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Background

In general, for a given transportation medium, modes of transport that are more densely populated are more efficient than those that are more sparsely populated. For example, it is not a complete comparison to say your Prius gets 50 mpg and the city bus only gets 5 mpg, therefore your Prius is an order of magnitude more efficient than the bus. This is because that bus also moved twenty people at once. So the per-person fuel efficiency of the bus is actually an order of magnitude higher than the per-person fuel efficiency of your Prius.

However, this does not necessarily hold up when comparing modalities from different transportation media (for example, flying versus taking the train). This is because of the differing challenges facing a given medium de facto, such as the fuel spent keeping an airplane aloft rather than propelling it forward, whereas the train is held aloft by static rails.

Question

In deciding on a vacation, how can I determine the more sustainable option between, for example, a train tour of the continent and a commercial cruise to Mexico? The ship likely has an order of magnitude more passengers than the train, but the expense of pushing the hull through water is significant.


Update for 2018

Recent research suggests that vacationing is not only bad for the environment in general, the energy usage from our vacationing is getting worse:

Here, we quantify tourism-related global carbon flows between 160 countries, and their carbon footprints under origin and destination accounting perspectives. We find that, between 2009 and 2013, tourism’s global carbon footprint has increased from 3.9 to 4.5 GtCO2e, four times more than previously estimated, accounting for about 8% of global greenhouse gas emissions.

Via Nature Climate Change

Thus, if at all possible, find the most ecological means of vacationing.

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    Related: Lowest impact transatlantic crossing – Jan Doggen Jun 27 '16 at 20:21
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    Comes down to choosing your battles. There are far more serious situations we humans need to address rather than the best and least environmentallly harmful way to vacation. Relax, vacationing will be rare to non existent. Just wait until you see what the big boats do to the ocean, the planes...then compare to Fukishima, millions of ports/docks with all the poo poo, airplanes...geoengineering. Homeowners who don't read the labels/lnformation nor have any idea was an MSDA sheet is...just wait until you learn where our SHIT goes. Do you know? Choose your battles, enjoy a vacation... – stormy Jul 2 '16 at 1:01
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    Possible duplicate of Impact of various travelling options – THelper Aug 10 '16 at 10:27
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The issue here is using the word "sustainable" as something like "healthy" - the more the better - when it actually refers to a balance between, say, resources used and resources generated. If I buy a plastic thing for putting leftovers in, and then throw it away, that's not sustainable, it will lead to landfills full of plastic. If I fly all over the world in a private jet burning jet fuel, that's not sustainable, I'll use up all the petro products. You want to choose a sustainable vacation option, but that doesn't really mean "the vacation option that uses the least fuel possible."

If you start saying "well, let's use as little fuel as possible" then as little as possible is zero. So your vacation should only involve walking. Even waling is kind of wasteful since you're wearing out your shoes and eating more than usual. If you ride a bike, the bike had to be made. If you go canoeing, unless you live on the lake you have to get your canoe to the water, and the canoe had to be made. All this uses energy.

So really what you have to do is compare two options that you would enjoy, and ask "which uses less resources?" Maybe one is a 12 hour flight on a large plane (more efficient) and one is a two hour flight on a small plane. You can even worry about whether planes on a particular route usually fly full or not. Don't get yourself distracted about "fuel to hold up the plane" vs "fuel to push the plane forward" since that's not really the physics, and anyway all that is reported is the overall fuel consumption. You can't reason out which is more efficient (those train rails didn't grow there, for example, and it takes resources to build airports and cruise ports too). You can just look stuff up and divide by passenger load. It's really a complex calculation.

Here's how I do it: I use money as a rough proxy for energy. If A costs twice as much as B, it probably consumes twice as much energy per passenger in running it, and in building its infrastructure (amortized over the infrastructure lifetime), maintenance, etc. This isn't strictly true; some price differences are because of better staff or free food or something, but it's close. So if you find two vacations that seem equally fun to you, but one has double the transportation cost of the other, the one with the lower transportation cost (whether the accomodations, attractions, food etc are more or less) is the more sustainable choice.

Another advantage of using cost as a proxy for energy is that it lets you evaluate the importance of the choice. Living somewhere that lets you commute by foot, bike, or public transit will save you far more money ever year (compared to driving a car) than most people's vacations cost. That means your efforts should be on that more than on vacation choices. OTOH if you already commute like that, maybe putting some attention to the vacation is a smart next step.

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    I recommend being careful with the method "more expensive = more fuel used". In our current global market, there are many examples of mind-boggling inconsistencies where a more energy-intensive option will be less expensive. When choosing airfares for example, it is common to find a cheaper flight with one or two extra stops and close to double the distance a direct flight would travel. Another example is the eco-friendly vacation options that are way more expensive than the standard fuel-intensive option, because of scale and the extra work needed to offer an alternative. – stragu Sep 7 '16 at 0:24
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Instead of comparing the impact of each travelling modality with 1 traveller, you want to compare modalities per passenger-kilometer or passenger-mile with average load. This means that you divide the total impact by the number of passengers and the distance travelled. Energy efficiency or CO2 emissions is almost always used as a proxy for impact as it is relatively easy to determine, but there are other environmental impacts as well, e.g. noise pollution, land usage, animals being run over, etc.

The following transportation methods are listed from low-impact to high-impact:

  • Walking
  • Bicycling
  • Sailing boat (no engines)
  • Train (electric)
  • Bus (petrol)
  • Car (petrol)
  • Campervan (petrol)
  • Airplane (kerosine)
  • Cruise ship (bunker fuel)

This list is loosely based on this chart by David MacKay as well as some articles I read in the past. Do keep in mind that this list is very much a simplification because in real life the exact impact depends on lots of factors, for example:

  • Exact type of vehicle (e.g. a large family car consumes more fuel than a small family car)
  • Type of engine and fuel (e.g. cruise ship running on gas oil, diesel oil, or bunker fuel)
  • Number of occupants (e.g. a car with 4 people has almost half the impact per passenger-mile than the same car with 2 people in it).
  • Distance travelled without stops (making intermediate stops can cost a lot of energy, that's also why short flights have a bigger impact per passenger-kilometer than long flights)
  • Weather conditions (headwind or not)

Some airplane companies and train operating companies list estimated CO2 emissions for your trip when you plan it with their software so that can give you some indication, but be careful when comparing results between different organizations as the used method of calculation can be rather different (see this answer comparing different sites for the same trip for example).

BTW if you are interested in other tips to make your holiday more sustainable I can recommend reading the free Lonely Planet's Guide to Responsible Travel

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    Most people don't plan holidays on kilometres though, but on days or weeks. So the analysis should probably balance days travelling vs fun had in some way, rather than assuming someone would actually say "should I walk from London to Cape Town, or drive, or fly?" - in that case the extra food consumed while walking for a year dominates the cost compared to one day flying. I know when I cycle tour the cycling is much of the point, for example, so "fun had per day cycling" is hugely higher than "fun had per day on the train getting to somewhere interesting to cycle tour". – Ⴖuі Aug 11 '16 at 22:27
  • @Ⴖuі That's one way to look at it. Another is to select a destination, determine which transportation methods can be used to get there, and then for each method make your own personal weighing between lowest environmental impact and most fun. – THelper Aug 12 '16 at 6:02

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