Saying Goodbye to the “Passenger Mile”

Last week I made a terrible mistake. I accidentally clicked on a link to a radio story from APM’s Marketplace, a “freakonomics” segment that argued that transit wasn’t as environmentally efficient as its supporters would like. Quickly, my brain began to fill with intellectual pollution.

Eric A. Morris, the “freakonomics” writer was basically parroting a line you often hear from bought-and-paid-for anti-transit shills Wendell Cox and Randal O’Toole (both of whom are funded by right wing think tanks), Morris concludes with the following sentiment:

“It is not clear that moving around large and largely empty vehicles is much of an improvement over moving around smaller ones. In fact, it may be worse. According to the Department of Energy’s Transportation Energy Data Book, in 2010 transporting each passenger one mile by car required 3447 BTUs of energy. Transporting each passenger a mile by bus required 4118 BTUs, surprisingly making bus transit less green by this metric. Rail transit admittedly fares better, at 2520 BTUs per passenger mile, but even this is not the kind of slam-dunk advantage over the auto that transit advocates might hope for. “

Anyone who’s ever glanced at a history book will know that economics is a notoriously unscientific discipline, dependent on unlikely assumptions (e.g. people make “rational” decisions). It’s the kind of thing that makes social science look good by comparison. (Very hard to do!)

This author’s argument is no exception. Like many economic models, it has to ignore a great deal in order to make a point. (See the recent IEA report which claims that the US will soon be “energy independent.”) In this case, Morris ignores massive externalities like land use effects (how mode choice affects our built environment), the (carbon) costs of freeways and parking lots, and a host of other things like public health. (He also assumes that transit is a partisan issue, when it absolutely shouldn’t be. Apart from the ‘Prius v. SUV’ debate, there’s nothing partisan about driving a car.)

But today I want to focus on one particular problem with this kind of analysis. Morris relies on the “passenger mile” as the standard by which to compare the carbon cost of two different modes. And that is a major problem. Relying on the “passenger mile” as a standard of measure implies that our transportation system is space neutral, that distance travelled is inherently good, that the expansion of transportation is an end in itself. Comparisons that rely on passenger mile statistics, instead of alternatives like data on a per trip or per hour basis, will automatically favor the automobile. To put it another way, passenger-mile measurements automatically benefit modes that travel the fastest, at the highest speeds. And speed shouldn’t be the goal of our transportation system. Permit me to explain…

Nobody is going to commute 25 miles each way on a bicycle.

I first started thinking about the passenger mile when studying bicycle safety statistics. Bicycle safety is a big issue. Fear of accidents is probably the #1 barrier to people riding bicycles in cities. And there are many reports which show that riding a bicycle is dangerous, more dangerous than driving. (Those studies have some problems, most noteably that bicycle accident statistics are notoriously unreliable.)

But most of those studies have a similar reliance on the “passenger mile” as a stasticic. That’s not quite fair to bicycles, because the average distance of trips for bicycles and cars are very different. Picture two different people, dependent on two different means of transportation:

If you live in South Minneapolis and ride a bicycle everywhere, your total miles travelled is going to be small. For your commute to work downtown, you might be travelling 3-4 miles each way to work. You might travel 1-2 miles each way to go out for socializing. You might travel 1/2 mile to the store.

If you live in Burnsville (a second-ring Twin Cities’ suburb) and drive everywhere, you are probably multiplying all those distances by a factor of 10. (If you live in exurbia, its even worse.)

The point is that, travelling by bicycle involves keeping trip distances to very small numbers. When considered on an accidents per/trip basis, or an accidents per hour travelling basis, bicycle will appear a lot safer. If you measure per/mile, cars appear a lot safer.

The same formula holds for people using transit. If you use transit to get around the city, you are going to organize your life to minimize trip length. Instead of shopping at the big box store at the edge of the city (12 miles away), you’re going to shop at the corner market down the block (1 mile away). Instead of going to a movie at a suburban cineplex, you’ll go to a closer theater. You’ll try to find jobs closer to your home. You’ll live in a compact, walkable neighborhood where your needs are close by. You’ll minimize your total miles travelled.

The automobile works in the opposite way. Once you’re on a freeway, there is little ‘extra cost’ to driving another 5 – 10 miles to a farther destination. That’s why carbon costs weighed by ‘vehicle miles travelled’ will favor the car, but carbon costs weighed by ‘trips’ (regardless of distance), transit will look much better.

9 thoughts on “Saying Goodbye to the “Passenger Mile”

  1. David LevinsonDavid Levinson

    In my mind, the most important complaint about the analysis Morris has is that of average vs. marginal cost. Just because one mode has a better average environmental record than another does not imply that the marginal passenger should choose it to save energy. It might be the marginal energy cost of an additional passenger in a car (driven by someone else) or a bus already traveling or a train already there is small, while the energy cost of a marginal traveler driving alone is much higher.

    This also does not look at the energy used in construction of new infrastructure, which is surprisingly enormous.

    At any rate, the poor performance of transit modes on average energy efficiency (in the US) is old and well known, we discuss it in The Transportation Experience. It does tell us something, we shouldn't run empty vehicles around if we can avoid it if we want to conserve energy.

    1. Bill LindekeBill Lindeke

      The key question is: where should our future investments be? Answering it involves making a lot of assumptions (e.g. % of people who can afford private cars, cost of ongoing maintenance for the infrastructure, price of energy, pollution associated w/ the energy source, mobility and demographics, public health effects) on top of the literal cost of the infrastructure.

      At a certain point, a strictly economic argument becomes rather silly. (A model turns into a black box from which no light can escape.)

  2. "T"

    Thanks Bill. You're great. I appreciate the commoner perspective. I always get annoyed by the privileged elitist discussions so prevalent these days. I'm concerned with fares stuck at $2.25 for so many hours per day right now. Most of us ain't thinking about carbon emissions or particulate matter as an issue in my neighborhood. Know what I'm sayin'?

    Maybe the Richfield crowd could start a streets.richfield website or something?

  3. Ivan

    The Freakonomics article *did* acknowledge the effects of building vehicles and infrastructure, and his point was not that an individual shouldn't choose transit, but that adding service is not better for the environment if it's underused. If you can stop using your car by taking existing transit, the marginal effect is wonderful, but if they have to start a new bus line just for you it might not be. The assumption here is that the origin and destination for a given user are fixed. Of course, there's a feedback loop, and if you use transit you won't live that far. But it takes time for the system to adapt, as it involves people moving or cities redeveloping.

    I do agree completely with your main point that instead of passenger miles we should be thinking of trips, or even people. The same argument has been made in the context of road safety: who cares where is safer to drive one mile? The thing you should worry about is where it's safer to *live*.

  4. Ian Bicking

    It's disappointing that all defenses against these kinds of critiques start with an ad hominem attack – that Morris is just a parrot, and that Cox's and O'Toole's critiques should be discounted just because they are part of right-wing think tanks, and so presumably their arguments are disingenuous.

    These people are making a simpler assumption – that the purpose of transportation is mobility. This is generally consistent with their intellectual approach, which does not try to peer inside the mind of individuals but instead trusts that they are making appropriate choices given their own diverse motivations. You can (and you do) make an argument that there is more going on this simple model allows for. But I think there is also value in this model, because with it you can actually make some statements: about mobility, about energy efficiency. And their model is in many ways more respectful of the individual – instead of applying moral judgement to people's transportation choices, it accepts and tries to understand those choices.

    Amidst transit advocacy there seems to be more effort expended explaining away people's choices than understanding them. And then of course there is energy efficiency – where your argument seems to boil down to: people are not willing to travel for long amounts of time, and transit is slow, therefore people will not travel as far: success! But now you've opened up another set of questions, the questions that the economists' model tried to avoid through the aforementioned simplification. What else have you lost by constraining these choices? How do you arrange to have your job, your partner's job, your friends, your schools, your extended family, all within this range? Or, having failed to achieve that, what then do you lose? You can start to answer this question by looking at people who have both a car and the ability to use transit (ignoring bikes and walking since they aren't transit!) This describes lots of people in South Minneapolis, to use your example. I don't know the numbers (I'm not sure they even exist on a local level like this), but it seems to me that among the population that can choose they seldom choose transit except for some very select destinations. That would indicate to me that individuals see a real advantage in the increased mobility cars offer.

    Also I really don't think it's fair to lump biking in with transit. Biking as a mode looks more like cars than transit – it mostly uses roads, it's a personal vehicle, there are no fairs or schedules, you generally own your vehicle, and you generally use it for the entire trip without mixing modes. It's not exactly complementary to cars, but when people choose bikes they are choosing an autonomous form of transportation. It's that very lack of autonomy that I think is transit's greatest disadvantage.

    1. Bill LindekeBill Lindeke

      Ivan: Morris may have briefly mentioned land use effects as an aside, but the vast bulk of his piece suggested that transit was not good for the environment, and ignored this "feedback loop".

      Ian: yeah, i often disappoint myself too. (that said, there are a few writers i can't take seriously, and cox, o'toole, and kotkin are three. I know what they're going to say before they say it. i'll get about a paragraph in and give up.)

      Also, the reason that bicycling is more similar to transit than to cars has everything to do with distance and personal mobility. Biking and walking and transit all have the following similarities: 1) how far you are generally going, 2) how you stop / shop / plan your day 3) exercise 4) tremendous desire to avoid automobile oriented spaces. I DO think there is an autonomy to transit. At this very minute, I just took the bus to downtown St Paul, and randomly hopped off to check my email. I am headed to the U of MN, and there's an express bus leaving every 15 minutes from two blocks away. I have tremendous 'autonomy' in this situation. Especially in a smart phone era, transit provides a great deal of freedom that you'll never find in an automobile.

      Everyone: i wanted to keep this post focused on the simple concept of the 'passenger mile,' which probably should be replaced by another concept (e.g. CO2 per trip) by someone more knowledgable about statistics than i am.

      But it seems like the passenger mile opens up another conversation. The crux of the matter seems to be conflating people's everyday lives with a 'mobility' exercise (e.g. moving raw material). For too long the city has been viewed as a system dependent on 'circulation' for its own sake. Picture the 50s diagrams of the city as a 'body', with veins and arteries of circulating 'blood' (cars, people, movement).

      This kind of assumption values movement, privileges it as the goal of transportation. But the goal of transportation shouldn't be about maximizing total miles travelled, as if people were raw materials to be transported around the earth. Transportation, or (even better), 'the city', should be about providing people with lifestyle freedom. There is a tremendous liberation that comes with leaving your car behind, and living in a true walkable/bikeable urban space. You have the ability to stop at a moment's notice, run out the door and to the store in a few minutes. You have social activities right at your doorstep. You have random encounters with people you may (or may not) want to see available to you.

      But leading this kind of compact, dense lifestyle will not 'register' properly within a transportation paradigm that privileges movement for its own sake.

      There's a good response to the Freakonomics argument online (incidentally, I first heard this argument about inefficient buses years ago from O'Toole, which is why I got a bit testy) up on humantransit ( He states: of course running mostly empty suburban buses makes little sense! Transit planners understand how to maximize energy efficiency. Most policy makers do too. You to things like: double the gas tax, double service times in the core cities (where all the dense ridership is), cut fares, and stop almost all service to outlying suburban sprawl areas. That would never happen politically, but at least its an honest though experiment.

      I guess the basic crux is this: does it make any sense to try and 'retrofit' transit into the suburbs? Or (if it was politically possible) would it be better 'cut our losses' and focus all transit expenditures into the core cities?

      Honestly, if it was at all possible, I'd vote for the latter.

  5. CC

    Trips is by far the best indicator for bicycles.

    I don't even have an odometer.

    miles are deceptive because they could be one way or round trip.

    However a trip is just one trip.

    trip to the store, trip to the library, trip friend house, trip work,

    Me that is less than 9 passneger miles, and when I go home it will only be 12. (but will be another trip)

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