The Projections Fallacy

Cross-posted from Streets.MN.

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.

The projections say California's I-710 isn't wide nearly enough yet. Photo: Cameron Bevers / ## Highways##

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 percent, 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 percent, 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.

12 thoughts on The Projections Fallacy

  1. I think your essay has a lot of interesting ideas but lacks a clear thesis.  I am left nearly empty handed.  That’s a shame, because I and a lot of amateur traffic observers would surely benefit from your experience.

  2. How about this: “We don’t need better projections, we need a system that is robust to modeling error.”

    That is probably a dissatisfying thesis to those looking for a way to do better projections, a frustration shared by a countless number of stock investors.

  3. What might be useful is development restrictions were tied to the growth models of the highways. Thus, if a highway is built to a certain capacity based on certain projections, new developments in the region which will depend on that highway for access need to fit within the parameters of the projections or else provide remediation such as alternative travel (like transit service) or additional capacity. What good is projecting traffic if you let development continue unfettered and unplanned until the projections are so far exceeded that the road ultimately fails?

  4. A lot of the problems would be solved with congestion tolling. It would provide a revenue source to maintain the road, and expand it if necessary, but it would also reduce the incentive people now have to build where there’s no traffic capacity, since they will have to pay the cost of connecting to the rest of the network, instead of everybody else paying to connect to them.

  5. Hi Charles, nicely addressed with the “auto-centric pattern” and projection issues. Unfortunately, solo-driving and traffic congestion are ever so persistent and unpredictable. 
    RideSpring has been attempting to delve into this sphere for a few years now by encouraging alternative commuting for businesses and moving away from solo-driving. I think you’ll find the following postings and video interesting in the development of the topic: 

  6.  Trains are more robust to modelling error than roadways, because of the massive capacity of even a single lane of rail.

  7. Here’s the thesis:

    “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.”

    And it’s exactly correct. Our goals for climate mitigation, air quality, and public health will all continue to go down the toilet if out year projections continue to guide transportation policy.

  8. What do the modelers say to this? It may be easy for us to blank on the idea, but a lot of smart people devote their career to modeling, projecting, and maintaining networks. Are you suggesting elimination of modeling from the transportation planning process? Because modeling is a federal mandate for all MPO’s in a TMA.

  9. Yes, I would eliminate modeling and all “build it and they will come” approaches preferring instead an incremental model where major projects emerge from a series of lesser successes. 

  10. Follow the money.  These days the traffic predictions usually are made by people who profit directly or indirectly from overstating the future demand — especially when passenger rail is in the equation.

    I’ve offered up to five figure wagers on some of the bogus projections in San Diego and California, but I can find no one willing to back their “predictions” with cold hard cash.  And rightly so — I’m offering sucker bets they know they won’t win.

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