【英语生活】乘公共汽车的边际优势

At the risk of turning this column into “The Undercover Environmentalist”, I need to return to that vexed question of carbon dioxide emissions. In my first column of the year, I vowed to reduce my carbon footprint from air travel – easy enough, given that it was 50 tonnes of CO2 last year. A kind reader wrote to reassure me that I needn't lose any sleep, because the planes were making the journey anyway. Glib, I know: I've often said it myself to wind up environmentalists.

The answer reminded me of a brain-teaser that's been entertaining me for the past couple of months. Since buses often run almost empty, two people sharing a car emit less CO2 per person than do bus passengers. Shouldn't we then be travelling by car?

The BBC's in-house environmental activist, Justin Rowlatt, aka “Ethical Man”, recently pondered this question and concluded that, no, he'd still be taking the bus. Why? Because the buses are making the journey anyway.

A pause to run through the statistics. According to my colleagues on the BBC's More or Less programme, cars emit 127g of CO2 per passenger per kilometre and buses 106g, based on average occupancy. Even London buses average a mere 13 passengers. This is one of the problems of a public transport system: in order to make the system attractive, frequent services need to run off-peak, and in order to make the system work at all, vast chunks of metal need to counter-commute, almost empty, to get back to the start of their rush-hour routes. They are “making the journey anyway”.

There is something strange going on when the environmentalist and the anti-environmentalist use the same excuse – one to justify taking the plane, the other to justify taking the bus. An admittedly unscientific poll of environmentalists at dinner parties suggests to me that they think “the plane is making the journey anyway” excuse is unacceptable but “the bus is making the journey anyway” excuse is spot on – and that they have no coherent justification for the distinction. Their favourite excuse is “you have to set an example” – but surely, before you decide to set an example, you need to be sure that you aren't setting a bad one.

To cut through the fog we need to rely on some technical language. We must distinguish between average cost, marginal cost, and average marginal cost. The average carbon cost of travelling by car or bus is the total emissions divided by the number of passengers: these are the numbers that are unflattering to buses. The marginal carbon cost is the extra emissions caused by one additional passenger. For planes, trains and buses this is low – unless, that is, the passenger is the straw that breaks the camel's back, and causes an additional bus, plane or train to be scheduled in future, in which case the marginal carbon cost of that passenger will be gigantic.

The average marginal cost averages out the marginal costs of a large chunk of passengers. (Exactly which chunk to use seems to be a rather black art.) The idea is to share out the cost between the passengers who do not provoke an extra bus or flight, and the passengers who do.

For all you environmentalists out there, then, here is the justification for the double-standard of taking the bus but not the plane: it is that bus schedules might be insensitive to passenger demand, while planes are highly sensitive – and ever more so since the budget airlines arrived on the scene. Your best argument for taking the bus is a perverse one: that, no matter how many people do likewise, it's the rare public transport tsar that will lay on extra buses.

I'll be cycling.

冒着将本专栏变成“卧底环保主义者”的风险，我需要再探讨一下二氧化碳排放这个伤脑筋的问题。在今年的第一篇专栏文章中，我发誓要减少我航空旅行的碳足迹——相当容易，因为我去年的这部分碳足迹达到50吨。一位好心的读者写信来安慰我说，我不用为此失眠，因为飞机无论如何都是要飞完那段行程。有点滑头，我知道：为了哄骗环保主义者，我自己也常常这么说。

这个回答让我想起了几个月来一直在思考的一个脑筋急转弯问题。由于公共汽车经常几乎是空驶，两人拼车的情况下，平均每人排放的二氧化碳量要比公车乘客少。难道我们不应该选择开车吗？

英国广播公司(BBC)激进的环保主义者贾斯廷?罗拉特(Justin Rowlatt)，又名“道德人”(Ethical Man)。最近他思考了这个问题，结论是：不，他还是会选择坐公车。为什么？因为公共汽车无论怎样都要走完那段行程。

让我们停下来，先看看数据。根据我在BBC《或多或少》(More or Less)节目的同事提供的数据，自驾车平均每名乘客每公里排放二氧化碳127克，而公共汽车按平均乘载率计算是106克。即便在伦敦，公共汽车也平均只有13名乘客。这是公交系统面临的问题之一：为了吸引乘客，公交系统必须在非高峰期保持频繁运营，而为了让系统正常运转，必须有大量几乎空载的公车向反向行驶，以回到高峰路线的起点。它们“无论怎样都要走完那段行程”。

当环保主义者和反环保主义者都使用同一个借口时——一个用来为坐飞机辩护，另一个用来为乘公车辩护——事情就有点奇怪了。我在晚宴派对上对环保主义者进行的调查（我承认方法不太科学）表明，他们认为“飞机无论怎样都要飞完那段行程”的借口让人无法接受，但“公车无论怎样都要走完那段行程”的借口则恰如其分——对于这种差别待遇，他们没有给出清楚的理由。他们最喜欢的借口是“你必须做出榜样”——但在你决定做出榜样之前，想必需要确保自己树立的不是坏榜样吧。

为了拨开云雾，我们需要借助一些专业术语。我们必须区分一下平均成本、边际成本和平均边际成本的概念。自驾车或乘公车旅行的平均碳成本是用总排放量除以总乘客数：这些数据让公共汽车不占优势。边际碳成本是指每增加一名乘客所带来的额外排放量。对于飞机、火车或是公车而言，这个数据都很低——除非那名乘客是压倒骆驼的那根稻草，导致不得不额外安排一辆公车、一架飞机或是一列火车，在这种情况下，那名乘客的边际碳成本将大得惊人。

平均边际成本计算的是一大批乘客边际成本的平均值。（究竟用哪批乘客来计算似乎是种相当隐晦的艺术。）这是为了让普通乘客和那些导致安排额外公车或航班的乘客分摊成本。

因此，全世界的环保主义者们听好了，这就是乘公车但不乘飞机这种双重标准的理由：公车的时间安排可能完全不受乘客需求影响，而飞机的时间安排则对此高度敏感——有了廉价航空公司之后更是如此。选择乘公车的最佳理由相当反常：无论乘公车的人有多少，除非公共交通部门的领导来体察民情，否则都不需要另外安排车辆。

我还是骑自行车好了。

译者/管婧