Ignoring Induced Demand is Engineering Malpractice

2016 10 29 11 48 28 View north along U.S. Route 15 Business %2528King Street%2529 between Loudoun Street and Virginia State Route 7 Business %2528Market Street%2529 in Leesburg%252C Loudoun County%252C Virginia

This was a lengthy document that was painful to read, but one answer stood out as being particularly awful:

Question: Explain how induced demand is relevant or not relevant to the potential widening of US Route 15, as proposed by Concept B.

Staff Response: The concept of induced demand suggests that adding capacity to a roadway creates or “induces” new demand for a roadway where demand did not previously exist. However, travel demand is not generated by the addition of network capacity, rather travel demand is created by existing and future jobs and household population produced trips. The distribution of trips produced by the jobs and population is then distributed on the available network based on a gravity model where travel time via various routes drives the distribution of trips. Certainly, the addition of capacity on a roadway could result in increased volumes on that roadway, but it also may reduce volumes on parallel roadways. Population and employment growth will also drive increased travel volumes.

When I read this response, I was kind of dumbfounded. To say this is antiquated thinking is being generous. It’s lacking in thought, introspection, and rigor—unless they don’t believe it, which is very possible. And disturbing.

Let’s break it down line by line:

The concept of induced demand suggests that adding capacity to a roadway creates or “induces” new demand for a roadway where demand did not previously exist.

This is not what induced demand suggests. It’s not that people or cars magically appear where none existed before, all other things remaining static. It means that trips that otherwise would not have occurred are now going to happen solely as a result of the added capacity. That’s the “induced” part of it.

Let me give a simple example: I’m cooking dinner and I’m getting low on milk. Prior to the new capacity, it’s a twenty-minute round trip and, because the kids are expecting dinner, we drink water instead of milk. I get more milk the next time I’m passing by the grocery.

Now, the planners and engineers add more capacity to the road network and suddenly it’s only a ten-minute round trip to get milk. Now I decide to go because I can get there and back in time to serve dinner. Nothing has changed in the overall environment, but I’m taking more trips than I otherwise would because trips are now more convenient to take (at least for a while). 

Adding capacity induces a trip that otherwise would not have occurred. That’s induced demand.

This example is the simplest form of inducing traffic, but just a tiny bit of brainpower will produce an infinite number of examples of people making decisions to take trips when doing so becomes easier and, conversely, reducing the number of trips they take when doing so becomes more difficult. 

However, travel demand is not generated by the addition of network capacity…

The concept of “travel demand” is where traffic engineers have stunted their own intellectual development more than perhaps anywhere else. And they’ve done so for two reasons. First, it makes their models easier to run. It’s really difficult (impossible, really) to create models that factor in the behavioral responses of humans. Better to just assume a static level of demand, even though that assumption is a farce (remember, traffic models are all about justifying projects, not actually modeling what is going on in the world).

Second, it allows traffic planners and engineers to position themselves and their craft as responding to demand, not creating it. That’s an important distinction because it allows them to be confident in what they do without having to struggle with the underlying reasons that things aren’t working. 

I’m not belittling them with that observation. Simplifying complexity is a reality that can be observed in many practices that, in the modern economy, have become simply about production. Economics, education, health care, etc.

Engineering in the auto age is about building—build, build, build—and not about optimizing or managing systems. When your ethos is merely to build more stuff, you develop myths and models that support that ethos. That’s what you’re seeing in the patently absurd assertion that additional capacity does not generate more trips.

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