Let me set the record straight, then. I do understand it quite well by this point. The idea is that people have a tendency to think that others are making choices based on "disposition," when in fact they are making them based on "situation." Both Michael D and Michael J argue that this isn't about irrationality, but when one person says that another chooses to drive based on their disposition, they are essentially saying that if the other were making a rational decision based on their situation, they would take transit. But they don't, so they must be irrational.
Jarrett clearly highlights Michael's post to warn people against assuming that others are irrational: "The point is to critique the widespread assumption that 'my' decisions are totally rational while 'their' decisions are cultural." The root of the question is whether transit advocates should assume that car advocates are behaving irrationally.
From my point of view, it doesn't actually matter what you assume, but as a general rule the polite thing to do is assume that everyone else is acting rationally. What I think is important is not to stick too closely to that assumption. As I wrote in my previous post, everyone has a mix of rationality and irrationality in them, and in fact every single decision is probably based on both rational and irrational factors. So sure, go ahead and assume that they're acting rationally, but be prepared to deal with the possibility that they're not.
Let me give an example of irrationality in this regard: parking minima in Flushing. There is a new residential development planned for the site of an ugly municipal parking lot in downtown Flushing. It would include an underground garage with more parking than is currently provided by the lot, but that's not enough for local business leaders. "There should be at least a parking spot per family or per unit," Terence Park, president of the Our Flushing Political Coalition, told the Times Ledger. "Mathematically we need more than 2,000 spaces ... people who live in Flushing want cars." There is this entrenched belief that people will own cars whether or not there is space for them, and that if the space is not provided they will park in other places. It has been well demonstrated that providing parking encourages people to own and use cars, but this is largely ignored by these business and community leaders. This line of thinking is not completely rational or completely irrational; it is somewhere in between.
So let's move on to a third Michael, the famous M1EK, who wrote in the comments to Jarrett's post:
Cap'n, I think it may be more helpful to view the choices in aggregate - each individual may have some irrational inputs into their decision, but those largely cancel each other out in the aggregate (just like they do in economics - the market itself is, while not completely rational, pretty much more rational than we give it credit for; so is the "transit market").
That thought did occur to me too. But it's still a little surprising to hear "the market" described as rational in 2010! Sometimes it does all wash out in the long run, but in the meantime you're stuck with 2000 shares of worthless stock in Pets.com - or your metro region is stuck with the FDR Drive. Sometimes it doesn't wash out in the long run; I encourage you all to read Jared Diamond's Collapse for examples of aggregate irrationality that brought down whole societies.
Finally, Jarrett points to the notion of "car culture" as an example of the "fundamental attribution error": "When we say that Americans drive because they're a car culture, we imply that that the choice of most Americans to drive isn't a rational one, in light of each person's situation, and therefore requires a cultural explanation."
I can't speak for others, but I personally am not saying this when I refer to "car culture." I'm referring to a complex pattern of things: literature, art and advertising that glamorize and romanticize driving; the tendency to choose to drive instead of take transit in situations where both are equally convenient; the widespread assumption that "everyone" drives; the preference for "roads and bridges" projects that encourage driving at the expense of transit, walking and other modes of transportation; the consequent restriction of choice that guides people to drive; and the generations who grow up not even conceiving of walking as a transportation option, let alone transit, which in turn leads to glamorization, the tendency to choose to drive, etc.
In other words, as I wrote before, the situations that make driving the rational choice are often the result of irrational choices. It's a cycle, a feedback loop. You can't talk about the egg without talking about the chicken, and also about laying and hatching.