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…

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.