On Do the Math, three previous posts have focused on transportation efficiency of gasoline cars, electric cars, and on the practicalities of solar-powered cars. What about personal-powered transport—namely, walking and biking? After stuffing myself over Thanksgiving, I am curious to know how potent human fuel can be. How many miles per gallon do we get as our own engines of transportation?
Okay, the “miles” part is straightforward. And we can handle the “per.” But what’s up with the gallon? A gallon of what? Here we have all kinds of options, as humans are flex-fuel machines. But food energy is not much different from fossil fuel energy in terms of energy density.
Nutrition labels in the U.S. use kilocalories (or Calories with a capital “C”) as an energy measure. One kilocalorie (kcal) is 4.18 kJ of energy. Carbohydrates and protein come in at 4 kcal/gram, while fat registers 9 kcal/g. For comparison, coal ranges from 4–7 kcal/g, gasoline is 10 kcal/g, and natural gas is 13 kcal/g. So fat is pretty much like gasoline.
On one extreme, we could use a gallon of lard for our fuel. Though not a delight to eat, at close to 2000 kcal/$, lard is probably the best value in the grocery store in terms of energy per dollar (just a guess). This is truly the closest thing we could get to eating a gallon of gasoline proper. But we’ll tone it down and explore more standard fare.
Besides the energetic components, water and fiber also contribute to mass/volume of our food. On a recent trip to New Mexico for my experiment, I did a food experiment for blog’s-sake: for several days, I ate exclusively packaged foods with good labeling. Excluding drinks, the food averaged 1.85 kcal/g. If we then assume that the food has a density similar to that of water, we get 7000 kcal/gal (1 gal is almost 4 liters, which itself is 4 kg). By comparison, a gallon of gasoline contains 36.6 kWh, or 31,000 kcal—about 4.5 times more potent than our typical food mix (but beware the aftertaste).
Couldn’t we do better by eating a bucket of lard? Energetically, yes. But would you really feel like walking or biking afterward? Or would you rather lie on the floor moaning in discomfort? I will default to packing our gallon of food with the “standard” fare of burritos, lasagna, etc. to make for a realistic calculation, but we’ll occasionally multiply our result by a factor of 4.5 to get the gas/lard equivalent.
For the bike, the approach is pretty similar to that for the car, in that the dominant effect is aerodynamic resistance. For the car case, we learned that the energy required to go a distance D at velocity v is ½cDρADv², where A is the frontal area presented to the wind, ρ = 1.3 kg/m³ is the density of air at sea level, and cD is the coefficient of drag (about 0.8 for a cyclist). If we take the area to be 0.5 m² (approximately a half-meter wide by a meter tall in biking posture), we find that it takes about 13 Newtons of force to push against the wind, turning into 13 kJ to travel 1 km (1000 meters) at 7 m/s (15 m.p.h.). Interestingly, the effective “drag area,” cDA ≈ 0.4 m² for a bicycle can be beat by an entire four-seat car having, for example, a frontal area around 2 m² and cD ≈ 0.2.
As an aside, a generic shape not designed with aerodynamics in mind—let’s call it a man-bear-pig—will have a drag coefficient around 0.8–1.4. A trout, on the other hand, has a drag coefficient around 0.1. What we really need is a trout on a bicycle! Then we’d really be smokin’. We can turn the famous feminist slogan that “a woman needs a man like a fish needs a bicycle” on its head with the observation that a bicycle really screams out to have a fish ride it!
Once you’ve recovered from that perversion, we should add rolling resistance, using a coefficient of 0.004 times the 800 N weight (based on 80 kg mass, or 180 lb for rider plus bike). This presents an additional 3 Newtons of force to overcome. Work is force times distance, so over 1000 m this turns into 3 kJ for a total of 16 kJ to go one kilometer on the bike.
A mile is 1.6 times as far, so that we require 25 kJ = 6 kcal of delivered energy to bike a mile. This is a tiny fraction of the caloric content of our gallon of food, implying that we could go for almost 1150 miles per gallon!
Alas, our bodies are not 100% efficient at converting food energy into mechanical output. But at about 25% efficiency, we’re surprisingly good considering that most cars are around 20%, and that an Iowa cornfield is only about 1.5% efficient at converting incoming sunlight into chemical storage.
Factoring in the efficiency, it takes about 24 kcal to cycle a mile, which yields 290 miles per gallon of food. 290 MPG! And the gas/lard equivalent is about 1300 MPG! But before our victory lap in the velodrome, we should put the bike activity in some context.
We have only counted the marginal energy required to bike. For short trips around town, this is the sensible number. But for a cross-country bike trip that may last days, you would consider the cycling to be your sole energetic activity and should count all your food energy as going in support of that mission (even sleeping and resting are necessary to keep going).
We all have a baseline metabolism to just run our bodies and carry out our daily activities. I’ll take the easy number of 2000 kcal per day. Smaller or less active people will need less, and larger or more active people will use more. Cross-country bikers classify as “more active,” but we’re accounting for the activity explicitly, so we can use the couch-potato baseline.
If you’re in good-enough biking shape to contemplate a multi-day cross-country trip, you can probably maintain about 100 miles (160 km) per day. That 100 miles will take an extra third of a gallon of food per day, or an extra 2400 kcal. Each day therefore consumes about 60% of a gallon of food, and you would therefore make 160 miles on the 7000 kcal gallon of food (720 miles per gallon of gas/lard).
So depending on the mode of biking and how you want to do your accounting, we got about 290 MPG in town, 160 MPG on the open road. Converting to the more universal and useful measure of energy per unit distance, these numbers map to 5 kWh/100-mi, and 2.8 kWh/100-mi. For comparison, electric cars turn in performances around 30 kWh/100-mi, and a 40 MPG car uses 90 kWh/100-mi (but beware a direct comparison between these last two: if the electricity is derived from fossil fuels, the fossil fuel investment becomes similar for the two: we’ll get to this for food in a bit).
We didn’t take hills into account, but to the extent that ups compensate downs, and you pedal at about the same intensity regardless, the power output is constant and we can use the average flatland speed to determine the rate at which energy is exerted. In the case of a net uphill climb, a daily gain of 1000 m (3300 feet) elevation adds 800 kcal of gravitational potential energy for our 80 kg (180 lb) rider-plus-bike—again using a 25% efficiency. This effectively adds a 33% energetic cost to the 100-mile day. We also neglected the start/stop energy dissipated in braking for the in-town rider, which will reduce the mileage a bit. So far, our bodies have not figured out regenerative braking.
These Boots Were Made for Walking!
And that’s just what we’ll do now. Walking is an entirely different regime from biking. Air resistance is negligible. Power expended against the air is proportional to the cube of velocity, so a biking speed of 7 m/s (15 m.p.h.) expends 7³~350 times as much power to fight the air as a walking speed of 1 m/s (2.2 m.p.h.).
But that doesn’t mean it comes for free. The energy cost of walking is dominated by the repeated lifting and swinging of legs. The smoothness of the terrain can have a noticeable effect on this energy cost. A table put together at BrianMac indicates that our 70 kg person (156 lb; dropped the bike) walking at 4 km/hr (2.5 m.p.h.) expends 3.8 kcal per minute. The mile will take 24 minutes to traverse at this speed, so 90 kcal will be consumed per mile. Recall that we could bike this distance for a smaller energy investment of 24 kcal. The same site has some figures for cycling, which all compute to about 45 kcal/mile for three different speeds. I find this to be suspicious, given that the drag energy varies by a factor of 5 for the three speeds. Their numbers are also well above what I compute, so I don’t know if the energy for walking is similarly elevated—but what’s a factor of two between friends?
For walking, our 7000 kcal gallon of food will propel us 75 miles down the road at 90 kcal/mi. So there we are: 75 MPG to walk, or 340 MPG on gas/lard. That means you should not eat a gallon of lard if there is not a hospital within 300 miles. Or maybe don’t do it anyway.
If we are on a multi-day trek covering 25 miles (40 km) per day, and lump the baseline metabolic energy into the mix as simply supporting the journey, we get about 40 MPG on food, and 180 MPG for gas/lard.
We’ve worked through a lot of numbers, under various interpretations. I hesitate to put these numbers into a table for fear that they will be taken literally and treated as definitive numbers. This is meant to be ballpark, people: good to a factor of two. So here’s a table.
|Activity||MPG food||MPG gas/lard||kWh/100-mi|
Another consideration to bear in mind: in most cases, cycling and walking involve a single “passenger.” A Prius loaded with four people effectively gets 200 MPG per passenger (18 kWh/100-mi per passenger), so make sure to account for this when making comparisons.
Fly in the Ointment
Our walking or biking economies look pretty decent stacked up against cars—especially if we considered consuming foodstuff as potent as gasoline. This is all well and good until one appreciates that because of the way Americans grow, harvest, distribute, and prepare their food, every one kilocalorie of food eaten has consumed about 10 kcal of fossil fuel energy (dominated by oil). Our 7000 kcal gallon of food therefore took 70,000 kcal of fossil-fuel energy to produce, or a little over two gallons of gasoline. So you would divide the “food economy” values we calculated by 2.2 to get the fuel economy that supported your bike trip or hike. Now walking consumes 18–34 MPG of oil equivalent, and biking comes in at 70–130 MPG.
This should in no way be taken to suggest setting aside the bike or boots for a car that gets better performance. Rather, we should consider ways to make our agriculture or eating habits less energy-intense. By necessity, we once spent less than one kilocalorie of energy on each kilocalorie of food delivered to the plate—otherwise we would have starved ourselves out of existence. So we know that we don’t strictly require a 10:1 ratio of input energy to output energy. Choosing our food sources and food type can make a big difference here.
For instance, if you eat locally grown vegetables that took one kcal of fossil fuel energy for each kcal of food energy, you could claim that the 7000 kcal gallon of food that lets you walk 75 miles only cost a quarter of a gallon of fossil fuels, so your journey effectively gets 300 MPG in fossil fuel terms. Now that’s something to smile about!
There’s another fly in the ointment. Unless death is an option for you, you’re going to be consuming those 2,000 – 3,000 kcal every day regardless of how many miles you travel under your own power. If you use some of that for transportation, it really doesn’t matter what ratio of fossil fuel to food energy; you’re still coming out ahead because that energy is being put to work rather than being wasted as hot air.
Now, factor in the health benefits and the resulting reduced cost to society….
If I can add on to that…the infrastructure required for cars is much more energy- and petroleum-intensive than bikes. Cars need a lot of pavement, and they chew it up something fierce. And there’s the initial energy investment in the vehicle itself…just a single car door is going to take more energy and raw materials to manufacture than the entire bike.
Don’t get me worng. There’s a lot to be said for cars. They’re a wonderful luxury, and there’s so much that’d be impossible without them. But we’d be much wealthier as a society still were we to use bikes where they make sense — and they make sense for a very significant portion of our transportation system.
I don’t think most first world people consider cars to be a luxury. I don’t think that’s likely to change in the foreseeable future.
And I don’t think bikes “make sense” for more than a tiny fraction of our travel. They’re too slow, uncomfortable and impractical.
Bicycles “makes sense” for more people than you say. I live in a car-centric city in the United States. When the price of oil spiked several years ago, one of my friends gave up his car and rode a bicycle to work. Another friend rides an old used $50 bicycle to work, about two blocks from his apartment to his job as a short order cook.
In the city Groningen of the Netherlands, 57% of the people who commute to work use a bicycle. The Dutch have some of the best bicycle road infrastructure in the world.
I live in Chicago and the number of bikers has gone up dramatically (purely my feel for seeing them) since the “crash” of 2008.
Even with the food cost tossed in biking is better than cars and walking is even with cars. When you factor in parking costs/availablity in a city, biking/walking is still better. I use my bike during the summer for lots of getting around the city stuff, walking too.
Based on this article, I am going to keep bacon on me at all times when biking for a little pick me up. To paraphrase the beginning of the article. Bacon is natures gasoline. 🙂
This really depends where you’re coming from. In the Netherlands, which is definitely a first world country and rather wealthy compared with most countries, over a quarter of all journeys are by bicycle. The population of this country, which is only about the same as New York, makes more than a million cycle journeys every hour. It’s not slow, not uncomfortable and not impractical.
Rather, it’s fast, inclusive, convenient for young and old, good for health, and very very much more convenient than driving. Unlike if you drive to get your shopping, by bike you can ride straight into the shopping centre. It’s a matter of how your cities are designed. Dutch cities have been redesigned to favour safe cycling for everyone.
Urban areas in the Netherlands are generally much denser and more compact than urban areas in the United States. So they’re generally much easier to navigate by bike and much harder to navigate by car. The climate is also much milder in the Netherlands, without the extremes of cold, heat and humidity that make biking in many parts of the U.S. very uncomfortable for much of the year. The Netherlands is also very flat, so bikers don’t have to deal with many steep slopes. For these reasons and others, it doesn’t really “make sense” to compare the level of bike use in the Netherlands with the level in the U.S., because the natural and built environments are so different.
If your bicycle is “uncomfortable”, you’re doing it wrong.
If it’s impractical, that may be an issue, but I used one as my primary transportation for several years (this century) so I think the set of cases where it’s practical is much bigger than most people imagine.
In my car, I sit in a very comfortable, adjustable bucket seat with leg, back, lumbar and head support. I’m protected from rain and snow. I can set the temperature, humidity and ventilation to my liking, as I enjoy a smooth, quiet ride while listening to the radio or music through my 6-speaker sound system.
On my bike, I sit on a small saddle-seat that supports only my butt. I have to lean forward in a prone position at almost all times to hold the handlebars. The ride is frequently bumpy, thanks to potholes, debris, speed bumps, grooves and other irregularities in the road surface. I’m exposed to rain, wind, snow, extreme temperatures, road noise and other unpleasant environmental conditions. Riding up hills requires considerable effort, leaving me sweaty and winded unless I slow down dramatically.
If you seriously think you can sell people on the idea that cars don’t have a huge comfort advantage over bikes, good luck.
Your arguments about density don’t work. Dutch cities are not nearly as dense as you think they are, and none are as dense as many American cities.
The density argument is just one of a long list of many excuses which people make and myths that they believe about how the Netherlands is different to their own country. They’re debunked here.
Standard or average density isn’t a useful measure of walkability or “bikability” because it doesn’t capture the extent to which people and destinations are clustered within a city or urban area. A much better measure is weighted density or perceived density. Here is an article that discusses the issue.
The urban forms of cities like Los Angeles and Amsterdam aren’t just slightly different. They are radically different. Los Angeles, like most other U.S. cities, was designed around the car. Amsterdam was designed around much slower forms of transportation. To make LA even remotely as bike-oriented as Amsterdam or other Dutch cities you’d essentially have to tear it down and rebuild it from scratch. Not going to happen.
I’d like to add one more point about cars vs. bikes since we’ve delved down to this level; You have to consider the cost per mile of owning the thing, whatever it is. That includes purchase price, insurance, maintenance costs, etc. The person who bought the $50 bike has the right idea. Your initial cost for the mode of transportation has to be averaged across the total miles you expect to “drive” the thing.
I did the calculation for myself. I figure that I ride my bike more than most bike owners, but because I bought it new my cost per mile (so far) is about the same as my cost per mile for my car (which I also bought new). Both are about a decade old. If I had bought used, in both cases, I would have done better. And by “used” I mean capitalizing on the rapid depreciation that things like cars experience in value early in their lives. It’s not like they’re wearing out faster when they’re new.
Bikes require a well maintained infrastructure too. I wouldn’t fancy doing my 10 mile cycle into work that I did this morning on roads/paths that are any worse than they currently are.
Walking yes you can do over rough ground, but cycling requires a good surface or the rolling resistance balloons
I second this. Think about someone who drives to the gym everyday to exercise on a machine that requires electricity. This person could simply bike to work to get their exercise! Less wasted time, too. I could go on and on.
Really good point. And I have seen that nonsense argument – that bicycling consumes more fossil fuels than driving because of the way food is grown – made in earnest. Of course our calory intake doesn’t depend much on the miles walked or bicycled. I would be surprised if the average SUV driver consumed fewer calories than the average bike courier. And an excellent way of conserving food calories, apart from using local and non-industrial food, is to cut the *food waste*. It is estimated that 40% of food in the US goes to waste post-harvest.
“We found that US per capita food waste has progressively increased by ~50% since 1974 reaching more than 1400 kcal per person per day or 150 trillion kcal per year.”
Good point on the embodied energy – I hadn’t thought of that.
Another critical consideration is that a healthy level of exercise gives a healthy metabolism, which means you can extract more energy per unit food… unhealthy overweight people usually consume more calories per day despite not exercising… and may have fewer calories available for body-work, the remainder go as waste. So you could argue that, at least for moderate values, biking/walking for transport actually *reduces* the burden on the energy intensive food system as while your personal output increases the inputs required to achieve a given output reduce.
I’m with Ben on the long haul issue too. Resting consumption would occur in a car as well. Of course, relative duration plays a part… Unless you bike really fast.
Anyway, neglecting the exercise is healthy and can result in eating less angle, I think the obvious answer is electric bikes 🙂 Cheap, fast, easy, and WAY efficient!
Rather than all-electric or electric assist bikes, I’ve wondered about the possibility of a hybrid electric bike where the pedals are completely disconnected from the wheels. Instead, your legs drive a generator hooked up to a battery, and an electric motor runs off the battery. That would let you constantly pedal at your optimum RPM (cadence) and power output, even when at a stop. The electric motor can then be used to maintain a constant speed over changing terrain; add in regenerative breaking for the descents, and you’re golden.
The big question would be efficiency losses. The mechanical bicycle drivetrain is nearly perfectly efficient. Even a 10% loss in efficiency would be enough to make this a not-so-good idea. Added weight is also a problem.
Anybody have any ideas if anything, even only in the lab, might work for such a machine?
Andreas Fuchs at the University of Bern, Switzerland, did some work on such a machine a few years ago. The best information I can find right now is the following page:
Some googling might produce more information.
You’ll need to have good balance to do it at a stop 🙂 I’ve thought several times about a series hybrid electric drivetrain for bikes to allow wide range optimal cadence though as you describe. As you predicted, efficiency is always the thing that kills it. You would be doing very very well to hit 90% at optimal operating point with large highly efficient machines, considering that even going straight through you need a mechanical to electrical conversion and an electrical to mechanical conversion, with at least one and probably two electrical conversion stages as well. This is before considering the battery (let alone any further power electronics associated with power flow control there, which you could probably do without).
At small scales, and non-optimal operating points (steep hills, long flats, etc) the mechanical – electrical efficiency especially will drop quite a bit and you’ll probably be looking at more like 60% – 70% or less.
For that reason I instead prefer the idea of a parallel hybrid architecture to acheive the same thing. You can get the exact same benefits (except for at zero speed) by using the electric path to provide a variable load on the mechanical path, and using the mechanical gear system (which, happily for our project, benefits from around 130 years of continuous refinement and is now phenomenally efficient, light, robust, and cheap) to optimise your cadence for given riding conditions as cyclists already do today.
This gives you extremely high efficiency for direct pedal to wheels, and a lower (but still optimal) efficiency (and motor/generator + power electronics re-use) for the smoothing stage.
The OTHER way is just to ride much faster on the flat and down hills, but considering the horrendous increase in aerodynamic losses (as Tom estimates above) at increased speed I can definitely see some merit in a parallel hybrid drive. If it’s light, cheap, and reliable.
For now though, I love my CHF100 around-town 18 speed.
I have several nitpicks with this article.
First, you’ve considered the efficiency of converting solar energy to chemical energy of an Iowa cornfield, but gasoline is taken as the raw MPG of a car. This ignores the EROEI of extracting oil, refining it, and delivering it to a gas station. Perhaps you can update your figures to give an equivalent comparison to gasoline?
I think it would also be useful to compare gallons of a standard fuel, in order to directly compare the energy required of travel. The true metric should be distance per unit of energy. Either keep the numbers in terms of energy required or convert them both to a standard fuel of known energy density.
Last, as a cyclist, I disagree with the assumption that on a long distance trip you should consider all of your energy intake as necessary to continue pedaling. 100mi at 15mph is near enough to seven hours of cycling. This still leaves 17 hours (70% of the day) as non-active. So why do the energy calculations for long-distance cycling include base nutritional requirements? Significant food intake is still necessary for you to continue living, which I think we can all agree is a base requirement for any form of travel. It makes no sense to ignore our biological requirement for food during short trips, but include it for longer ones.
With an EROEI of typically 20:1 for oil (5% effect) and maybe a 25% hit on refining to gasoline, I think we need to relax a bit. The bike numbers are ballpark, so let’s not get bound up in small corrections.
As for counting base metabolism in one case and not the other, you are free to do whichever you want. I presented two methods of accounting that bracket the numbers. Anywhere in between is something I’d consider reasonable.
Those two contribute to slightly under a 30% drop in the efficiency of oil before we even get to last-mile distribution. You compute those numbers for bicycles to point out they’re not as good as they look, so why not perform the same analysis for cars and compare apples to apples?
To the same sentiment, comparing quantities of two different kinds of fuels isn’t really useful. Imagine if we were comparing gallons of fuel for nuclear reactors instead. Using a fuel with equal density is (I think) a truer comparison between transportation methods. The concern with fuels isn’t typically the quantity, IMHO — it’s the energy requirement.
“presented two methods of accounting that bracket the numbers. Anywhere in between is something I’d consider reasonable.”
The way I see it, there are two sensible ways to look at bicycle efficiency. The first is to look at the raw energy input per unit of distance, as you did (arriving at 24 kcal per mile). The result is that bicycles are by far the most efficient means of transport. The other way would be to do an apples-to-apples life cycle assessment. That would be more ambitious than what you attempted. I agree with Stephen and Ben Goren above that your “long distance” approach is flawed. No offense, this is just a criticism in good faith. To get a real comparison, we’d have to compare the habits and diets of a typical motorist with a typical bicyclist and see how calorie intake varies with bicycle miles. I expect that you’d see a difference in calorie intake between moderate and heavy bicyclists but not necessarily between moderate bicyclists and motorists. The latter will be more likely to be obese, for example. Or they might work out more to compensate for their lack of exercise. In any case it would be meaningless to count base metabolic rate as a cost of transportation in the case of the bicyclist but not of the motorist.
Point taken. I just wanted to know: if we had to put a sticker on our foreheads describing our fuel economy, what would the number be? I wanted to look at it from a variety of sensible angles. Any way you slice it, human transport comes out looking pretty good, energetically.
The beauty of my having done the calculation both with and without base metabolism and laying out the assumptions explicitly is that if you don’t like my choices, you can roll your own calculation or selectively ignore the parts you don’t like. The all-inclusive number is useful for long backpacking treks: you want to go 100 miles in four days? Pack about 2.5 gallons (17,500 kcal) of food.
As to the point of including metabolism in car drivers, some quick numbers: if an average car goes 12,000 miles per year at 30 mpg, it uses 400 gallons of gas. A 2000 kcal/day diet, where 10 times this much is consumed in fossil fuels for food production, leads to about 200 gallons of gasoline equivalent to feed the person for the year. So the metabolic portion is 50% of the transportation piece for cars (much smaller for the SUV driver), and therefore not as big a deal as for human-powered transport.
Worth remembering that if you are a fit cyclist that your body will as it’s base metabolism consume more.
A fit heart takes more energy to operate than an unfit one, that’s why the human body scales down it’s muscles unless they’re needed. Otherwise why would we have evolved to scale down our muscles to only the amount absolutely needed, big/lean/fit muscles are expensive energetically every single day.
@Ben: bicycles require pavement too–not nearly as much as cars but near the turn of the 20th century some of the original advocates of paving roads were cyclists. Also not exactly what you mean by cars ‘chewing up’ pavement but if you’re talking about damage and its eventual repair and repaving then cars actually don’t do much damage. Road damage scales as the fourth power of axle weight which means trucks and buses account for nearly all of it.
On the fly in the ointment: if we’re going to scale the MPG of walking and cycling by the fossil fuel requirements that go into making our food, then for comparison we also need to know the scale factor for getting gasoline from oil well to refinery to gas stations, because our sense of what a good MPG doesn’t include this. Then again the scale factor for food would need to include this factor as well, depending on the energy source distribution that goes into the 10x factor for food. (Side note: reviews I’ve read of Simon Fairlie’s _Meat: A Benign Extravagance_ suggest that the energy input to food numbers commonly used may be way off.)
And on perspective: I would argue that energy per unit time, and not energy per unit distance, is the relevant metric. The old assumption by traffic engineers is that the distribution of trip origin/destination pairs are generated without consideration of how easy any particular trip is, but through the well-documented phenomenon of induced traffic this has been shown to be false. But recent work at University College London by Sally Cairns, Carmen Hass-Klau and Phil Goodwin has come up with an alternate model, in which, on the whole, people are constantly re-assessing their transportation decisions. If you have a tolerance for a 30-minute commute, and you decide to drive, you’ll tend to pick a residence or workplace that are 30 minutes apart by car, and if you decide to cycle, you’ll tend to pick a residence or workplace that are 30 minutes apart by bike. This wouldn’t apply to every immediate situation, but there’s evidence that averaged over a whole regional population and over a sufficient time span, decisions would trend this way.
The energy per minute metric is actually a pretty interesting idea. I’d known about the thirty-minute commute tolerance (from reading Traffic), but hadn’t thought about using that to solve the dilemma that stems people using different modes of transportation tend to select their location of residence differently (e.g., I moved in closer to the city after I began cycling).
Another awesome post.
I riffed on it, fwiw:
Information update: I added a brief statement after the table about per-passenger considerations (fitting 4 in a car, for instance). I also corrected the number for long-haul walking to 20 kWh/100-mi (was 15 before for some reason).
Wikipedia suggests that the average is about 1.3 passengers per car, which matches my own guesstimate. That rounds to 1 for one significant figure, which matches your original numbers.
The more interesting calculation would be for a tandem bicycle, especially since you don’t go twice as fast on a tandem. And how does the efficiency of the peloton come into play? Regular commuters often cruise in the ballpark of 20 mph, and from experience you can easily add at least 5 mph by putting enough such riders in a well-disciplined pack.
The National Household Travel Survey has found pretty consistently that the average occupancy of passenger cars and light trucks (SUVs, pickups, etc.) in the United States is about 1.6. So the average efficiency in passenger-miles per gallon is the average efficiency in vehicle-miles per gallon divided by 1.6.
The per-passenger metric is a really good one. Bicycle advocates like me tend to forget what kind of impact having multiple passengers can have on the energy efficiency of cars.
True, but a low number there is probably THE top selling point for automobiles, right?
Thanks for another great post. I really appreciate that you’ve taken the fear-mongering about peak oil out of the equation and simply stick to the facts. It’s a refreshing change, and I think it’s a far more productive way to begin an informed discussion than I see elsewhere on the net.
I hope you don’t mind if I share a quick anecdote..
I recently visited your beautiful home town of San Diego to check out the America’s Cup trial races that were being held in the harbour, and I couldn’t help but be awed by these high tech machines. If you’re not aware, they are essentially the most advanced sail boats ever built, designed from the ground up to efficiently slip through the water while maximizing the energy captured from the wind by using a wing sail. I think it’s fair to say that they are the epitome of technological innovation in their field, and they cost approximately $1M each. Watching these boats in action is inspiring, and makes it seem like weaning ourselves off oil won’t be that all that bad. After all, look at how we have been able to harness technology to create machines that move so fast without using a drop of oil! That illusion lasts until you look more broadly at the event and realize that each sail boat is being followed by a gas chugging motorboat that surpasses the sail boats in nearly every measure of performance – speed, manoeuvrability, amount of training required for use, number of crew, and most importantly cost (by a factor of 50). The only place the sail boats come out on top is in range, and when you consider the amount of physical exertion required to them moving, even that is debatable. Applying the same principles to our more common methods of transportation and it’s painfully obvious that the down slope of the fossil fuel consumption curve is going to be quite an interesting ride!
Cars represent a three-stage consumption/pollution lifespan, each requiring energy in their own way. A mid-1990s study by the Environment and Forecasting Institute in Heidelberg, Germany outlined the three stages: manufacture, road-life, and disposal. They used a Ford Escort for their case study. They found that before the vehicle had even left the production plant, its construction had generated 29 tons of waste and 1.2 billion cubic yards of polluted air. This represented half of the polluted air emitted over the lifespan of the vehicle. Over 85,000 miles of road-use, the car emitted another 1.3 billion tons of polluted air, and shed 40 pounds of tire and brake debris alone. Finally,as the automobile was disposed of in a typical manner, the process generated another .13 billion cubic yards of polluted air plus PCBs, hydrocarbons, and heavy metals interred.
Of course, as already mentioned, there is the matter of infrastructure for automobiles, not the least significant characteristic of which is huge amounts of SPACE. Parking space, sure- state-sized swathes of arable land appropriated for car storage- but also roadway. Cars, when moving, take up a whole lot. Not just the ~100 square-foot perimeter of the automobile itself, but effectively up to 300 square yards afore and aft of the moving vehicle, depending on speed.
Then you get in to the hard-to-quantify losses such as health/life that Ben touched on above. 40,000 people dead from collisions each year, up to 10 times as many from heart disease, lung disease and cancer (benzene, toluene, xylene are but a few of the carcinogens introduced to your lungs from automobile exhaust) in the US alone, plus millions of wild animals every day.
To thm: “Also not exactly what you mean by cars ‘chewing up’ pavement but if you’re talking about damage and its eventual repair and repaving then cars actually don’t do much damage. Road damage scales as the fourth power of axle weight which means trucks and buses account for nearly all of it.”
I’m not sure how you arrive at that last statement from your first statement, without quantifying the ratio of cars to trucks and buses on the road. Furthermore, if cars don’t do much damage at ~2000lb, then bicycles, which by your formula, at 200 lbs (bike+rider), do virtually _none_ at (1/10)^-4 as much.
Apologies for typo, (1/10)^4.
I’m not sure that MPG of a human is the important factor when it comes to a bicycle. Given the constraint that the power source must not sustain output greater than the sustainable output of a human, I think any wheeled vehicle designed with the constraint in mind should reach a similar MPG?
To overly simplify, going slower tends to be more efficient at travelling a distance than travelling faster.
This article was great, except for one point. At the very end of it, you subscribed to the “local food” myth:
“Choosing our food sources and food type can make a big difference here… For instance, if you eat locally grown vegetables that took one kcal of fossil fuel energy for each kcal of food energy…”
Local food requires as much energy as global food, if not more. Whereas local food avoids the energy expended on long-distance transport, this is a negligible fraction of the total energy expended on food production and distribution. Long-distance transport consumes only about 2% of the energy used for food, with most of the remaining energy being used on fertilizer production, farm machinery, and transportation from the supermarket to your house. Although local food would save that 2%, it would more than offset that amount through increased inputs resulting from growing crops in less-favorable regions.
The reason local food is the same or worse as global food, is because large ships are incredibly energy-efficient. Most large ships can transport cargo at 1000 ton-miles/gallon of fuel. In contrast, your car at 20mpg with 100 pounds of food would transport only 1 ton-mile/gallon of fuel, using 1000x as much fuel per ton-mile. If you drive 6 miles to buy your groceries, you have burned more fuel than transporting a similar amount of food from Beijing to Los Angeles.
Worst of all is the farmers’ market. Farmers’ markets are swarms of small lightly-loaded inefficient vehicles from farmers and consumers all converging on a fairly distant place. Farmers’ markets don’t even have permanent inventory, so farmers must transport unsold product back, on their lightly-loaded small inefficient trucks.
With regard to the comment about large ships. I should have said that long-distance transport is generally much more efficient than short-distance transport. Whereas your car gets about 1 ton-mile/gallon (with 100 pounds of groceries), a large tractor-trailer truck gets about 90 ton-miles/gallon, a train gets about 400-500, and a large ship gets about 1000. Of course these figures are estimates, and vary based upon things like the size and speed of the ship, the length of the train, and so on.
Don’t like local farmers or small-scale food markets much, huh? Enamored with “Efficiency”? Aren’t you in for a suprise.
When done right, organic farming practices can produce greater yields (in caloric value or mass) acre while using a fraction of the fossil fuels, life-cycle. But pickup trucks are less fuel-efficient than ocean-going vessels, as you point out.
Coincidentally, I just read this article about velomobiles and their increased efficiency relative to bikes:
Here is a calculator which lets you compare what speed you’d travel at for the same energy input for various different bicycles including several commercially available velomobiles. I find that this corresponds very closely with the difference between my bicycles and velomobile.
And yes, they are quite remarkable. It is only since I had a velomobile that I have been able to race at an average of nearly 25 mph for nearly hours. It also makes a wonderful all weather commuting machine.
“Our 7000 kcal gallon of food therefore took 70,000 kcal of fossil-fuel energy to produce, or a little over two gallons of gasoline. So you would divide the “food economy” values we calculated by 2.2 to get the fuel economy that supported your bike trip or hike. ”
I’m probably being slow, but why divide by 2.2, and not by 10 or 11?
If the 7,000 kcal gallon of food took 70,000 kcal to produce, and gasoline contains 31,000 kcal per gallon (36.6 kWh), then it took 2.2 gallons of gas to make one gallon of food. It’s 10:1 in energy, but 2.2:1 in volume, since the energy densities are pretty different.
@tmurphy: Your efficiency values for biking and walking are in close agreement with those found in Ivan Illich’s “Energy and Equity” (1973). He came up with the following: 0.15 cal per km per gram for biking and 0.75 cal per km per gram for walking. If I did the unit conversions correctly, this works out to 20 kcal/mi for biking and 85 kcal/mi walking (assuming 180 lb for the biker and bike and 156 lb for the walker). “Energy and Equity” is one of my favorite essays on energy use and society, but Illich doesn’t explain how he arrived at these numbers, though I see them quoted from time to time on bicycle forums. Thanks for doing the math!
‘Messy’ street patterns provide the most functional urban space:
“Venice has 1,725 intersections per square mile. “It’s very complex, it’s very messy, and people walk,” said Allan Jacobs, urban design consultant, former San Francisco planning director, and author of Great Streets.
Brasilia, near the opposite end of the spectrum, “has 92 intersections, and you don’t walk there,” The Vancouver Sun reported Jacobs as saying. “Irvine, California is the classic automobile city. It has just 15 intersections, the lowest I’ve ever counted.””
“‘Messy’ street patterns provide the most functional urban space”
Depends what you mean by “functional.” You seem to be equating it to walkability or compactness. I don’t think that’s a very useful conception of functionality.
Some time ago I found interesting study on electric assisted bicycle, compared to walking, cycling, car and sorts of public transportation. May be a bit one-sided though:
Actually, the above mentioned source compares only cycling and cycling with electrical assist, basicly comparing human efficiency with different batteries chemistry including manufacturing and shipping. Source in this study is another study of dutch transportation efficiency:
Thank you for your efforts.
My two cents on this post would be to point to the extraordinary efficiency of recumbent bicycles (practically halving the CdA). Lots of figures for “do the math” types @ http://users.telenet.be/fietser/fotos/VM4SD-FVDWsm.pdf
There´s one thing you have forgotten:
The food you can pick on the way.
On an insane ride with the bicycle from Oldenburg (Oldb) to Bielefeld,
I encountered not few pears, blackberries and hazelnuts.
Would we be using these fruits lieing on the ground or hanging in bushes at the side of the way if we were not going to make the ride anyway?
“We didn’t take hills into account, but to the extent that ups compensate downs, and you pedal at about the same intensity regardless, the power output is constant and we can use the average flatland speed to determine the rate at which energy is exerted.”
A point about which racers (bikers, swimmers, rowers) are reminded: because of the square law relationship between drag and speed, travel at some constant speed V uses less total energy per trip/race than will higher and lower speeds that also average to speed V.
I suspect a more accurate comparison would include the Calories and expense required for the staff of bicycle operators required to provide the equivalent of a diesel powered truck hauling 30,000lbs of freight, multiplied by the number of trucks on the road at any point in time providing the lifestyle we take for granted. Plus the Calories and costs required to provide for the needs of the staff of bicyclists.
Today’s “developed’ world requires far more work than humanity would willingly provide, even if we could afford the pay it minimum wage.
Delightful essay! Thank you!
The «gallon of lard» (well, whatever the unit) is not a complete gross-out. As long as there is more in the diet! On family backpacking trips (many decades ago), I learned that «a little fat goes a long way». And an adequate supply of calories weighs less. My kids, under other circumstances the most fastidious of gourmets, utterly loved the diet of ≈75% calories being fat. Glom the vegetable oil into the breakfast oatmeal!
But then, the efficiency thing can be quite meaningless. We would drive 600 miles to hike a 30-mile stretch of the Appalachian trail.
Another thought that occurs to me: The big adjustment for food distribution and farming here is very important, but isn’t it also true that only muscle-power forms stand to benefit much from improvements in that factor? Less oil-intensively car-drivers don’t make much difference on car economics.
But I am impressed by the big change in the numbers Tom calculates for bikers and walkers.
Great post. You arrive at roughly the same figures that I present in my book The Cyclist’s Manifesto. Traveling by bike is even more efficient than travel by train, is about three-four time more efficient than walking, and is a helluva lot of fun.
Some quibble — the passenger in the car is going to consume roughly the same amount of food energy as the casual bicyclist.
A number of comments in this thread have gotten me to do a fair amount of Googling, and I find myself drawn to this:
It would actually seem to come surprisingly close to meeting many (but certainly not all) of my minimum requirement for an electric vehicle, but oh-by-the-way it “just happens” to primarily be a high-performance human-powered vehicle. It looks like it would get the job done as an around-town pure electric vehicle and like it’d be an absolute blast for human-powered “just for fun” riding as well as be a pretty good cross-country touring rig. I can see using it for in-town trips where clothing requirements would rule out a bicycle as well as for cross-town trips where it’s okay to get sweaty but the distance is far too great to cover on a traditional bike in the requisite time. And it’s unlikely I’d even notice the extra electricity usage, as opposed to with a full-sized EV (though, granted, I’ve got lots of capacity to spare from my PV system).
I need to do a lot more research, but, at least at this early stage, it seems deserving of such research.
Any chance somebody reading these words might have some insight to offer?
I would love it if you would do this for manual wheelchair users.
MPG of human-powered transport vs machine-powered is certainly an interesting thing to discuss, and I appreciate Tom Murphy’s thoughts and hard work on it.
Going from comparing MPG to saying which kind of transportation is better, socially, would require a great deal more information, as well as other perspectives, than the relative consumption of energy per mile.
For example, in the times when one calorie of food required less than one calorie to bring it to the table, humans spent all of their working time on food gathering, with the rest of the time spent on cave painting and other low-tech activities, in societies that couldn’t afford to allow people to specialize in say, studying physics full time. The extra energy we use to support our lifestyles is not all waste, pollution, and extravagance, but allows us to live lives that are not dominated by coping with the environment that Nature hands to us.
Oh yeah, you sure do consume more food when on a cycle tour! I can’t stop eating! Your body turns into a machine when you get really fit. Just add energy and it performs. You rely less on your reserves.
This is my favourite cycle touring blog. He’s ridden all over the world, currently in California.
I went on a trip by bicycle last year, going a little over 6,000 miles total across Canada and the US. I had about 60lbs or so of gear with me and while I did find that I had a healthy appetite while on my journey, I did not find that I ate significantly more than I do now, especially due to finances at the time (though I do ride a bicycle as main transportation in my hometown, it is not ~80 miles per day). I weighed about the same when I returned as when I set out.
Tom, I wonder if you could provide links to your point that for every calorie of food energy it takes 10 of fossil fuels to produce it. That’s something I have been pondering a lot lately and I would love more sources for that.
A good overview of food energy is at http://pge.uchicago.edu/workshop/documents/martin1.pdf, and references therein, which indicates that 17% of the U.S. energy budget is used for food purposes (planting, fertilizing, irrigation, harvesting, transportation, refrigeration, preparation, etc.). This ends up producing about 3800 kcal of domestic food per person per day—perhaps 2500 of which is eaten per person per day (yes, there is food waste). 17% of our energy turns into 1700 Watts per person, or 35,000 kcal per day. So there is a 10:1 ratio. I’ve seen several other estimates coming at it from different directions always getting similar answers. Pimentel has done significant work in this area, for instance.
Fun work! There are of course lots of interconnecting dimensions that are hard to get a good handle on.
How about extending this to a full thought experiment: a city built for cars vs a city built exclusively for bikes?
* comparative energy costs of manufacturing bikes vs cars
* comparative energy costs of the road infrastructure.
* the areal requirements of road surfaces on the extent of cities and thus the average travel distance
* the energy savings of health effects on a population raised on bikes vs cars — obesity levels have a direct impact on health expenditures
To integrate all of this we might need a simulation; lots of interacting parameters.
We’d need waterways or rail for heavy transport. Historical examples: maybe Suchou in the ’50s? Or a Dutch town?
I live in Santa Monica and bicycle to work on a borrowed bike — the marginal energy costs are really close to zero, since the increase in food intake is negligible over driving or taking the bus. I conclude the mpg is close to infinity!
I don’t think I need to do the calculation: bikes would win by a wide margin in total energy. But try to get up the votes to transition a city to bikes/walking from cars. Energy is not important enough to us—yet.
If we ever do decide to transition out of the cars and into public transit and onto bikes, we’ll be in luck. Our current road system wuld be amazingly luxurious. Even half our current road capacity would probably be overkill. And all those parking lots could be planted or developed, and all those garages could get converted into additional living space (or workshops or storage or whatever). Parking garages might be hard to re-porpoise, but there’s always the wrecking ball if nothing else.
Hell, just getting back the on-street parking would be huge…turn all that into dedicated full-sized exclusive bike lanes, and you’d see lots of people too scared to bike today suddenly decide biking is a lot more fun than fighting over that last spot in the garage.
Reasons why a large-scale transition from cars to mass transit and bikes is extremely unlikely in the foreseeable future:
1. Huge advantages of cars (speed, comfort, convenience, privacy, flexibility, etc.)
2. Huge sunk investment (trillions of dollars) in car-oriented urban forms.
3. Huge reserves of fossil fuels. Not just conventional oil, but also shale, tar sands, natural gas, coal, etc., plus technologies to convert coal and gas to liquid fuels suitable for transportation.
4. Huge potential to increase the energy efficiency of cars. Current average efficiency of U.S. automobile fleet is only around 20 mpg.
5. Huge potential to shift from internal combustion engines to electrical propulsion, further increasing the range of potential energy sources (nuclear, renewables).
6. Huge potential for automation. This would produce many benefits, including greater safety, less road congestion, and more efficient use of roads and parking areas. Self-driving cars would allow automated taxis to replace most or all existing urban mass transit.
You factored in the energy cost of producing food and added it to the energy in the food, but you didn’t do the same for gasoline. Extracting and refining require energy as well. For some of the costly operations like the oil sands I think the amount of energy may be close to or greater than the amount of energy in the gasoline produced. The reason it is still profitable is because they are burning cheap natural gas which is usually a byproduct anyways to produce expensive gasoline..
I’m getting a lot of this one. Conventional oil has an EROEI of 20:1, so a 5% “tax” to get it out of the ground. Refining gasoline exacts another 25% tax. Food production, on the other hand, comes with a 1000% energy tax—the way we produce it now. It wasn’t worth the effort to factor in minor effects on the gasoline/oil side when my overall estimates are only factor-of-two quality. For tar sands, etc., the EROEI may be in the 3–5:1 range, resulting in something like a 20–33% tax. Still not in the ballpark of our net-energy-sink food story.
Could you maybe do a full post on the energy costs of food? I’d like to know how all that energy gets spent, what it’ll mean when petroleum gets expensive, and how we might continue to be able to afford to eat when petroleum runs out.
Mexico right now is having a terrible drought. Farmers are devastated. The lucky ones in some areas are harvesting a tenth of what they normally would. Cattle are dying of thirst by the tens of thousands. All their woes could of course be solved by transporting water or food into the region or people out of it, but the energy costs (let alone the financial costs) for such an undertaking would be incomprehensible.
What will happen to the planet’s several billions if even non-drought farming requires proportionally-similar extravagant energy resources?
Of course are cars better in terms of mobility but only for long range trips.
When you are inside your city I think riding a bike makes definitely more sense. You will save money, exercise and do something for your body and mind and also decrease the CO2 emissions. Moreover, it sounds funny but sometimes you are even faster with a bike than with a car (if you dont leave the city)
If you factor in all the time an individual spends working to _pay_ for one’s car (2 hours per day on average, for Americans- and that’s a conservative estimate), and consider that American drivers sit in their cars for an additional hour each day- then factor in health and tax cost detriment to the individual, and spread across society as a whole, the bicycle is faster than the automobile far more often than anyone would think.
Although a car costs about 2 hours’ wages per day, on average, it also saves a tremendous amount of money, in other ways, at the same time. Owning a car allows a person to live in suburbia, which saves far more money than the cost of the car.
A bicycle doesn’t go any faster simply because it costs less than a car. If people didn’t think the benefit of cars was worth the cost, they wouldn’t buy them or support car-oriented public policies. Cars have come to dominate the transportation systems of all or virtually all wealthy democracies because people find them so much better than the alternatives. Obviously bikes have some advantages — they’re very cheap and they can promote better health, for example — but those benefits are not important enough for bikes to attract more than a tiny share of the market.
I really enjoy reading your articles, but one variable you may not have factored into your equation is that a calorie does not have the return value as science may think it does on the human metabolism. Read Scott Abel’s article.
One of the prevailing industry myths that just continues falsely chugging along is the myth that calories measure metabolism. They do not. They never have. If you look up the definition of “calories” in a good dictionary or reference you will see no such reference to metabolism. The simplest definition of a calorie is “a larger unit of energy – this unit of energy is equal to the heat required to raise the temperature of 1KG of pure water by 1 degree Celsius.” – Sounds pretty complicated and mathematical, and it more or less is. But what you don’t see here is a reference to metabolism and food correspondence. That’s because this is not what calories measure and never have. At best calories can “generally” assess the energy content of a food value, but this is once again unrelated to metabolism. In fact even as a measure of food values it is not very accurate. In other words, if you look in terms of the energy of vitality, or vital energy delivered by food, this calories assessment is not all that accurate.
I’m glad you said this: I have always suspected a broken correlation, but have never dug in to the topic. But I believe that Caloric content of food has historically been assessed in a “bomb calorimeter,” which is intended to completely combust/oxidize the foodstuff. That’s clearly not exactly what happens in your gut, but still may be a good approximation for the maximum energy extractable from the food.
Consider, though, that if you ask a nutritionist what happens when you eat protein, they will likely say that we co-opt the amino acids to re-use in building cells and muscles, etc. Ask them how much energy we get from protein, and they’ll give the standard 4 kcal/gram answer. But that number assumes total destruction/oxidization of the protein: nothing resembling the large amino acid building blocks will be left. So pick one: both can’t be true simultaneously. I think the right answer may be that we can use protein for energy, when our bodies need the energy more than the amino acid building blocks.
So yes, I think we should treat caloric content of food as an upper limit to metabolic activity. I would be fascinated to learn more about the efficiency factor by which we convert the resource into actual metabolism.
A simple thought experiment should be enough to confirm your suspicions that all kCal of foods are not created equal.
Imagine identical twins who have, until this point, lived essentially identical lives, especially including diet and exercise. We now place one twin on a 2,000 kCal / day “Mediterranean” diet, and the other gets 2,000 kCal / day of sugar water plus the minimum RDA of micronutrients.
On the ‘Net, it’s common to come across what gets derisively labeled as the “physicist diet,” because it naïvely assumes that, if the calories on the package label equals the physical work expended by the person, the person will maintain body mass; increase the work and the mass goes down, increase the food calories and the mass goes up. The actual food calories consumed are irrelevant.
While this is a good first-level approximation, it is the worst possible nutritional advice one can give. As you note, it assumes that human metabolism is perfect and functions as a bomb calorimeter. The reality is far more complex. Even similar simple hydrocarbons, like fructose and glucose, have radically different metabolic pathways…and, in that particular example, the one pathway creates significant amounts of blood-borne LDL cholesterol while the other does not. And, of course, they have different EROI figures as well.
Now, start that same sugar-water “meal” with a good-sized salad, and the mechanical effect of the vegetables in the digestive system along with the chemical effect of the soluble fibre alter the equation even further — some of the sugar will now simply get eliminated before it gets absorbed in the intestines. Bind the fructose in ripe fruit consumed whole and the deleterious effects of the fructose essentially vanish.
I hope you do decide to further investigate the energy budget of metabolism, because I’m confident you’ll be a strong voice in countering the deleterious effects of the “physicist diet.”
Increasing exercise rate from sedentary to light levels results in LESS food consumption. An electric bike with an electronic exercise coach would encourage more sustainable travel and exercise habits. Diagrams and references are included here: