The Projections Fallacy

We spend billions every year in this country on our transportation network, large percentages of it based on traffic projections. This despite the fact that we have a long record of not being able to accurately project traffic. The answer isn’t better projections but a better transportation system, one that is robust to modeling error.

My home town newspaper recently ran the standard repeat-what-the-engineer-says article on traffic projections. Essentially, the report indicated that we’re going to be inundated with traffic. As things continue to “full build out” (it was in quotes so I’m assuming it is an engineering term), traffic is going to increase by 75%, an astounding amount since most locals will attest we are already drowning in traffic (we’re not, but most would attest that we are). The recommendation for dealing with all this traffic seems sensible: make some prudent investments today to acquire more land for future road expansion and then, as they are built, oversize the roads to meet this future demand.

A lot of the rationale for these projections — as well as the public’s acceptance of them — comes from the fact that growth has been robust. In fact, if you go back decades and look at the projections that were made for the present time, they are laughable in how dramatically they underestimated the amount of traffic. We projected out based on what our experience had taught us to anticipate, but we were wrong, and it cost the city a lot of money to retrofit all of the places that were inundated with cars.

This reality fits a national trend. My experience is backed up by studies demonstrating that, the higher the functional classification and the larger the traffic volumes, the greater the degree of underestimate. This correlates with work by Patron Saint of Strong Towns Thinking, Nassim Taleb, who has made the same observations of economic systems, governments, etc… (For one example, go to the 5:10 mark of this recent video.)

Amazingly, the fact the we have been so consistently wrong doesn’t make us any less confident today, either in my hometown or nationwide. We’ve “enhanced” our models now and believe we have it figured out this time, revising the data upward to reflect what we have experienced in the “real” world. This is the essence of modeling, and what else could be more rational?

Or more foolish. In these models, we’ve taken something that is unpredictable — driver behavior — and treated it as if it were actuarial science, akin to estimating life expectancy or your odds of drawing a face card when the dealer is showing fifteen. The idea behind our hubris is that, while one driver may be unpredictable, the average driver will react in a predictable way and, thus, we can model based on a normal distribution. These models are failing to account for things like consumer preference, the ability to access financing, overall market growth, cost of construction materials, gas prices, government employment levels, and on and on and on…. We assume all drivers make predictible traffic decisions. They don’t.

I’ll take my hometown as an example. Our old traffic projection models assumed that cars multiply like people (I guess that’s what’s going on in all those two car garages), so we projected traffic based on historical growth rates the same way we projected population. Then what happened? I live in a resort/tourist area with cheap, abundant land and lots of natural resources, just the kind of place retirees and near-retirees wanted to move to starting in roughly 1990. Fuel that desire with a stock market bubble (I wrote a lot of permits for people who paid cash for their new lake home after selling stocks) and then with a cheap credit housing bubble, and — unexpectedly — we are inundated with traffic.

Today we already have Super Walmart, Super Target, Home Depot, Menards, Fleet Farm, Kohls, JC Penny’s, Best Buy, CostCo (under construction) and a myriad of chain restaurants, gas stations and other highway parasites sucking off of the hundreds of millions we’ve invested in the adjacent high-capacity highway system STROAD (street/road hybrid). The baby booomers are stuck in their existing homes, which are now worth much less than they had hoped, while many are also stuck in their job as their retirement savings suffers much the same fate. The cheap shoreline is gone, sold off decades ago to people who are now reaching the age where maintaining a lake home may not be worth the effort. The cheap gas is gone too and it doesn’t seem too likely that the state is going to throw hundreds of millions more into shortening travel times from the Twin Cities.

Has any of this new reality informed our projections? Of course not. Like a mad scientist adding random chemicals to a potion they just don’t understand (and then testing it on unwilling human guinea pigs), we’ve tinkered with our models now, convinced ourselves that this time will be different. And you can bet that there is no way we’ll be caught underestimating. We’ve “seen” what happens and so we’ve “fixed” that problem. The new answer is simple: estimate high (which is, in the perverse vernacular of the trade, called being “conservative” with the estimate).

We have a couple of discussions going on right now at the Strong Towns Network about traffic projections. One of my friends there sent me the following:

We’re working on a traffic study for a small development. According to their comp plan (from 2007) we need to offer recommendations for a county road equivalent assuming 4 percent annual growth. We looked up the numbers and found that VMT has (of course) declined steadily in the area since 2003/4. Someone at our firm contacted the county about the growth rate (a 26 yr old traffic engineer by the way) and, from the message that was relayed back to me, we were instructed to just proceed with the Comp Plan’s assumptions. Long story short, we’re probably going to have to recommend a turning lane. 

In another conversation, one of our members from California is knee deep in the debate over widening I-710 to 14 lanes. In a note to the spokesman for the Los Angeles County Metropolitan Transit Authority, he asked:

I know that the analysis of vehicle trips and container traffic for this project were done prior to the global financial system collapse in 2008, and so the idea of traffic volumes or container shipment volumes decreasing by 2035 were not considered.

 However, when the Panama Canal is open, it looks like our region will be receiving less cargo simply because going through Panama is much cheaper than shipping to LA or Long Beach and moving via trucks, trains, etc.

 Is anyone at Metro thinking about this project differently now? Shouldn’t the “No Build” alternative for this project (and all the alternatives) be revised?

Here is the answer he received:

The Ports re-evaluated their growth projections after the recession and made adjustments. The revised projections show slower growth in Port activity in the next decade, but the 2035 total TEU estimate remains the same: 42.7 Million. In other words, the Ports anticipate to handle 42.7 Million TEUs by 2035, even if growth is slower in the next few years.

In other words, it might slow down for a while, but we’re confident it will go back up ultimately because our projections say it will.

There is a certain contingent out there that is already pounding out their email to me demanding that, if I’m so smart, I provide a better way of estimating. There’s the fatal flaw in our current system. I’m not so smart, but neither is anybody else. The big difference here is that I’m not pretending to be able to predict the future.

Inherent with the auto-centric pattern of development that defines the Suburban Experiment is the hierarchical road network. Like a river swelling during a steady rain, changes on the periphery have an enormous impact on the trunk components of the network, as do many other things that have nothing to do with driver behavior. The reason we put so much time and effort into projections that we know will be wrong — that we have no consistent history of getting right — is because the cost of being wrong is so great. When the projection is off by even 10%, the level of service on major roadways can plummet. Since nearly every trip is funnelled into this network, failure is catastrophic. This is an incredibly fragile approach.

We don’t need better projections, we need a system that is robust to modeling error. We need a system of growth and development where we don’t need to project correctly in order to succeed. We need a system where we build incrementally in a replicable and evolving pattern, one where each fractal evolves continually and naturally over time. We need a system where we’re not required to place huge bets on the future, oversizing infrastructure in service of projections, but instead can invest in high return endeavors where the likelihood of success is great.

We need a strong towns approach which, if you stop and look, is a lot like the pre-automobile approach that served us well for thousands of years. I’m not saying we get rid of the automobile, but when we build our entire environment around its propagation, we are slave to our own hubris and lack of understanding.


If you want to chat with Chuck and many others about implementing a Strong Towns approach in your community, consider joining the Strong Towns Network. The Strong Towns Network is a social platform for those working to make their community a strong town. Get expert assistance, consult your peers and stay current on the latest techniques and analysis.

Charles Marohn

About Charles Marohn

Charles L. Marohn, Jr. PE AICP is the President of Strong Towns, a Minnesota-based 501(c)3 non-profit organization. He is a Professional Engineer (PE) licensed in the State of Minnesota and a member of the American Institute of Certified Planners (AICP). He has a Bachelor's degree in Civil Engineering from the University of Minnesota's Institute of Technology and a Masters in Urban and Regional Planning from the University of Minnesota's Humphrey Institute. Strong Towns supports a model of growth that allows America's cities, towns and neighborhoods to become financially strong and resilient.

10 thoughts on “The Projections Fallacy

  1. Ian Bicking

    It's kind of easy to just be cynical about the failings of modeling. Can't modeling be better than it is? Good models should incorporate probabilities, different scenarios, and the problems of each of those. They *don't*, of course, but they *could*. We could model in terms of trips, instead of just vehicle counts on roads, which would identify some of the advantages of a more flexible road system instead of just offering a positive feedback cycle for current arteries. We could consider not just average times, but outlying times, how the system responds to events. It's not unreasonable to treat drivers as rational actors in a system, and that gives us a lot of tools to model with.

      1. Ian Bicking

        What that Fourth Quadrant article describes as Extremistan (situations where a Black Swan result has significant impact) I would describe as situations where a cycle of positive (or negative) feedback can magnify what are at any one time seemingly reasonable incremental changes. Even in the case of the stock market, there are a large collection of rational actors, which most of the time balance each other to create a result that in aggregate is reasonable. But in cases of bubbles and crashes there is a feedback cycle where their independent choices align to create extreme results.

        But just throwing up our hands isn't a reasonable response. Something still has to get *done*. Roads must be built, or repaired, or removed, we can't just say "this is too hard to understand." Even if the answer is a reactive approach, or building a system that offers lots of different fallback options for those actors (individuals making transportation choices), that is a decision. There should be some model, some theory, that underlies that; or if we lack such a model we should create one. Maybe the results of that model can't be summarized in a bar graph, maybe it does not converge on one ideal response, but that doesn't mean we must make decisions blind.

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  3. Brendon SlotterbackBrendon Slotterback

    Chuck – what you seem to be saying in the paragraph about a "hierarchical network" is that we need a grid pattern, rather than the tree limb pattern of suburban subdivisions.

    1. Charles MarohnCharles Marohn Post author

      The problem we have with hierarchical transportation networks is the same problems we have with rivers that are prone to flooding. Somehow we seem to frequently get it wrong. There are many reasons — too many to model — because the cumulative peripheral action has such an impact, good and bad.

      I can see hierarchical networks on real roads (connections between places), but not on streets where a grid of some type has been shown to result in higher value creation. We need to completely stop building STROADS.

    2. Jeb

      Even as a driver who uses an automobile a lot (no fixed public transit options here), street grids are much preferred to a hierarchical model, at least for the most part. (Freeways and major highways that are meant for through traffic don't need to adhere to the grid, but the vast majority of roads should.)


      Grids are much, much easier to navigate and get from any point A to any point B relatively quickly. Granted, there's few/no hypotenuse roads in the grid system, but if I'm traveling a long distance, I should already be expecting to use the freeway/highway system.

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  5. Barry KLein

    Traffic models lack a feedback loop to reflect road user behavior in response to congestion.

    Commute times stay under half an hour in most locales because of

    1. driver resistance to long commutes.

    2. employers who move their offices closer to where their workforce lives to make employee retention and recruitment easier

    3. retailers who plant new outlets close to underserved residential enclaves

  6. Charles MarohnCharles Marohn Post author

    I think you are misunderstanding Extremeistan, as defined by NNT. Where you have things that fall into Extremeistan, you can't model them but only build up resilience in the face of uncertainty.

    In many instances, making decisions blind would provide better outcomes than making decisions that we know to be wrong. You have to move on from the current pattern of development, where models provide comfort to the decision-makers but not results that benefit society, and have a different model for growth.

    I'm going to be explaining this all in detail this entire week on the Strong Towns Blog, starting today:

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