Communal travel decisions

HighwayIn my last post, I wrote about carbon emissions for different modes of travel on my trip to WI. Arduous picked up on my last paragraph about how these are potentially unreliable estimates and expanded that into a thought-provoking piece on calculating carbon emissions.

Basically, the problem relates to fixed versus marginal costs. For you non-economists, marginal costs are the cost for one more unit, whether producing another widget at your factory or, in this case, adding one more person to a bus. My article focused on fixed costs, but the real question is which marginal cost is more. Even if a bus produces more CO2 per person on average than a full car, it’s going to produce basically as much whether I ride in it or not. As Arduous puts it:

… driving a car is only the most carbon efficient method IF the carbon emissions Will and his friends’s weight cause on a plane, bus, or train is GREATER than the TOTAL emissions of the car. Which seems pretty unlikely.

Since none of us weigh much, especially compared to a bus, I think Arduous makes a really good analysis. The problem at an individual level is that your decision depends on factors you can’t know about in advance (basically, how many people will join you on a bus, plane, or train, and whether or not these trips will be cancelled if you don’t go). This seems like a good place to apply rule utilitarianism, even if it doesn’t make sense in the general case.

In basic utilitarianism, you do what creates the most happiness (or in this case, produces the least CO2). Since it’s so hard to figure that out, rule utiliarian provides some simple rules that help you act in a timely manner. Things like “don’t kill innocent children” or “always ride the bus when possible” increase happiness overall but also makes it so that you don’t have to actually spend the time to run the calculations every time. Which is good, because we can’t, as I mentioned above.

To move from utilitarianism to rule utilitarianism in this case, we need to stop looking at it from an individual point of view and start looking at it communally. The question changes from “how can I travel to reduce my CO2” into “how can I travel to reduce my community’s CO2?” It’s a subtle distinction, but it makes the math easier. :)

To decide that, let’s calculate the break even point between a bus system and a system of cars. Unfortunately, the source for my carbon calculator determines the CO2 production of planes and trains by dividing their total CO2 production by the total number of passenger miles which makes the data useless for this. For buses and cars, I was able to grab the Greenhouse Gas Protocol Initiative (GHG Protocol) data that my calculator from the other day uses.

According to their spreadsheets, a 30mpg car produces 186.6g of CO2 per km (yeah, different units from last post, but it doesn’t matter; it’ll all come out in the wash). A bus, on the other hand, produces 1492.5g of 2! The difference is so large because buses get terrible mileage (6.7mpg average in the US). This difference between running a bus and running a car is almost exactly a factor of 8. This means that it takes eight cars driving the same distance to equal the emissions of one bus.

If you assume that there are generally about two people in a long-distance car trip, then the bus will have to have 17 people in it before it actually reduces carbon emissions.

This gives us a good break-even rule for travel. If you take a bus and it has much fewer than 16 people in it, don’t ride that route anymore. That’ll discourage the bus company from keeping that route going. If the number is closer to 16, it might be worthwhile to keep riding the bus and encouraging others to join you. If it’s more than 16, you can relax, secure in the knowledge that not only are your carbon emissions low individually, but you’re helping reduce your community’s emissions. Note that this is true no matter how many people you would otherwise cram into your car because whether or not the bus produces less CO2 depends only on how other people would act, not on how you’re acting.

On a community-wide scale, this helps you make decisions about when to add buses and, perhaps more importantly, what type of buses to get. Small buses, like those sometimes used as school buses, can never be better than cars. It only makes sense environmentally to create a route with a large bus and only if over 16 people will ride each direction.

Increasing bus mileage would help too. Bloomington has started trying out hybrid buses and some nearby parks use propane-powered vehicles. These methods can help reduce the break-even point.

This same analysis should be possible with planes and trains if you can get hold of overall emissions rather than data per passenger-mile. And, naturally, carbon emissions are only one facet of the much larger issue of sustainable living. Even if a bus produces more CO2 than a car, it might be worth it for other reasons, like traffic reduction.

Personally, I find the carbon calculators valuable for determining which technologies are approximately the same. Driving a packed car versus riding a bus are approximately equal, so I don’t feel bad about the occasional long roadtrip and I also feel good about encouraging additional buses.

On the other hand, rail and plain emissions are so much higher than car emissions that I don’t feel like we’ll be able to meet their break-even points anytime soon. So while it’s true that I, individually, won’t produce more CO2 if I fly, I’m helping support an industry that might require up to 60 people to break even.

The break-even point is also a sliding scale. As cars get better, it requires more and more people on a given flight, train, or bus to reduce the overall amount of CO2.

To make a long (and perhaps boring) story short, it makes sense to encourage high-volume busing and discourage low-volume busing even if that puts some more cars on the road. At least from a carbon perspective.

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4 Responses so far »

  1. 1

    Student Doctor Green said,

    April 13, 2008 @ 3:18 am

    Will, I can say honestly that this post answered a question that I thought was unanswerable or at least not easily answerable. You explained the nitty gritty behind carbon calculators seemingly effortlessly. I had been ranting on No Impact Man months ago about stuff like this and called for this sort of analysis but there wasn’t interest. Thank you so much for proving something I knew wasn’t right: taking the bus or a train isn’t always better and taking a plane isn’t necessarily worse. It just depends, which I guess isn’t as compelling as PLANES ARE THE DEVIL but it’s certainly more honest. Thank you for this post.

  2. 2

    arduous said,

    April 15, 2008 @ 11:10 pm

    Yes, Will this was a great post. Thank you VERY much for this.

    The only thing about those small school buses is that, to be fair, while there are two people in the car driving to school, it’s usually a parent and a child. Parent doesn’t actually need to go to school, they’re just driving because kid can’t drive. So yes there are two people in the car, but one of those people doesn’t count. When you factor in that the parent drives BACK home without kid, I think it works out so that if four children are on a school bus, then the school bus is worth it. Are my calculations correct?

  3. 3

    Will said,

    April 17, 2008 @ 2:49 pm

    SDG: Thanks! I’m glad it was useful. The carbon calculator I used made planes seem especially bad by using a 2x multiplier to adjust for the fact that planes release their CO2 in the upper atmosphere. I’d still avoid planes for that reason, but if you can’t avoid planes, I think your marginal difference in CO2 emissions (even with the unknown additional affect of where planes are producing their emissions) is going to be pretty low in a large plane full of people. I know that Boeing and presumably other airplane manufacturers are trying to come up with better fuels. Maybe in the future, we’ll ride veggie oil planes!

    arduous: No problem. I usually like running through numbers. My analysis doesn’t really cover school buses anyway, since all of my numbers are for inter-city busing. I don’t know how intra-city transportation compares, although my guess is that it’ll be worse because intra-city buses stop more often. But you’re right overall. If you think that buses are replacing individual drivers rather than pairs, then they don’t have to have as many passengers to provide a net gain.

    I think that intra-city buses are much more likely to replace single drivers, both for the reason you mentioned and because most people who drive to work drive by themselves. While on longer trips, I tend to see at least two people in most cars, which is why I used that as my estimate.

    Which all just goes to say that yes! You’re absolutely right that short-range buses probably don’t need as many passengers to break even.

  4. 4

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