughly 22km/14 miles each way)(rest of year it's too cold/dark so it's public transportation). I am definitely not thin, my BMI is roughly 27. I don't notice myself being hungrier during the warm months, but I do eat more since it's also the barbecue season.
So if I switched from bicycle to car, I would most likely move from slightly overweight to obese and experience a decrease in health.

My position is that what we eat is fairly constant independent of exercising unless we go for extremes.

Yancey Ward writes:

Andrew,

It is going to be a rough approximation since the inputs for both are manufactured within the economy. Just based on that calculation, I would not be a bit surprised if walking is higher emission.

MingoV writes:

@Jessie: I have heard 10 or 20 times the fossil fuel input is typical for agriculture vs. the calories of the food actually produced.

I believe that estimate is far too high, but I'll accept it for the sake of this argument. If you're going to count the calorie cost of transporting food as part of human energy use, then you also have to count the calorie cost of manufacturing and maintaining personal vehicles and the calorie cost of pumping crude oil, shipping it, refining it, distributing it, maintaining the gas stations that sell it, etc. These costs will be much higher for all the personal vehicles than the trains and trucks that transport food.

Finch writes:

> I don't notice myself being hungrier during the
> warm months

Would you notice? People are notoriously bad at estimating their caloric intake.

> My position is that what we eat is fairly constant
> independent of exercising unless we go for
> extremes.

If this were true, one would think it would be a nice solution to the obesity problem. That said, I briefly googled for a cite and couldn't find something strongly supportive of a connection. I found a number of things that were not supportive and said "exercise and energy intake found unconnected in a study of twelve 11-year-old girls." I.e., unsupportive, but not a very powerful test. Many things said "it is commonly believed [that there is a connection]," but that's not really helping. So perhaps the point is not as supported as I thought, and may actually be false. I saw nothing about intense exercise for athletic people, so maybe that's different.

Finch writes:

Were there evidence that just going for a walk would make you eat more to compensate for it, I think I would have found it. So I now think there is not evidence for that. I'm not sure I would read the literature as providing strong evidence that is not true, but there's at least some evidence it is not true.

So McKenzie has a nice story, but you can't have confidence in it.

I'm glad I went to look. All that said, a good training session seems to make me want to fell and eat a horse, so I await a study of trained athletes and various more intense forms of exercise. :)

Greg Jaxon writes:

What is "pollution"?
The article attempts to reduce all costs to units of supposedly "fossil" fuels. The assumption is that fuel use = pollution, despite the considerable variation in combustion end-products of the many processes used across the problem space.
This is an inhumane thing to economize. In classical economics, we serve the human race: we consider the net subjective utilities of two alternatives and rank them.
And the market does this everyday, and currently favors driving as a dominant (but hardly monopoly) method for racking up person-miles.
Taxes pollute my world. They seem to be by-products of every social movement that comes along, sapping net utility by pre-empting the personal choices that would have gone toward further maximizing the sum of human happiness. Some chemicals in my air and water are also pollutants, and arise in similar ways (third party effects I seem unable to control).
Since we haven't yet found a way to curb taxation, by correctly pricing everyone's consent to be taxed (a seemingly computable, addressable problem), I don't see how we're going to leverage that lack of solution to tackle the alleged problem of pollution. Abandoning utility and price as our planning tools in favor of carbon units and a fossilized Malthusian dialectic about fuels running out and Earth being unable to tolerate our civilization is a foolish excursion to inhumane collectivism.

Richard McKenzie writes:

From browsing the comments on my article on walking v. driving, I have the impression that several commenters (here and elsewhere the column has been reposted) believe that I intended some overt, if not hidden, political (even anti-market) message about how people should get to work, or about environmental policy. Not the case at all, as readers who know me will surely attest. I intended only to make a counter-intuitive argument, as it was for me when I first stumbled on it a number of years ago: There are circumstances under which walking short distances can be more polluting driving, with everything written conditioned by the presumption that the walkers replace the calories expended. I conclude with as many circumstances under which walking could be less polluting than driving as space would allow. Those who think I am unaware of the country's weight problems and the various benefits of walking should take a look at my book on the unheralded economic explanations for, and economic consequences of, the country's weight issues. In my article I seek to stress the ways in which appearances can be deceiving, more so than many seem to think. Because of space limitations I could not make the argument exhaustive. Unfortunately, I can't make my response complete either, for good old economic reasons.

Krishnan writes:

@Richard McKenzie

I agree that so much of what "we believe to be true" is simply not (!) - and counter-intuitive arguments are always, ALWAYS good ... My take on your piece was to highlight how everything gets politicized - and today, anything that indicates that "humans are evil" (or a close approximation) is picked up by environmentalists ... Sure, you are pointing out that in some instances, driving may be better ... but I suspect that some will conclude that the world will be better if we can build robots that can generate their own energy using photosynthesis - so use sunlight and reduce CO2 (unlike humans who create CO2) (wonderful as those robots could be, we are far from that state of nirvana - but yet environmentalists insist on demanding policies that would harm those that are living today)

Jesse writes:

@ MingoV

If you're going to count the calorie cost of transporting food as part of human energy use, then you also have to count the calorie cost of manufacturing and maintaining personal vehicles and the calorie cost of pumping crude oil, shipping it, refining it, distributing it, maintaining the gas stations that sell it, etc. These costs will be much higher for all the personal vehicles than the trains and trucks that transport food.

The estimates i have seen are around 80% efficiency of well to tank for oil. This includes all direct emissions of transport. You can make arguments for and against including general mtce, and economy wide impacts in any given scenario. If you do the most reasonable way is probably to multiply the cost of all indirect inputs (excluding all taxes) by the economy wide rate of carbon emissions per dollar GDP. Not sure where this would end out, but a reasonable question to ask then would be what is the cost per mile of the calories required to walk? 200 Calories would be something like 1/3 of a fast burger - or a dollar or so per mile? To be accurate, the direct energy inputs would need to be accounted for and subtracted out of the price, and only the indirect accounted for this way.

Richard Mckenzie writes:

I hope readers of this blog will actually go to my full article and read it:
Several readers have commented about how many walkers may not replace (and maybe should not replace) the calories burned in walks. Such comments miss my objective, which was to point out that walking CAN BE more polluting WHEN the calories burned are replaced. I had no intention to advocate driving or walking for weight loss, health, or other reasons. My analysis is what it is.
The more serious concern raised by commenters is that I did not include an assessment of the energy used, and pollution emitted, in the “car-supply chain.” I stand corrected for not saying more. However, the subject of my article had to be constrained for purposes of focus and clarity of argument. I did not intend a definitive discussion, and I could not I have provided one had I set out to do so. Complex systems are what they are, complex – and, in the case of the energy system, extraordinarily complex. In some regard, the problem at issue is that of “where do you truncate the analysis?” Should I have included an assessment of the energy used in producing farm equipment and the trucks that deliver groceries to stores? If I include the energy used in producing cars in the analysis, should I have extended the analysis to include an assessment of the energy used in the development of walkers’ bodies? I am not so sure there is an end to it all, while, at the same time, I grant that focusing on just the gasoline used in driving is probably narrower than it should have been.
Having conceded points, my objective was to suggest a number of ways that comparing the gasoline used and the resulting gases emitted in the act of driving with the total lack of gasoline used in the act of walking is myopic and untenable. Calories are burned in walking, and it takes energy to produce calories (contrary to the protestations of several commenters). That means that there can be some gases associated with walking, and my summary analysis understated the energy used and gases emitted because I did not add the energy used to produce the food that doesn’t make it to people’s stomach or the multitude of gases released by farm animals (although I make note of them).
I hope readers will see that I don’t intend here to push a political or environmental agenda, and I have tried in my article and here to stay away from the global warming controversy. I don’t advocate or endorse either side, at least not in my article or here. I am simply trying to suggest here that people who are confident that driving is everywhere and under all circumstances more polluting than walking should reconsider, maybe try a hand at making a more complete comparison than I provided within the space allowed.

Richard McKenzie writes:

One point I forgot to cover in my last response: One commenter mentioned that walking a mile requires 200 calories (above the normal daily expenditure)while driving the mile requires 1,570. I don't contest the numbers, but I don't think that is the comparison I was trying to press.

I won't go into detail here, but my issue is really how much energy is expended in moving the car the distance and expended in the food-supply chain when the expended calories on the walk are replaced. In my article, I try to explain various ways a lot of fuel is used in the production, distribution, and consumption of foods, and then not all of the food calories produced, distributed, and consumed are actually used in walking (or other activities). Seen this way, walking is not as "energy light" as many seem to think.

And then there is the pollution issue. My point is that there are a lot of greenhouse gases that are emitted in the food-supply chain that are not commonly associated with walking but need to be considered in making a fair comparison (and do note my concession on making more complete comparriisons in my earlier responses).

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