How Traffic Jams Decentralize Cities: Scientific American

Sarah Fecht writes about How Traffic Jams Decentralize Cities:


“In a new paper in Physical Review Letters, [Marc] Barthelemy and his colleague, Remi Louf, have constructed a mathematical model to explain how cities and their surrounding suburbs evolve to be polycentric. Their findings suggest that population size and automobile traffic congestion play large roles in driving the creation of alternative hot spots, even in small- to medium-size cities. “It’s an interplay between how attractive the place is, and how much time it takes to go there,” he says. At first everyone goes to the city center, but as the city becomes increasingly crowded it becomes more difficult to get there. Eventually subcenters spring up toward the city’s outskirts, providing more convenient locations for residents to work and shop. Cities with accommodating transportation networks remain centralized longer, but once population density passes a certain threshold, cities inevitably become polycentric, Barthelemy says.



David Levinson, a transportation engineer at the University of Minnesota, says it is not altogether surprising to find a relationship between population size and the number of urban subcenters. A group of economists made that assumption a few decades ago. Barthelemy counters that the economic models were “fuzzy” and untested. “After 20 pages of calculation, they don’t have a prediction and they don’t test their model,” Barthelemy says. “We can test our results against data.”


Having a clearer understanding of the evolution of metropolitan polycentricity could prove useful, Levinson says, especially considering that two thirds of the world’s population is expected to be urban by the year 2050. “There’s a lot of urbanization left to happen,” Levinson says. “If planners imagine a city to take a particular form, but that’s not the way the city wants to behave, we’ll be making unwise investments.”“

Reactions to `What happened to traffic’

The End of Traffic and the Future of Access: A Roadmap to the New Transport Landscape. By David M. Levinson and Kevin J. Krizek.
The End of Traffic and the Future of Access: A Roadmap to the New Transport Landscape. By David M. Levinson and Kevin J. Krizek.

Recently I wrote a blogpost: What happened to traffic? which, for a few days, increased traffic on my site 10-fold. So asking the same question, with an appendage “What happened to traffic on my website” I wonder, why did this happen? Well the short answer is that it was linked to by the curators of the blogosphere.

Reihan Salam is as far as I can tell, a regular reader and occasional linker. But then Tyler Cowen of the well-read blog Marginal Revolution picked it up and traffic exploded. Streetsblog also picked it up separately (I am guessing they do not read National Review Online). It expanded from there.

I got asked to do two podcasts (Asymcar with Horace Dediu and Jim Zellmer and Ricochet with Jim Pethokoukis), and two radio shows (Larry Elder on KABC (podcast behind a paywall), and KFWB NewsTalk 980).

Some key links discussing the post and the ideas therein are listed below:

There seemed to be a lot of attention to the changing nature of work as the driver. I did spend a couple of paragraphs on that, and the linkers tended to pick up on the issue. I am surprised that so many people (commenters etc.) think leaving the full-time workforce at age 60 is an unreasonable prediction, I thought that early retirement was a highly likely outcome, since most older workers will not be as up on current technologies as younger folks, and training will have less return on investment. In many countries, grandparents help with childcare in their 50s and 60s. There was also some pushback on “office workers” working “only” 4 hours from home. I should have phrased it as getting credit for 4 hours of paid work from home, obviously the amount of work that people actually do and what they get paid for has only a loose correlation.

Some people were sad with the scenario, thinking it meant decline. I don’t think it means decline, though there will certainly be shifts. (I do expect twenty-somethings will probably be worse off economically in this world where employers can pay people like interns rather than regular employees for a longer period. I am observing this among graduate students. But they will still have computers, internet, mobile phones and large screen TVs, and will have deferred having children and mortgages and maybe cars.)

My sense is that if population roughly levels out, and technology advances, average (mean) incomes should rise. The distribution of incomes is a more difficult question, but this feels like a macro-economic cyclical problem, which depends in part on how many redistributionist policies there are, whether revolutions redistribute property, the relative scarcity of capital and labor (and thus the returns to capital vs. labor), and items like that as well as the functioning of the market. These items are certainly well-beyond the scope of a travel demand forecast.

How you pay for a workforce that is reduced in size, and especially for retirees, is a topic I did not address (also beyond my scope … I suppose I should pretend to be an expert on everything.). My short answer, for the short-term, is immigrants (though of course that would create more traffic, but the nature of work may be very different nevertheless, so it might not be peak-period traffic).

(General comment: If I knew the post would be popular, I would have been more careful writing it. If I were careful writing it, it probably wouldn’t have been so popular. Find the equilibrium).