Fossil fuel per human mile assumptions & math
March 30, 2018 10:20 AM   Subscribe

A while back I asked how much fossil fuel it took to walk. I built a calculator. Can you look over the math and sources of information and tell me if there's something better/more accurate to use, and more generally if there are things that I'm overlooking that I could include (without getting too into the weeds).

For an average American: At rest over 24 hours your body uses about 1523 calories. Every hour of the day you use about 63 calories. It takes 3.4 times your resting calorie rate to walk at 3.1MPH; so, every hour you walk you use about 214 calories. Which means that for every mile you walk you use about 69 calories. For omnivores it takes 10 calories of fossil fuel to create every calorie you eat. So to walk a mile you use 690 fossil fuel calories. There are 30400 calories in a gallon of gas. So, your HumanMPG is: 44.1.

I'm using the The Mifflin St Jeor Equation for BMR. Is there an age/weight range that this equation is valid for? Putting 1000 years old in puts garbage out.

I'm using the METS numbers for walking from Wikipedia. I interpolated the intermediate values.

The 10:1 fossil fuel calories to human calories for omnivores is mentioned a bunch of places, and I've found a couple of papers that say slightly different things; here's a bit from the most recently published:
By our estimates, therefore, it takes about 7.3 units of (primarily) fossil energy to produce one unit of food energy in the U.S. food system. This estimate is somewhat lower than others presented. Pimentel and Hall both put the ratio of output food energy to input energy at 1:10. -- Page 42


That paper also mentions that a vegetarian diet would use about 5:1:
Pimentel calculates that providing a 3600 kcal diet with 1000 kcal from animal products requires about 35,000 kcal of fossil energy whereas a 3600 kcal vegetarian diet (with more than sufficient levels of protein) takes about 18,000 kcal of fossil energy – almost half that of the non-vegetarian diet.
A gallon of gas has about 30,400 calories. Should I subtract the number of calories it takes to make gasoline from this? How much energy does it take to make gasoline? How about the fact that most gasoline (in the US) has 10% ethanol, and ethanol has fewer calories per unit than gasoline?

Here's the calculator HumanMPG.
posted by gregr to Science & Nature (10 answers total) 3 users marked this as a favorite
 
That sounds at least an order of magnitude too low to me.

Have you double checked every value for hidden cal/kcal problems? Because it’s not at all rare to find ‘calories’ used to mean ‘kilocalories’.
posted by SaltySalticid at 11:05 AM on March 30, 2018


Also you should’t really compare walking to not existing, you should compare walking to not walking, i.e. subtract out the resting rate.
posted by SaltySalticid at 11:11 AM on March 30, 2018 [1 favorite]


cal vs kcal -- I'm using calories to mean kcal since that's the way most people talk about them. The BMR & gasoline numbers are both kcal. The numbers from the referenced paper indicating the 10:1 ratio are mostly kcal, some being BTU or other measures of energy. The paper is through and matches other estimates reasonably closely so I don't think there are any cal/kcal errors. If you have papers indicating other ratios I'd love to see them.

walking vs existing -- you consume greater than your resting rate of calories while driving, so I figured I would just ignore that value since it's there in both the driving and walking figures. I'll look at including that.
posted by gregr at 12:01 PM on March 30, 2018


I just walked 3.74 miles in 1:03 and my Fitbit says I used 537 calories, or 153.5 per mile. I weigh 162 pounds. So either the Fitbit is way high or your 69 calories per mile is way low.
posted by beagle at 1:49 PM on March 30, 2018


Fitbit -- In one study Fitbit significantly overestimated the time spent in light, moderate, and vigorous activity. See table 1 in the PDF. Spending a few minutes searching google people seem to agree that Fitbit overestimates calories burned.
posted by gregr at 3:02 PM on March 30, 2018


How about give a range? Because with so much uncertainty, any one number is almost surely not right. 7.3:1 will yield rather different results from 10:1.

Also a good idea to publish your data sources and methodology.

Finally, you do realize that 0 fossil fuels per mile is a possible answer, right? Or, alternatively, infinite miles per gallon, because no gallons of gasoline were used. Realistically, a large source of fossil fuel input to food is through the Haber process for making N fertilizer. If people avid that, through organic foods etc, then that goes away.

Definitely an interesting project, but to have much credibility you’ll have to list some methods, sources, assumptions, and variabilty.
posted by SaltySalticid at 4:23 PM on March 30, 2018


Every gallon of gasoline that you buy took an enormous amount of energy to find, extract from the ground, refine, and transport. That’s not including things like the US Navy keeping the straight of Hormuz open.
posted by rockindata at 5:49 PM on March 30, 2018 [1 favorite]


walking vs existing -- you consume greater than your resting rate of calories while driving, so I figured I would just ignore that value since it's there in both the driving and walking figures.

Yeah, no. I can write you a 12 page dissertation on why if you like, but it sounds like you're figuring it out already. You're going to compare walking to either resting/normal background calorie expenditure or the alternate activity you're comparing it with, like driving.

Note that there is going to additional calorie expenditure with driving vs complete rest but unless you are driving the Indy 500 or something, the additional rate above resting is going to be miniscule in comparison with walking. Driving is basically sitting while moving your arms slightly once in awhile. Not much more than sitting & watching your TV, really. If you use your arms occasionally to work the remote or wave a fly away.

Best is probably just to use additional calorie expenditure for walking vs average daily background rate.
posted by flug at 11:14 AM on March 31, 2018 [1 favorite]


Every gallon of gasoline that you buy took an enormous amount of energy to find, extract from the ground, refine, and transport.

This X 100.

When you calculate an "MPG" for walking, you are implicitly comparing the fossil fuel/energy inputs to walking with those of driving.

But you are doing a massive total-systems analysis of the energy inputs of walking while considering only a single isolated input towards the energy cost of driving.

The input you are using in your comparison (direct fuel cost) is actually 10% (or even a little less than that) of the total cost of driving.

I don't happen to know of any really rigorous and comprehensive studies that compare the total system energy inputs of walking vs driving.

But I know a really excellent source that works out the total economic cost of walking vs driving vs many other commonly used types of transportation.

The source is ithe Victoria Transportation Policy Institute's Transportation Cost and Benefit Analysis: Techniques, Estimates and Implications [Second Edition]. It is the best summary I know of for figuring the total systems costs of most common types of transportation. It calculates these costs with a considerable degree of sophistication and detail and is very well sourced.

In the U.S. economy dollars are actually a pretty good factotum for total energy use in the economy, so you'll actually get a pretty good answer to your question regarding total energy use of driving vs walking by looking at the total economic cost of the two activities.

At any rate, if your energy-based calculations diverge hugely from the calculations based on economic cost (and they do, to several degrees of magnitude) you'd better take a hard look at your procedures and calculations.

The total system cost of various type of transportation is summarized in Chapter 6.

There you will find basic facts like:

- External and intrinsic costs of motor vehicle ownership are over 2/3 of the operational cost per mile when compared with operational costs (page 6-6)

- Fuel cost per mile is about $0.10 but total economic cost per mile ranges from $0.94 (rural) to $1.51 (urban peak travel times). In general, fuel represents 10% or less of total cost of driving motor vehicles on a per-mile basis. (Table 6.3-1, page 6-11)

- Ownership costs of motor vehicle use are nearly 3X as high as fuel costs, cost of crashes almost 2X as high, parking 1.4X, cost of roads nearly as high as fuel cost, a bunch of other external costs are small individually but total to quite a bit more than the cost of fuel, etc etc. (Table 6.2-2, page 6-6)

In short, the costs of building and owning the vehicle, its share of the cost of motor vehicle crashes, the cost of parking (keeping mind not only the place you park it at home--but creating & maintaining the places for you to park it wherever you drive to, such as work, shopping, school, etc), cost of building & maintaining the roads you drive on, etc etc etc all total to far exceed the cost of the fuel.

The total energy cost of all of these inputs will also far exceed the energy cost of just the fuel.

Finally Chapter 6 includes a summary of external cost savings per day attained by switching from typical motor vehicle driving to other options (Table 6.6-1, page 6-15):

- Peak shifting $6.55
- Compact car $0.60
- Electric car $1.18
- Rideshare passenger $12.35
- Diesel bus $4.79
- Electric trolley $4.15
- Bicycle $12.99
- Walk $13.37
- Telework $10.63

This doesn't by any means capture ALL of the economic savings of these various forms of transportation. This is only looking at external costs, which are about 1/3 of the total cost.

Part of the reason walking & bicycling come out so far ahead of most other options is they have a rather significant positive health benefit, while most other options have no health impact or even a rather hefty negative health impact.

"Improved health doesn't have any impact on energy use" you're going to say. But calculate the total energy inputs into a typical open heart surgery, which costs something like $100,000 and you'll see they are very significant. By way of comparison, the average person is going to spend less then $100,000 on gasoline over an entire lifetime.

Don't underestimate external and indirect energy costs. They amount to far more than the direct energy expenditure in most cases and definitely so, by a factor of 10X or more, in the case of motor vehicle travel.

Part of the reason I point out this final table is that your calculator finds the result that the MPG of walking about the same as that of a fuel efficient compact car. But this table finds compact car use is just a slight improvement overall compared with the average or typical car, and walking is more than 20X more beneficial compared with the compact car (0.60 vs 13.37).

This is just another indication that your comparison is off by at least 1-2 orders of magnitude.
posted by flug at 12:20 PM on March 31, 2018 [3 favorites]


Also it is worth pointing out that the average person living in an industrialized, motor-vehicle-centric society can and should be walking about 5 miles more and/or bicycling about 20 miles more each day while not changing what they eat one iota.

We're already eating the calories. We're just not putting them to any good use. They're padding our waistlines rather than getting us where we need to go.

Part of the reason I'm pointing out all of the above is that it is cool and neat to calculate MPG of walking vs driving in a narrow and rather technical point of view. But people are going to look at that result and draw conclusions, like "Oh well I'm just going to drive my Prius for the 0.5 mile trip because it is every so much more efficient and friendly to the earth than walking" when it fact that conclusion is wrong in ever so many ways.

Even if you just point out at the bottom of your calculator that this is just one interesting piece of the picture and provides some links that put this result in the greater context. Or calculate the relative MPG but also use the summary figures from the VTPI article to also calculate total cost of operation for walking vs. driving just to show some greater context.

I remembered this walking vs bicycling vs driving calculator that adds up the annual savings just for one example of how it could be done. That calculator assumes that the average person could walk or bicycle a mile or two a day without changing their eating habits. But that is a totally realistic assumption.

Most of us in the U.S. need to walk or bicycle a mile or two a day and also simultaneously reduce our food consumption.
posted by flug at 12:37 PM on March 31, 2018 [1 favorite]


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