Q & A on Induced Demand Biased, Depends on Out of Date Scholarship

At the center of the materials published for the Wednesday Work Session on the SRC is an omnibus Q & A, a staff report arranged as a series of questions and answers. It is almost medieval in form, a modern quaestio or Sentences! It aspires to be a Summa.

You know that's an ironic setup. The report is supposed to be magisterial and neutral, but it is biased and unrepresentative. Too often it is ideologically motivated. Rather than the best of medieval scholasticism, it is the worst of it, narrow, cramped, and oriented with a determined teleology.

Here we will look at Induced Demand.

The Sources are too Few and are Biased

In the discussion of induced demand and traffic forecasting the material is shaped in order to position autoism as a "neutral" description of reality, the natural order of things, rather than a highly ideological and heavily subsidized position resulting from multiple policy decisions over years and even decades.

(Ian Lockwood, via Public Square)

Though the tone of the discussion in the Q & A is ostensibly even-handed, in the discussion of induced demand the last bit on a "preference for suburban living" gives it away. But the preference for suburban living requires heavy subsidies for the car-dependence. If "lawn and driveway" zoning weren't mandated in our vast swaths of our single-family housing districts, we might look at highway investment a little differently.

"preference for suburban living"

As it happens, the study cited here by "Professor Cervero" (it is frequently a sign of status anxiety when it becomes necessary to attach this honorific, and this is a tell) is old, from 2003, and published by a notorious autoist and anti-transit advocacy group that advocates for sprawl.

Are we really gonna cite the American Dream Coalition?

There is nothing neutral about this stance and strand of scholarship.

If this were but one of many citations, discussed and sifted in a truly even-handed way, that would not be a problem, but the Q & A here relies on a very small number of studies and authors, mostly by Cervero, and does not represent a fair assessment of the current state of scholarship. The sources in this section of the Q & A are highly biased.

Here we have referenced several times the foundational paper, "The Fundamental Law of Road Congestion." It is nowhere to be seen in the Q & A.

That they make no reference to it is a sign of their selective citation and ideological bias. A neutral document would do a much better job with full survey of the relevant literature, and at the very least would conduct both sideism with a full he said, she said.

Significantly, a paper by Todd Littman (referenced at footnote 141, and therefore known to the Salem authors, but minimized by them, as it does not support their ideological commitment to highway expansion) does engage the literature. The first two items are the Cervero paper (2003a) and the Duranton/Turner paper (misdated here as 2008, but dated correctly in Littman's bibliography):

It goes on for much more than this...

Large state agencies elsewhere are echoing new research on induced demand.

Adding lanes doesn't work (CalDOT)

In "Closing the Induced Vehicle Travel Gap Between Research and Practice," a recent survey of the literature published by the Transportation Research Board authors said:

Several studies have rigorously documented the induced travel effect, in which added highway capacity leads to added vehicle travel. Despite the evidence, transportation planning practice does not fully account for this phenomenon, with the result that estimates of the potential congestion-reducing benefits of added highway capacity may be overstated and estimates of potential environmental impacts understated.

The Q & A is very one-sided and does not represent the current state of the research. Our own discussion of induced and latent demand is fundamentally flawed and clearly biased.

Preference for Latent Demand Shows the Bias

The bias then shapes the way greenhouse gases are discussed later in the Q & A. (There will be more to say on the way they assess, minimize, or try to evade assessing, greenhouse gas emissions.)

Section 19.e on greenhouse gases

As a concept and term, it is significant they prefer "latent" demand to "induced" demand. Latent demand is pent-up, already existing demand that is frustrated and which a new bridge might solve and satisfy. "Induced" demand is new demand brought into being by a new bridge. The difference expresses an ideological preference for the bridge as a solution to a problem. It is another instance of bias and failure to be even-handed.

ODOT’s transportation modeling section prefers to use the term “latent demand” rather than “induced demand.” Latent demand refers to trips that would occur (that is, trips that are desired), but these trips are not taken because of traffic congestion. For example, an individual might want to do some shopping after work. However, because of traffic congestion (i.e., a “constraint”) coupled with constraints on time available to sit in traffic, that individual may choose to forego the shopping trip. If the congestion is decreased by widening a road, widening an intersection, reducing the number of vehicles on the roadways, or improving the signal timing, the same individual might then decide to do that shopping trip. This new trip would be a latent demand trip.

Induced demand implies that new trips (trips that would otherwise not occur) are generated (“induced”) by the presence of the extra capacity provided by a road widening or new facility. For example, an individual might rarely go out to a movie theater owing to the distance involved. However, if the route to the theater is made shorter by a new road or bridge, that same person may might go to the theatre more often. This new trip is an example of an “induced demand” trip.

The difference is subtle but important, reflecting that road facilities do not by themselves create additional trips. Rather, it is specific situations and the behaviors of individuals combined with the changes in constraints that determine if a trip is latent or induced. [Section 14.a]

Here they explicitly deny new induced demand and claim that "road facilities do not by themselves create additional trips." This is an ideologically motivated conclusion and is merely asserted.

(The move to individualize also is one we see a lot in policy discussions. We see this with our civic conversations about racism right now. Is racism the malignant attitude by individuals and the actions of bad actors? Or is racism the aggregate system that makes certain outcomes more probable for certain classes of people? Can polite, nice, and well-intentioned people also be racist? Do we have a system problem or just one with bad actors? The discussion of induced demand is not so extreme as this, of course, but the move here to individualize trip decisions minimizes the system effects of new roads and works to disparage "induced" demand.)

Our Current Model is Inadequate and Should Have Margin of Error

There is more. The authors admit there are important questions "beyond the current capabilities of the travel model." This is an important element of uncertainty that should be incorporated as error bars or something similar in the traffic forecasts.

Section 19.e: "beyond current capabilities"

We will come back to this in a discussion of modeling and forecasting. But in short, the claim here is that like any forecasting, there is a range of uncertainty. Systematically our public-facing traffic forecasts resolve all that uncertainty to a single number and give policy-makers a false sense of precision. We make million, billion dollar decisions on a single number for 2040 traffic, and the range of uncertainty on these projections is erased. That is a real problem.

On Induced Demand the Q & A is like a Ptolemaic System with Epicycles

All in all, the perspective in the Q & A is one from the 20th century, struggling to keep up. It is old, it is being discarded or modified, and it does not represent current best practices or the state of scholarship. It looks, in fact, like adding epicycles to the old Ptolemaic system with earth at the center when a heliocentric system explains things much better.