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Transport and ecology: explanations by a world traveler-engineer

After graduating as an engineer, Clément, Julien’s little brother, undertook (also) a world trip by hitchhiking with a friend. On the road, Clément developed a reflection on transport and ecology, a subject that is close to his (and our) heart… Not having a blog but things to say, we wanted (with his blessing) to share his thoughts, giving him total freedom of speech – for a result that reflects its author: punchy, scientific and totally offbeat!

Short introduction

Hey, you! I saw you pointing your finger at this guy with his hot-dog in one hand, his coke bottle in the other: I saw you mocking him, this poor guy who is full of dopamine (because it’s his only source of pleasure, which he dares to call happiness) and who ends up throwing his bottle next to the nearest trash can. You think that he told himself “someone’s going to clean up after me”, that he must be proud of his gesture “which creates jobs”. You’re completely freaked out, you don’t even have the words… “Asshole”, “Another idiot who doesn’t respect anything”, “What a bastard, he deserves to be punished!”
His act is certainly obnoxious, disrespectful, disgusting, selfish. Strangely enough, however, people like you or me who despise him also look kindly on all the others everyday – those who are not judged, but respected. “Look at this one, how well he has made his life for having such a beautiful car”! “How fortunate he was to Instagram his trip to Bali! I wish I could do the same”… Yet, we should all look at the same person who throws his plastic bottle on the ground, that the one who takes his car daily, alone, to drive 10 miles; or the one who makes himself proud to fly around the world every year and comes back full of modesty (because he saw the poverty of the world, you know). You don’t see my point? Wait a minute, I’ll explain!

Let’s talk science

Only because I like it. Okay, it might be a little boring at first, but I promise it’ll get better afterwards. And if you’re not happy, it’s the same thing!
So I may surprise you, but any movement of an object with a weight (kg) greater than zero (don’t make me believe you weigh nothing, you seed eater!) requires energy. Friend of physics, I see you coming: I know, we don’t say “weight” but “mass”… Stop bothering your world, and just replace those words in your head while you read this post. Welcome to the wonderful world of energy conservation! “Nothing is lost, nothing is created, everything is transformed” as that brave Lavoisier would say. So you’re going to tell me: “Yes, but when I’m riding my bike or driving down a hill, I don’t consume anything”. Right, but let me explain a few things…


It took you a lot of energy before you went down that slope, right? You had to push your bike so hard and sweat like in a sauna to get up that hill and enjoy the reward of being able to come down effortlessly? This hard-accumulated energy (at least some of it) is called “potential gravity energy”. And the higher you climb, the more it increases! It’s pretty good, huh? As the name implies, this energy can potentially be transformed into another energy. Typically, when you climb to the top of the mountain with a lot of potential energy, it’s not really any good to you – because you’re stuck to the ground that keeps you from falling. But if the mountain suddenly disappears, believe me, you’ll take a big fall and gain speed! You see, you’ve just converted the potential energy of gravity into real kinetic energy (speed energy): well, it’s the same thing with the downhill slope!

But let’s get back to our story. Every movement consumes energy – even your little legs that have to push your weight in every step! This energy comes from your muscles, which are fed by what you eat (I’m not a biologist, but I’m pretty sure of this). In theory, walking more means eating more. I doubt the simplistic linearity of the thing, because we tend not to specially adapt our diet to our activity…

Also note that walking is not very efficient in terms of energy per kilometre: your legs make the ultimate effort to push you, with all your weight, FOR EVERY STEP. You take one step, and it’s over: your body stops moving. The invention of the wheel will change all that (and this one is, in my opinion, one of the most beautiful inventions of our history). Such a simple thing that preserves (or almost preserves) our movement, it’s beautiful! Now we just have to push the object up to the desired speed, and maintain that speed by counteracting all the friction opposing its advance.

Let’s take the example of your niece on her ice skates: to propel her at 1 km/h (I know, it’s slow), you will have to provide 1 unit of force. But if you want to propel her at 10 km/h (it starts to blow her mind), you’ll have to provide 100 units of force this time. Basically, you’ll need an energy proportional to the SQUARE of your speed.

I’ve already told you about it: it’s the energy you accumulate when you climb a mountain, for example. In an ideal world, you should get all that energy back in a hurry when you go down, but since we live in a shitty world, you’ll only get back some of it (fucking frictions).

Friction-related energies will slow down your movement: for example, friction related to mechanical parts (engine, transmission, axles etc), wheel friction on the ground, air friction… Aside: the energy of the air resistance (also called air friction or drag) is proportional to the CUBE of the speed. For solid friction (aka mechanical parts), it’s “only” proportional to the speed.

Energy supplied to move forward (your legs, the engine…) must compensate for all the energies mentioned above. It’s also necessary to provide more because the efficiency is never 100%… Well it’s more complicated, but that’s roughly it. I see you coming back, physics nerd: I agree, we should reason in force… But frankly, forget it. I feel that I have already lost you with my energies, little trickster, but I will give you an example and you will quickly understand: when you take your bike, you experience without knowing it all these energies. Cycling for a long time is hard, stopping and restarting frequently is hard, going up is hard (but you go down again), riding fast is hard too!


From these simple observations, we can already draw one fundamental thing from them: increasing speed requires a much, much, MUCH greater input of energy. In other words, doubling one’s speed (x2) requires much more than double the energy (x4 for kinetic energy, x8 for air friction energy).
First conclusion:

Moving slowly reduces MUCH the energy consumed

“Yes, but if I go faster, I arrive earlier so I consume less: QED”. YOU DON’T GET IT, IDIOT! Since we have energies proportional to the SQUARE or CUBE of the speed, the linear temporal “gain” (twice as fast = twice as little time) is completely massacred by the energy loss (twice as fast = much more than twice the energy to be supplied).

Weight (in kg)

There is also another variable that has a great impact on the energy to be supplied: the weight (kg). And it’s understandable: moving a fridge will be much more difficult than moving your phone! Weight (kg) is less significant than speed, since it’s only linearly proportional (no squares or weird stuff) to kinetic energy and mechanical friction (except air friction).

Now, let’s put it together: we will calculate the weight per passenger of each mode of transport – in other words, the share of the vehicle’s weight to be attributed to each passenger:
Average weight/passenger for 70kg passengers: [vehicle weight + passenger weight(s)] / [number of passengers].

  • Walk: 70kg
  • Bike: 80kg
  • Car with 1 person: 1570kg
  • Car with 4 people: 1780/4 = 445kg/passenger
  • Bus: 170kg/passenger
  • Tramway: 200kg/passenger
  • Train: 750kg/passenger
  • Fast Train: 1000kg/passenger
  • Aircraft: 450kg/passenger



Last parameter: time! I’m not teaching you anything: moving longer at the same speed requires more energy. And it’s almost linear! (well, actually, it’s not totally true, especially for planes that will consume much more energy at takeoff, but well…)

Double time = double energy 

I prefer to tell you that it’s not simple… Especially because of the frictions that depends on many things: surface, aerodynamics, mechanical parts, wheels or rails… For example, trains will be more efficient in aerodynamics and friction than cars. There are also stops and restarts that require energy to be recharged… It’s also complicated for an aircraft, which has to go very fast to take off and climb thousands of metres, but for which the friction forces have nothing to do with road transport: not the same mechanical parts, not the same aerodynamics, not the same air density, no contact with the ground… So my approach remains simplistic!

And what about ecology?

So the dude puts in headline “transport and ecology”, and he just talks about his science knowledge. Where’s the ecology? Wait, here it comes! The reason I mentioned energy before I mentioned ecology is because the two concepts are linked. Excuse me idealists, but free energy that doesn’t pollute, it doesn’t exist. So let’s start with the basics: polluting less starts by consuming less energy.

A question of choice

Admittedly, in our panel of energy consumption choices there are some good and some bad – but it’s above all a question of choice, or rather a question of “which pollution seems most acceptable to you”.
For example: for an ecology matter, is it better to take a bus or a train over the same distance? All the figures will suggest that you should take the train, as its CO2 emissions per passenger are much lower than those of a bus. It’s normal: if we take a look in France, trains run on electricity, 75% of which is produced by nuclear power, and about 15% by renewable energy, so in terms of CO2, it’s top-notch! Here’s the choice you have to make: do you prefer to emit CO2 into the atmosphere (which contributes, among other things, to climate change) and pollute the air you breathe with micro-particles, or do you prefer to contribute to the burial of radioactive waste (which will remain so for hundreds of thousands of years) and live with the possibility of having a nuclear accident near you?
And I have no answer to give you; it stops there. Comparing these two modes of transport is extremely complicated… The best way to do it, in my opinion, is to be informed, to educate yourself and to get your own opinion.

I would just like to share the scientific consensus on this. Nuclear power with its very low CO2 emission is considered as a good alternative to fight global warming for several reasons (I have no action in the nuclear business, and I am not pro-nuclear either): although there is no such thing as zero danger, this technology is well known and produces huge and controllable amounts of energy. On the other hand, the great disadvantage of renewable alternatives is not so much their lower power, but their intermittency (most of them do not produce energy in a constant way). The management of an electrical network is very complex and without a large-scale storage solution, it’s necessary to have a large part of our production controllable in real time. Although the political involvement is also important, these 100% renewable countries (Iceland, Costa Rica, Norway, etc…) are 100% renewable because they can do it thanks to a huge controllable renewable hydraulic/geothermal capacity. It’s a bit more complicated for countries like France.

CO2 equivalent/passenger

Educating and informing oneself also means understanding the data we are regularly bombarded with. Here, I am going to criticize the way we see the Earth issue: to simplify political decision-making and society’s understanding, we have focused our attention on ONE indicator of transport pollution, CO2 equivalent/passenger. What is CO2eq? It’s simply a calculation method that allows all greenhouse gases to be combined into one. Some gases have a greater or lesser greenhouse effect than CO2, but it all comes down to CO2 equivalent. Example: I emit 1 kg of CO2 and 1 kg of methane. Methane has a greenhouse effect 21 times greater than CO2, so I emit 22kg CO2eq. Simple, effective. We have therefore classified the different modes of transport by calculating their average CO2eq emissions, and dividing by the average fill rate. It’s a bit of a hassle when you know that driving a 4×4 alone has nothing, but really nothing to do with driving a full small city car… Choosing your mode of transport based solely on the CO2eq/passenger impact is like assessing a patient’s health just by his pulse. It’s necessary, but terribly insufficient!

Note: there are many other types of pollution that I will not talk about (soil acidification, loss of biodiversity, rising water levels, plastic pollution, pollution of drinking and marine waters, noise pollution, light pollution, microparticles, odours, deforestation, soil deterioration and so on…).

The current Earth issue is complex, and is not just about climate change and CO2. Making a ranking of pollution from the worst to the least worst is, I think, impossible. These pollutions are often linked to each other…

The different modes of transport and their pollution


With a pretty bad energy per kilometre ratio (strangely enough, evolution hasn’t given us wheels on our legs), walking is not the most efficient way to travel, but it has a lot of advantages:

– It’s our natural mode of transportation (we mustn’t forget that)
– It’s still very slow, with a low weight, so it consumes little energy per kilometer
– It’s good for ecology because it doesn’t pollute or very little (I guess here we don’t double our food intakes of Argentinian beef because we walk 10km)
– Physical activity is good for your body and your mind
– It’s not very bulky: you can walk everywhere and make wonderful hikes!
– You don’t need much infrastructure and equipment (just a pair of shoes).

The only drawback: it’s quite slow, and our feet don’t have wheels to go downhill.


As you can imagine, it’s my favorite mode of transport! It has a lot of advantages:
– a low speed but not too much
– low weight and so low energy consumption
– it gets you physically active
– it pollutes little or not at all
– it uses simple and often repairable / reusable equipment

The problem is that cycling requires infrastructure (roads, paths); but I like to attribute this to the cars that monopolize it.


Well, now we’re getting into the tricky subject… Because regular car use is one of the 4 biggest causes of global pollution. Without talking about pollution right away, a 1.5t thing to transport a few 70kg dudes is not very crazy… But why building 1.5t machines, you might ask? Well, you’ll prefer to have a big trunk to do your shopping, 4 other seats to carry your whole family, have the possibility to go at 200km/h to race with your friends, use the air conditioning when it’s hot, the safety equipment on board in case of accident… All that, we pay for it at the end in terms of energy to supply. Well, I’m not going to blame safety, but the rest of it…
If we take a ratio of minimum 445kg/passenger (car with 4 passengers), the classic car is not very optimal for our daily use. Well, if you have to drive 100km to move your whole family with a full trunk, and at a reasonable speed, then this may be the most suitable transport. On the other hand, using your car to go to work alone at 10km is not good for your lungs and ecology.

If you have understood my energies story, you understand that braking, accelerating, braking, accelerating, is all about the energy to provide. Driving in the city is a source of a lot of useless energy consumed by our frequent stops. Surprise (or not): driving in the city can consume more energy than on the highway where the speed is stable and constant!

Let’s talk about pollution now. With a nice combination of noise pollution, light pollution, air deterioration, greenhouse gas emissions, noxious gases, micro-particles, the use of more and more electronics (often to limit our emissions, ironically), the car is certainly not the first in the transport class. All the same, I welcome technical progress, allowing for much better efficiency (i.e. using the maximum amount of energy from our petrol to move forward, rather than forgetting the half that escapes into the air). Having said that, pollution will also depend on the type of fuel, so it’s time to get into the diesel/petrol fight.

In France, there is a large and well-known diesel lobby. We are now slowly changing course by balancing taxes… But what’s the difference with petrol? (another scientific parenthesis, but I’ve already told you I like it!). A diesel car is simply a car whose engine runs without a spark plug. WHAT’S ARE YOU TALKING ABOUT? Simply put, a gasoline or diesel engine is a box where we produce a lot of controlled mini explosions, in order to push pistons that transmit this energy to the whole car. When I say explosions, I mean spark plugs that start the process: with diesel, it’s great, you don’t even need spark plugs, it explodes automatically under pressure, easy. But let’s go back to our pollution issue. Here again, it’s difficult to make a classification: these two fuels simply have very different pollution levels!
– Gasoline will essentially emit large quantities of CO2: not great, eh?
– Diesel emits little CO2 (I specify: harmless for humans to breathe), but in return it will emit gases harmful to the environment (nitrogen oxides) and many microparticles very bad for human health (you know those that get lodged in your lungs… You like it, I know it!)
If cities are full of diesel cars, where the population is concentrated, you can see the problem… No, you can’t? Then we can’t do anything more for you, I’m sorry. So which one would you rather do: destroy the planet with CO2 and continue this infernal machine, which, without change, will cause the slow and painful extinction of our species, or give birth to cancers and asthmatics? IT’S AWFUL, ISN’T IT? YOU DON’T KNOW WHAT TO DO? (and I’m not even talking about production pollution, all those disgusting old car breakdowns everywhere, all those micro-particles due to braking, and above all, I won’t even talk about all the road infrastructures!…)

Behind this extremist moralizing speech, let us be relevant and promote sobriety, a proper use of car, the one that makes sense!
– Take your time: speeding is stressful, it kills and pollutes;
– Use your car to carry a lot of people or more weight over long distances. Think about carpooling!
– Drive zen, favor smooth routes to limit braking. It will make your trip more pleasant for your passengers, you and the planet!
– If you use your car once a week, think about sharing the ride with friends or neighbors… It’s great for your wallet too!

And what about electric cars?

It’s always the same: there’s no better, or worse. We are told that electricity is good, it’s beautiful, it’s clean. NO! Electricity is energy too, it pollutes too… You’ll have to charge your electric car on a charging station that uses your country’s electricity grid – but depending on the country, there will be huge differences in pollution (whether the electricity comes from gas, coal, hydro, solar, nuclear…). Again, it’s a matter of choice! 

The advantage of the electric car is to export urban pollution elsewhere (but is it really a solution?) and improve air quality and noise pollution in cities. Let’s keep in mind that increasing energy transformations increases losses as well. Yes, every step of the way, from the raw material to the electricity produced in the power plant, from the power plant to your city, from your city to your terminal, from your terminal to your battery and from your battery to your engine… It’s starting to be a lot, and it has to be part of our decision making. Coming back to the fuel, it is injected directly into the engine (no loss as opposed to a battery), but it had to be transported with trucks, boats… which implies other energy losses and pollution. Which of the two systems is more efficient? It depends mainly on how your country produces its electricity… What if we had a nice energy mix that could offer a cleaner alternative to combustion cars in the whole production chain? 

For instance in France, the electrical conversion efficiency is 39% – it takes 1 unit of primary energy (nuclear, hydraulic, fossil, etc..) to obtain 0.39 unit of electrical energy on our plugs. But are the efficiencies of electric motors much better than those of combustion engines?

  Conversion efficiency Engine efficiency Battery efficiency Summary
electric 39% 90% 90% 32%
combustion 87% 30% 26%
Comparison between electric and combustion engines



Electricity production chain seems to be more efficient than the one of diesel and petrol, in the sense that an electric motor in France will consume less primary energy than a combustion engine (see summary column). Moreover, with nuclear power in France, electric powered engine seems to be a good candidate to combat climate change compared to diesel and petrol. The big big problem (because I feel that I made you dream!) is obviously the battery which is very, very bad for environment in terms of recycling, costs… We then have a combustion car which pollutes a lot during use (by fuel combustion) compared to an electric car which pollutes a lot during manufacturing (especially by the battery). In other words: a new electric car required much more energy to manufacture than a combustion car, but its engine pollutes less. This is why an electric car becomes interesting if the environmental cost of manufacture (estimated per kilometre) has been amortised by its use. It is then necessary to drive a lot of kilometres (between 50,000 and 100,000 km according to a study). You can also take a look at this TEDx on these grey energy issues (total energy of an object’s life, from extraction to recycling, including manufacturing, maintenance, use, etc.): Why keeping your old car pollutes less than buying a new one.


Bus is the same as car, except that the kg/passenger ratio is much better: between three and nine times better (170kg/passenger), which makes it much less energy consuming despite the frequent stops. Be careful however to the extreme sensitivity of the load factor: being 4 in your car is much more interesting than being 10 in a bus! This is a crucial point, almost political, in the implementation of public transport. Can we justify maintaining a bus line during off-peak hours to transport a handful of people? It may make sense politically, but it does not make sense energetically. And yet, there are solutions… Why not deploy mini-buses when demand is low?


Train has its advantages and disadvantages of course. As seen above, it’s very very heavy, weight per passenger ratio (even with a good load factor) is bad. The french high-speed train, TGV, goes very fast which increases the consumption a lot. You could think that it’s not that efficient but it has a considerable advantage: it’s profiled structure limits the importance of air friction (major responsible for its consumption) and once divided by the number of passengers, its value makes it a very efficient transport! It has other advantages such as a good filling rate, more interesting rail friction than tyre/road, straight line runs, limited stops (especially for the high speed train).

Although fast and heavy, high speed train is the most energy efficient motorized public transport. Followed by subway and trains which, because of their frequent stops, require more energy per kilometre traveled and per passenger.


You’ve been waiting for this, don’t lie! It’s the most sensitive subject, the one that makes you feel guilty, makes you angry during family debates or at the corner café. Like cars, aircraft is in the podium of polluting industries. Let’s take a look at it…

In terms of weight (kg) per passenger, we’re in the middle of the ranking (I expected worse to be honest). As for speed, it’s getting awful: an aircraft goes fast… This is both the objective and a necessity: to make a beautiful 60-ton beast fly, you have to go very fast. We have seen that speed requires a lot of energy. But beware! Friction forces are different (lower air density at high altitude, advantageous aerodynamics), and mechanical friction has nothing to do with it too (here, the only parts responsible for the thrust are the engines; there is no transmission, axle or wheel to fly). And this friction is often more energy efficient! On the other hand, enormous speeds (1000km/h) quickly make up for this advantage – I remind you that with an air friction energy proportional to the cube speed, a speed 10 times higher implies an energy to provide 1000 TIMES HIGHER. And we’re going to have to climb those 10,000 metres… That means a huge cost in gravitational energy! It’s not for nothing that take-offs are responsible for a large part of the energy consumed and therefore the pollution of airplanes.
This is why the CO2eq/km/passenger indicator shows that short journeys have a much higher pollution per kilometre than long journeys. The share of the take-off (which will remain the same take-off for a short or long trip) in the total trip is enormous for short distances – therefore the CO2eq/km will be higher. Once again, be careful: it’s important to know the pollution per kilometre but increasing the distance will increase your impact EVERYWHERE, and this despite the best CO2eq/km ratio! A lower CO2eq/km ratio should never be an ecological argument for driving more kilometers!

Scientists are just starting to worry about the effects of contrails (you know, those white trails behind planes that conspiracy theorists say are chemical clouds voluntarily dumped by governments on our faces!). It turns out that they are accentuating global warming by cooling our days and warming our nights (warming effect at night is greater than the cooling effect).

We can also notice that for long distances, CO2eq/km is similar to that of a car. Isn’t that surprising? You have to look at it in several ways:
– If you compare an airplane, with a good load factor, flying long distances, with a single driver in the car, you can find similar CO2eq/km. The trick is that you rarely drive 1000km alone! With a full car, you divide by 4 or 5 its emissions per passenger and per km compared to a plane (which many press articles forget to mention…).
– We generally travel MUCH more distance by plane than by car (Bali is not nearby) so it’s without hesitation a huge source of emissions compared to the car – even once a year!
Pollution is quite different, and the long-term effects of air pollution from aircraft are still being studied, but it seems fairly clear that this pollution at altitude has a greater impact than on the ground in terms of the greenhouse effect (see condensation trails above). Don’t forget to take into account noise pollution and all the space required for infrastructures (airports vs. highways).

Aircraft does have one definite advantage: it flies! Flying limits the distances to be covered, sometimes by a factor of two. When travelling across Europe by car, train or plane, the pollution balance is not obvious and flying is not necessarily the worst. Flying also makes it possible not to have to stop, and therefore not to have to re-obtain the kinetic energy once acquired. Although it’s necessary to limit the plane to the maximum, it can be in some cases, if the trip is URGENT and VERY DISTANT, the most suitable means of transport.
But you can do better than worse with a few precautions:
Fly by day to avoid nighttime heating from condensation trails,
Favor non-stop flights because take-off is the biggest kerosene eater
Avoid aircraft for short distances if possible

Compensating for pollution

I deal with this subject because I have found that it’s possible to offset our carbon emissions when buying a plane ticket. It’s called “offset”. Basically, you are offered to pay a little more and bang, your carbon emissions disappear! In concrete terms, you give your money to associations, foundations, companies that have “carbon capture” or “emission reduction” projects such as reforestation, renewable energies… We’re touching on a sensitive point that personally bothers me: because it’s good, even better than nothing, green capitalism uses this very good tactic to make travelers feel guilty and encourage, or rather not “discourage”, people to take a plane. Oh, I almost forgot: CO2 does not disappear because we pay a little more. Reforestation is nice, but the real effective CO2 traps are forests that are hundreds or thousands of years old. Renewable energies are also nice, if they do not become a reason to consume more, as history tends to show us. There is no secret: pollution is often irreversible with extremely complicated chain effects… You, who are nature lover; marvel at its complexity and beware of simplistic solutions.

By hitchhiking

Difference with carpooling

What a smart idea! Use the transportation system already in place to improve it, meet new people and all for free! But what does it change in terms of ecology? Is it so different from carpooling?
There is one difference that I find enormous: the carpool driver will (not always, it’s true) define his route for economic reasons. It’s indeed more advantageous to carpool than to take a train ticket, but driving alone is not advantageous in terms of ecology! There is therefore a financial argument that motivates the carpooling market: it’s in the expectation and hope of having passengers that the driver will also travel by car. In this case, the pollution is shared equally among all the passengers: normal! And that’s good enough!

Hitchhiking and ecology

Hitchhiking is a bit hazardous, as it does not encourage any system of supply and demand (it bypasses the market laws). Nobody is going to take his car in the hope of picking up a hitchhiker! This mode of transportation simply optimizes, with people’s generosity, the system already in place without changing the drivers’ decision. As a result, pollution distribution is different: we don’t equitably allocate car emissions to hitchhikers and drivers. But don’t think you don’t pollute at all when you hitchhike! You have to consider the added weight (kg) because the car will consume more.

A simple example of a classic car that consumes 10L/100km: an hitchhiker who weighs 80kg with his bag represents about 5% of the weight of the car (1500kg). If we consider the consumption of the car as being linear to its weight (which is actually not accurate, but almost), the car will consume an additional 5% or 10.5L/100km. The hitchhiker’s share is therefore 0.5L/100km, which is not so bad!

More about the calculation of our carbon footprint
when hitchhiking


I see you coming with your stupid arguments “yeah, but if everyone hitchhikes, it doesn’t work anymore! You can’t think of that on a large scale, and you depend on others, you parasite!” Wait a minute, wet blanket! It’s true that if everybody hitchhikes, there wouldn’t be any hitchhiker left. Are you also going to blame those who collect food scraps from supermarkets? The same ones who try to limit food waste? The system wouldn’t work properly either if everyone used expired left-overs! It’s the same with hitchhiking: a system with faults is bound to offer an alternative to its optimization.

People who take advantage of these alternatives are faced with several disadvantages: for the waste, it will be less fresh or less beautiful products; for the hitchhiking, it will be waiting or discomfort… But hitchhiking has the advantage of offering the driver a pleasant company, an opportunity to have nice conversations, sometimes to create friendships. Never forget that the pleasure of giving is stronger than the pleasure of receiving (and it’s not me saying it, but science!).

Let’s assess all these!


In order to support my thought, I created a simple energy calculator that takes into account all the energies mentioned so far as well as important parameters such as filling rate, number of stops/restarts, speeds. This calculator is open source – and if it amuses some of you to check/calculate, you can click on this link:

Energy efficiency review

The idea of this graph is to show the differences in energy efficiency. In other words, for one kilometre, for one passenger, how much energy is needed?

backpacking Jul&Gaux SerialHikers stop autostop world tour hitchhiking aventure adventure alternative travel voyage volontariat volonteering transports écologie ingénieur clément tour du monde énergie pollution autostop avion train bus marche vélo graph
Energy balance per km of transport (Wh / km.passenger)
in purple: drag energy / km – in orange: kinetic energy / km
in blue: potential energy of gravity / km

Unsurprisingly, car with one person on board and a full plane are off the hook, while the bus, full cars, and trains are more efficient. Also note that for buses and cars, a smoother drive in the countryside consumes less energy thanks to a better stop/peak speed balance: in city, the number of frequent stop/start operations implies a high energy consumption despite the low speed; while on the highway, the low number of stops is not enough to avoid a high energy consumption to go fast.

Another criterion that needs to be studied and which is close to my heart is the way we use transport: as stated above, transport can be relatively efficient but very energy-intensive, depending on how it’s used over very long distances and at high speeds.

To do this, we will look not at consumption per kilometre but per unit of time, here per hour. How much energy is needed per passenger travelling in an hour?

backpacking Jul&Gaux SerialHikers stop autostop world tour hitchhiking aventure adventure alternative travel voyage volontariat volonteering transports écologie ingénieur clément tour du monde énergie pollution autostop avion train bus marche vélo graph
Energy balance per hour of transport (kWh / hr.passenger)
in purple: drag energy / hr – in orange: kinetic energy / hr
in blue: potential energy of gravity / hr

No comment: aircraft use is awful, simply because it covers many more kilometres in one hour and therefore will consume much more. Similarly, highways are generally used for long distances. Who says long distance says more energy!

Once again, better energy efficiency should not be an argument for travelling more kilometres. It’s simply a criterion (admittedly an important one) to be taken into account. The best solution remains, to date, to limit our travels. And if we have to go very far, then in my opinion we should focus on the best energy efficiency.

Just because a transport is efficient doesn’t mean you can/should travel a lot of kilometres; it’s because you have to travel a lot of kilometres that you need to favor a more efficient transport!

** For all experts out there, these graphs are expressed in final energy. I have created a tab “primary energy” in my open source file which takes into account the conversion efficiencies in France, giving a slight disadvantage to electric transport…

Ranking by transport mode

To sum up, I had fun making small tables by evaluating each mode of transport by use (small, medium, long distance) according to the 3 parameters that interest us: energy efficiency per kilometre, energy consumption per hour and pollution generated. I then classified the results according to 4 grades: green (for the best), yellow, orange and red (for the worst).

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First table: urban use (<10 km) – Second table: average use (10< km <200)
Third table: long-distance use (>200 km)

Regarding the environmental impact, as we have already seen, French nuclear power offers enormous advantages in terms of CO2 emissions. As for other pollution (fine particles versus nuclear waste, impacts related to extractions, transport, leaks, water and soil pollution, etc.), it’s extremely difficult to integrate them into the calculations due to a lack of sufficient knowledge. I will consider here these pollutions as almost equivalent to nuclear pollution.

The transport cost

Don’t you think it’s very easy to travel by plane, even if it pollutes a lot? Almost too simple? I’m not an economist or an aviation expert; I’m not going to get out of supply/demand theories or do a financial analysis of the industry… But let’s be logical for a moment: the cost of transport should, in theory, be correlated with its speed – if we “save time”, we have to pay for it, as well as its comfort (standing, sitting, lying down, extra services etc).

Let’s take the example of a one-way Paris-Marseille journey (~775km) using the different existing modes of transport and with only one passenger. For public transport, price range observed is defined for a regular economic fare. Concerning the car, we will estimate its cost on the highway with one person. If there are several passengers, the price would have to be divided by as many people as the car carries.

backpacking Jul&Gaux SerialHikers stop autostop world tour hitchhiking aventure adventure alternative travel voyage volontariat volonteering transports écologie ingénieur clément tour du monde énergie prix avion tgv voiture bus
Price (in Euro) versus time (hour)
in orange: aircraft – in green: TGV (high-speed train) – in light blue: car
in electric blue: bus

With almost the same comfort level, we can see that the faster we go, the more we pay. But the plane is an exception to this rule: you can find much cheaper offers than the high-speed train. Why? From a purely scientific and energetic point of view, it makes no sense and it doesn’t really help save the planet. Perhaps the problem is not the plane itself, but its price, which is too low? Like everything else, a technology is not fundamentally wrong, but its use that can be toxic… We could argue about why: kerosene taxes, policies that are good or bad… But I don’t have the knowledge to do that. According to the rational and ecological concept of “polluter pays”, it would be a matter of seeing our aircraft tickets go up before this exponential growth in air traffic becomes irreversible…

I’ve done the same job on several routes, mainly European ones for the sake of simplicity and the results are very similar with some variability in the bus-car-train order – but the idea remains valid I think.

My conclusion

Moving around pollutes anyway. So we need to take a thoughtful approach on our travels. Moving fast and far is a source of big pollution anyway. Waiting for science to find a new technology that would take us to the other side of the world cleanly, or to solve our current ecological problems, is a way to get rid of our guilt and plunge straight into the wall. It would be a way to stop kidding ourselves with our “yeah, I went to Cambodia for a week to save orphaned children and offset my CO2 emissions” speeches. Go back to feeding yourself on insects, you quack! So I was saying that we should take responsibility for our choices: you travel around, you pollute, so you take your clicks and slaps, and your responsibility for your choices, it will relieve your brain, I promise you! Or you can rethink the way you move around, and be aligned with your values:

1. By moving less, less far, less fast and more fluidly
2. By limiting motorized vehicles, which are inevitably more fuel-intensive
3. By minimizing the kg/passenger ratio by filling our vehicles
4. By minimizing our need to own everything, thinking about sharing (bikes, cars, etc.).
5. By trying adventurous alternatives such as hitchhiking!

And I think that this, is the beginning of happiness.

Useful links:

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Clément left France in 2017, with his travel buddy Raphaël and thumb up towards Asia and Australia. Back in France, Clément became a project manager for a citizen’s renewable energy cooperative.


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