Looking for something else, I came across a web page that makes this rather startling claim:
[W]alking actually uses more fossil energy than driving, if the calories burned from walking come from a typical American diet.
The crux of the claim is that the North American food system uses so much fossil fuel -- for manufacturing fertilizer and pesticides, running farm machinery, transporting food from farm fields to stores and homes, powering refrigerators and stoves, etc. -- that producing the food calories to power a one mile walk uses up more fossil fuel energy than a typical car burns in a one-mile drive.
That seemed counterintuitive, to be sure -- but not completely ridiculous. So I spent some time looking at the issues.
As far as I can tell, the web page is probably wrong: walking is more energy-efficient than driving.
However, they're closer than I might have thought.
Some quick calculations: according to the Earth Policy Institute, the U.S. food system consumed about 10.25 quadrillion BTUs in fossil fuels in 2002 -- the energy equivalent of about 3/4 of a gallon of gas for every American, each and every day. (Very little of that energy is actually gasoline, by the way.) Based on this, the U.S. food system burns about six to seven times as many calories of fossil fuel per day as we consume in food.
In other words, we don't just eat food; we also eat oil, coal, and natural gas.
(There's some uncertainty in the figures, to be sure; and I'm still trying to figure out a wrinkle or two in the data. I'll post more if I find anything interesting.)
Obviously, your mileage may vary. If you eat lots of grain-fed meat, in particular, you're likely to consume more energy in your diet. Then again, there's practically no diet that's truly benign; there are just darker and lighter shades of gray.
As for walking: for a 150 pound person, walking a mile burns about 43 calories above and beyond what the body would burn just loafing around. (See, e.g., here for calories per minute per pound of body mass for walking; and see here for what you burn while watching the tube.) Doing the math -- and accounting for the fact that about three food calories are wasted for every seven that are actually consumed (see, e.g, fig. 10, p. 30 of this big ol' pdf) -- I get that a person who walks a mile gets the equivalent of about 75 mpg.
That is, if you lump together all the fossil fuels that go into growing, transporting, selling, storing, and cooking your food, the human body uses a little less fossil fuel, mile for mile, than a high-tech Honda Insight.
However, walking is arguably less efficient -- in per-passenger terms, at least -- than an Insight with someone riding shotgun. In fact, in one way of looking at things, walking a mile is about as fuel efficient as driving a 15 mpg SUV with all five seats filled. Go figure.
Still, if you're weighing whether to walk to the store or drive, walking is clearly the environmental winner -- and it's healthier, to boot. And more to the point, a neighborhood where you can do lots of your chores conveniently on foot is bound to be a fuel-efficient place -- not just because you can walk or bike (which is more efficient than driving), but also because, day to day, you don't have to travel far to get to where you need to go.
And the real efficiency comes from arranging our lives so that we don't have to travel so far every day. That's where the rubber (or shoe leather) really meets the road.
Comments
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Biodiversivist Posted 7:10 am
15 Aug 2006
In the end, it all comes down to biodiversity. Help acquire and protect ecological hotspots, give to a conservation organization: http://www.saveourbiodiversity.com
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plantface Posted 7:54 am
15 Aug 2006
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Bensch Posted 8:27 am
15 Aug 2006
First up, the users of that vehicle are still burning calories while driving. And their average speed for a short trip must include time spent at stoplights and parking - time must be calculated door to door. It's also unrealistic to compare, say, a five minute drive to a fifteen minute walk - while the pedestrian might be living a little longer, at least nine of those ten minutes are calories they don't burn for the rest of their life.
Second, the oil necessary to build the car and to build and maintain the roads it's on account for much more than we seem to be considering.
Third, that pedestrian is a lot more likely to be buying organic or local - throwing quite the monkey wrench in that walking average.
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Laurence Aurbach Posted 12:23 pm
15 Aug 2006
Riding in a vehicle: 1 kcal/kg/hr
Driving a car or light truck: 2 kcal/kg/hr
Driving a heavy truck: 3 kcal/kg/hr
Slow strolling: 2 kcal/kg/hr
Walking, 3.0 mph, level, moderate pace: 3.3 kcal/kg/hr
Walking, 3.5 mph, level, brisk pace: 3.8 kcal/kg/hr
(Divide each of these by 2.2 to convert to kcal/lb/hr)
And for fun (values listed in (kcal/kg/hr):
Sailing, general: 3
Skateboarding: 5
Kayaking: 5
bicycling, 10-11.9 mph, light effort: 6
Horse racing, trotting: 6.5
Jet skiing: 7
Bicycling, 12-13.9 mph, moderate effort: 8
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Laurence Aurbach Posted 3:31 pm
15 Aug 2006
Walking a mile burns 43 calories.
About 29% of the food supply is wasted, so walking a mile actually consumes 61 calories.
It takes seven calories of fossil fuel to produce one calorie of food, so it actually takes 427 calories of fossil fuel to walk a mile.
427 nutritional calories of fossil fuel is equivalent to 1694 Btu. It takes 1694 Btu to walk a mile.
How many Btu does it take to move a car one mile? We've used "fields to feet" accounting for walking, so let's use "wells to wheels" accounting for driving, just to be consistent. A standard-engine gasoline vehicle uses 6000 BTu to travel one mile. A typical gasoline vehicle uses 350% more Btu than a walking person to travel one mile.
(Please correct my calculations if you see any errors.)
Following on Bensch's excellent comment, one should consider the energy costs not only of building and maintaining roads and highways, but also of parking (5-9 spaces per vehicle), infrastructure (drainage systems, lighting, etc.), policing, emergency, and medical response (43,000 killed and 2.7 million injured annually in U.S. traffic crashes).
Furthermore, dedicating more and more urban land to vehicles results in more sprawling, less dense cities where it is less pleasant and convenient to walk and the modal share of walking (and biking and transit) is reduced. The characteristics of transportation infrastructure have synergistic and cascading effects on travel choices and behavior. Those in turn affect the overall energy efficiency of the tranportation system.
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KathyF Posted 5:32 pm
15 Aug 2006
So I'll keep on walking, thanks.
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caniscandida Posted 6:37 pm
15 Aug 2006
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bookerly Posted 9:11 pm
15 Aug 2006
The comparison also misses energy spent on increased medical costs due to obesity and lack of exercise (for drivers much higher than for regular walkers).
There is also the matter of particle pollution from rubber tires and from brakes.
And the increased asthma and other breathing diseases (all of which require energy to treat).
The energy required to raise food generally contributes less to global warming than that required to move a car.
This kind of statistical analysis is akin to comparing apples and watermelons. It gives science a bad name. Shame.
patrick
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amazingdrx Posted 9:30 pm
15 Aug 2006
Stuff a porkchop in it Limbaugh fans.
Last time I checked the vast majority of us exersize and healthy food addicts don't want anything to do with fossil fueled, chem food.
http://amazngdrx.blogharbor.com/blog
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mihan Posted 4:53 am
16 Aug 2006
With the numbers given by plantface, this would mean that, biking at 5.5 mph, the woman would burn (36 cal/10 min)/(5.5 mi/60 min)=39 cal/mi. She would burn 77 cal/mi walking 3.5 mph.
Leaving aside the question of who exactly bikes as slow as 5.5 mph or walks as fast as 3.5 mph, it seems that by distance, biking burns fewer calories. I don't know if this makes sense: the wheels allow you to coast where your tired feet would have no choice but to plod along, but on uphills, you have to raise not only you but also your bike up the hill. I'd have to see more documentation.
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mihan Posted 5:06 am
16 Aug 2006
You burn the least amount of energy walking 3 mph or biking 10-12 mph.
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redjenny Posted 7:43 am
16 Aug 2006
Also, I don't know any environmentalist who promotes big-agra farming... Most of us want to improve the efficiency of transportation AND food production/distribution.
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redjenny Posted 7:51 am
16 Aug 2006
Also, have you see the claims made by the Department of Energy on efficiency of various modes of transportation as reported on wikipedia: en.wikipedia.org/wiki/Fuel_efficiency (scroll down the page to the chart)? According to this, driving is more efficient than taking the bus or intercity train. Does this make sense?
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bookerly Posted 10:44 am
16 Aug 2006
Mihan, it takes longer to walk the same distance than to bike it. So, biking should burn less calories. If you biked the same amount of time as you walk, you will cover much more distance, and you will burn much more calories.
Bikes are supposed to be efficient!!!
The energy saved can be applied to other things.
RedJenny, the idea that driving is more efficient than trains or buses is silly (I can't reach the page), you should note that one of the reasons Wikipedia can't be trusted is just because of this. Someone has gotten in to it, and manipulated it. Try not to use it as a serious source of information.
Walk if you have time, bike if you don't. Both are good for you and the environment. It is useful to walk sometimes even if you bike a lot (different muscles).
patrick
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mihan Posted 1:38 am
17 Aug 2006
I, too, find it hard to believe that driving is more efficient than bussing, especially given the source: witness.
Also, as redjenny points out, there's the energy to produce the car in the first place. Wear and tear on shoes means new shoes (which have their own ecological production costs), but this can't possibly be anywhere near production costs of a car.
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dtscook Posted 11:36 am
17 Aug 2006
At 3 mph, it takes 3.3 kcal/kg/hr / 2.2 lbs/kg = 1.5 kcal/lbs/hr
For an American man between 20 and 29, the median weight is 168 lbs here.
So, the median American man then, consumes 1.5 kcal/lbs/hr * 168 lbs = 252 kcal/hr while walking 3 mph.
Walking at that pace, it takes 20 minutes to walk a mile, so the median American man consumes
252 kcal/hr * .333 hr/mi = 84 kcal/mi
Converting this to gasoline gallon equivalents, this would be 84 kcal /mi * 3.966 btu/kcal = 333.144 btu/mi / 114,100 btu/gge = 0.00291975 gge/mi
= 342.5 miles per gge. Remember this for later.
Although that number looks great, there is a range of estimates for how much life-cycle energy is required to produce each unit of we in the U.S. consume, but I will use the estimate of Heller and Keoleian (page 42 here: 7.3 units required for 1 unit consumed.
This means that in eating those 84 kcal to walk a mile, we actually consumed 84 * 7.3 = 613.2 kcal
Converting this to gasoline gallon equivalents, we have 613.2 kcal /mi * 3.966 btu/kcal = 2431.95 btu/mi / 114,100 btu/gge = 0.02131 gge/mi
So the average American man between 20 and 29, walking 3.0 miles per hour, uses 46.9 miles per gge.
However, we can't really compare this to posted automotive fuel efficiencies. Remember the 342.5 miles per gge that I calculated at first? That figure came about from neglecting the total life cycle of the energy used for walking. To make a meaningful comparison, we ought to also be looking at the total life cycle of driving. And when you consider manufacturing, insurance, fuel, fuel cycle, and especially infrastructure, the 33 miles per gallon that a 2006 Honda Civic gets is similarly misleading. I couldn't find a good automotive life cycle analysis, so if anyone knows of one, it would be great if they could post it.
Tyson
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KathyF Posted 6:28 pm
17 Aug 2006
Or perhaps we should say only humans who fuel themselves using low petrol-based diets should walk instead of drive.
Someone could come up with a calculator that takes into account all conditions, including food fuel as well as weather conditions, i.e. a stiff wind requiring more calories, walking in the cold vs walking in mild weather, etc. This would allow environmentally correct decisions to be made in every instance.
Is there a grant available for this?
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Laurence Aurbach Posted 11:31 pm
17 Aug 2006
a) Remember that cars are a system of cars AND drivers. The caloric cost of the driver should be accounted for. If we want to figure the energy use of the car alone, we need to subtract the driver's caloric expenditure from both sides of the equation. That means the walker uses 3.3 - 2.0 = 1.3 kcal/kg/hr more than a single driver. (The average car also has 0.5 passengers and you could incorporate that if you wish -- I am going to leave it out of this example.)
b) In the "Doing the Math" post above, I cited 6,000 Btu/mile as the wheels to well energy cost of driving. The link is to the Argonne National Laboratories' GREET model. As far as I can tell, GREET accounts for waste like spills and flaring, so it makes sense to account for waste in the food production system (29% as cited above).
c) How about the energy used to manufacture the car? Here's a study that finds that 120 GJ are used in manufacture. But it is based on 1990 prices and uses a 1990 Ford Taurus as the example. How about more recent numbers? This model can calculate the energy use of the automobile and light truck manufacturing sector. If I plug in $25,000 as the average price (when new) of cars on the road today, the model says the energy used in manufacture is 216 GJ. Assuming the average car goes 135,000 miles in its lifetime, that works out to about 1500 Btu/mile.
d) Putting it all together. Using 1.3 kcal/kg/hr, a 150 pound person burns 29.5 calories to walk a mile. Add wastage and that's 42 calories. Add fossil fuel inputs and that 305 calories, or 94 mpg.
A car uses 6000 Btu/mile in its fuel cycle plus 1500 Btu/mile to be manufactured = 7500 Btu/mile or 15 mpg. The walker is 6.3 times (or 626 percent) more efficient than a car with a solo driver.
All figures and assumptions are estimates, your mileage may vary.
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mihan Posted 3:33 am
18 Aug 2006
"The walker is 6.3 times (or 626 percent) more efficient than a car with a solo driver."
I would just round it to "six times more efficient." A bike would be even more efficient, of course.
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pyrobowe Posted 5:08 am
18 Aug 2006
Consider whether you are wasteful in the amount you eat. If you eat 3,000 kcal a day (2,200 is the FDA recommendation) you're pretty much wasting the energy if you sit and drive a car. However, the same meals would give you enough to do all that walking or biking, at no extra food cost. So, by this measurement, walking to work is an infinite improvement over driving.
Peel back the layers of calculation and speculation and there's still one thing we're assuming: the food is grown using fossil fuels. Well, if you buy locally grown organic produce, or animal raised the same way, your efficiency is going to be much, much better.
One way to seek the ultimate walk-vs-drive ratio is to look at global population. Our present population of 6.5 billion is a result of fossil fuels, directly or indirectly. Before the use of oil, gas, coal, peat, etc. the world population was around 300-400 million and either walked or used "organic" tools like horses and plows made of wood. So by measure of populations the benefit of doing anything "organically" versus using fossil fuels to do the same thing is between 15:1 and 20:1.
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dtscook Posted 8:37 am
18 Aug 2006
In response to the points you made,
a) Simply subtracting the driver's caloric expenditure from that walker's is misleading in that driving is much faster than walking. The walking is 3.3 kcal/kg/hr at 3.0 mph. If you change the speed it also changes. Driving on the other hand, we can assume takes the same amount of caloric energy no matter the speed. The maximum driving speed recommended by the EPA for best fuel efficiency is 60 mph. In this case, it takes only one minute to drive a mile, which amounts to 2.0 kcal/kg/hr / 2.2 kg/lbs = 0.91 kcal/lbs/hr. For the person I was considering, we then have 0.91 kcal/lbs/hr * 168 lbs * 1 min / 60 min/hr =
2.55 kcal burned while driving a mile.
Subtracting the driver's caloric expenditure from that walker's would then give 84 kcal/mi - 2.55 kcal/mi =
81.45 kcal/mi for walking as compared to driving.
b) This should definitely be included in the calculation, and would bring the real walking efficiency to
36.37 miles per gge (for the median American man between 20 and 29 walking 3.0 mph)
c) 135,000 lifetime miles for a car is based on 1983 model-year cars. To be consistent, either the 1983 manufacturing cost should be calculated, or the average lifetime of 2006 model-year cars should be used (I don't know what this is, but I would hope it has improved -- my car has 140k, and I don't want it to die!). I think a good compromise would be to use all data exclusively from the ilea. This found that the fuel cost is about 73% of the energy used.
d) If we really wanted a comparison between walking and driving any car on the road, we would now increase the efficiency of walking to 81.45 * 7.3 kcal/mi * 3.966 btu/kcal / 114,100 btu/gge = 0.02666 gge/mi =
57.38 mpg for walking (compared to EPA automotive MPG calculations).
This would be accurate up to including the fuel, fuel cycle, service, insurance, and manufacture of automobiles. Unfortunately it would neglect, among other things, the cost of infrastructure for automotive travel.
Since the automotive energy life cycle is still incomplete, I think the most useful (and accurate) thing to do is to note that the median American man between 20 and 29, when all is said and done, gets about 36.37 miles per gge when walking at a moderate pace.
As far as automotive fuel efficiencies go, they are certainly much less than what the EPA lists if the entire energy necessity is considered.
At this point, it is quite reasonable to assume that walking is more efficient than driving, but how much I couldn't say.
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Laurence Aurbach Posted 11:38 am
18 Aug 2006
So let's simplify things and skip accounting for the driver's caloric expense. 3.3 kcal/kg/hr for a 150 pound person walking a mile is 75 calories. (I use 150 pounds because that's the average weight of all adults, male and female.) Add wastage to get 106 calories; add fossil fuel inputs to get 771 calories, or 37 mpg.
c) If you look at the last sentence from the citation, it says "Although this estimate is for 1983 model-year cars and may vary slightly for cars of different model-years, it is a good general estimate for passenger cars that are in operation today..." The date is 1998, same as the ILEA study. Also, the ILEA study includes some questionable costs like the labor to provide car insurance. I'll use the manufacturing cost of 1500 Btu/mile until something better turns up.
d) I don't understand what you did there.
The comparison I get now is overall efficiency of 15 mpg for the typical car vs. 37 mpg for the average walker. The walker is more than twice as efficient as the car with solo driver.
All I know is, I'm ready for "renewable food": food produced, transported and marketed with renewable energy.
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