Search this Site
Hidden Stuff
« Best of Blog: Crazy Ideas | Main | Best of Blog: PennDOT Fiddles »
Tuesday
Dec182012

Best of Blog: The Projections Fallacy

Yesterday I noticed that comments on the site Greater Greater Washington referenced our Conversation with an Engineer video as well as my essay Confessions of a Recovering Engineer. The initial article was about speed cameras and whether or not speed limits should be set based on solely engineering criteria or whether other considerations should be brought into the conversation. 

I'm still amazed at how the Conversation video continues to resonate two years later, having been watched over 172,000 times, shared at numerous conferences and college classrooms. There is a disconnect between the people building our places and the people living in them that is palpable. It is captured in that video in a way that causes people to completely empathize with one of the characters and have hostile feelings towards the other. Regardless of which one you align with, know that there are many out there that think the joke here is on you.

Lets never forget that the traffic engineering profession is a mere six decades old, barely enough time to accumulate anything resembling historic wisdom, let alone apply it. That this collection of professionals speaks with such confidence and authority about a future they have proven inept at predicting speaks more about us than it does them. Why have we chosen to serve them instead of the other way around?

This essay began what ended up to be a four part series on traffic projections. To grasp the entire conversation -- which directly challenges a lot of standard orthodoxy about our built environment -- I recommend reading them all in this order.

---

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 would like more from Chuck Marohn, check out his new book, Thoughts on Building Strong Towns (Volume 1)

 You can also chat with Chuck and many others about implementing a Strong Towns approach in your community by joining the Strong Towns Network. The Strong Towns Network is a social platform for those working to make their community a strong town.

PrintView Printer Friendly Version

EmailEmail Article to Friend

Reader Comments (3)

Charles, I love the way you think about these topics.

You gave me serious food for thought this morning--it was only a single word that got me going, but here is what I was thinking....

You criticized oversizing trunk roads, as We need a system...where each fractal evolves continually and naturally over time.

Several years ago, my mind was blown by Constructal theory, which has been proposed as a Fourth Law of Thermodynamics by Adrian Bejan.

Constructal Theory looks at flow--the flow of water in rivers, the flow of nutrients in trees, the flow of cars on streets. From Wikipedia, here is one of the brain-shaking notions.

The constructal law states that every flow system is destined to remain imperfect. The direction of design evolution is toward distributing the imperfections of the system, such that the “whole” flows easier (e.g., river basin, animal body, human vehicle). Evolution never ends. Optimality statements (minimum, maximum, optimum, end design, destiny), have no place in constructal theory. Nature does not move toward an optimal end design. The natural phenomena is not the elimination but the distribution (better and better over time) of imperfection. The distribution of imperfection generates the geometry (shape, structure) of the system.

So, constructals are awesome--I think they are an earth-shaking new way of looking at patterns in the world, so they suit your work well. And the thing about fractals--Constructal theory can actually optimize a leaf to suit the tree and conditions. Fractals can generate leaf-like patterns, but are actually just math. If you zoom in, you can see another leaf, and another and another. Obviously, this does not happen in nature. There are no super-tiny leaves within leaves. Fractals are just math, but constructals are explanatory and predictive. They can help us find scale.

And one of the things they have been applied to is road design. The thing that caused me so much furrowed brow this morning was your criticism of trunk and feeder traffic systems. Nature uses trunks all the time--in trees, in our lungs, in river systems, so why are trunks bad for cars?

The answer I came up with is, as you noted, evolution of road design has only had 60 years, whereas nature has had hundreds of millions. Also, humans stop growing, and so you can actually size bronchial tubes for the organism. Cities seem to think it is a good idea to keep growing, and so they keep outgrowing their circulation systems.

So I guess this reinforces the notion that we should stop growing towns. When the town is the right size, make one a little ways away and start from the beginning again. It is better to have more small, human-scale places, than a few big, ugly, congested, stinky and noisy places.

December 18, 2012 | Unregistered CommenterRuben

As always, Ruben, I enjoy your thoughts. We need to get together soon.

-chuck

December 18, 2012 | Registered CommenterCharles Marohn

Perhaps we should have a Skype beer over the holidays. Or didn't someone in Vancouver BC tell me they were bringing you out?

December 18, 2012 | Unregistered CommenterRuben
Comments for this entry have been disabled. Additional comments may not be added to this entry at this time.