Charles Marohn

The projections fallacy

Engineer, president of Smart Towns, Jul 24, 12.

Tagged with transportation, urban planning, roads, construction, engineering, transport planning

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 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.

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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.