Recent posts on density, efficiency, and other metrics often used as buzzwords in urban policy have sparked some of the most active and interesting comment threads on this blog in recent memory. These ideas clearly touch a nerve with many of our readers, and blogs written by Strong Towns members this week showed no shortage of reflection on related topics.

One crucial Strong Towns theme is that cities are complex systems that respond to feedback—economic, environmental, cultural—in hard-to-predict ways. And one thing that the "density" discussion really illuminated is the difficulty of describing the behavior of such complex systems in concise, straightforward rules that can inform policy. Density, in its concrete simplicity, can be an appealing shortcut to urbanists trying to explain why, say, San Francisco works better than San Bernardino. But to try to use density as a prescriptive metric for success is often to confuse correlation with causation.


My Parking Sign Blog illustrates this pitfall in a recent post. Urban density, it turns out, is not putting the dent in U.S. carbon emissions in the way its cheerleaders wanted it to: 

It’s largely accepted that “smart growth” approaches, which push for more compact urban residential development, will naturally lead to decreased vehicle emissions. “However,” says Gately, “the most pronounced declines in per-capita emissions are only observed in cities that are already very dense such as New York and Boston, while for low- and medium-density cities the effects of densification are much more varied.”

Consider Salt Lake City, where the population density hasn’t varied over time, but per-capita emissions have skyrocketed, thanks to population growth in suburbs and areas beyond the suburbs. And in cities like Denver, for example, denser housing, more bike lanes and improved transit systems haven’t automatically resulted in a decrease in emissions either, since suburban populations continue to sprawl and suburbanites continue to rely on their cars, explains Quartz.

Image from Peter Blanchard

Image from Peter Blanchard

Does density reduce car usage (and thus carbon dioxide emissions)? Well, it depends. On a lot of factors that aren't easy to quantify. The unfortunate conclusion of the story under discussion:

DARTE suggests that areas must reach a density of approximately 1,650 people per square kilometer, in order for vehicle emissions to drop off quickly.

I wouldn't hold my breath for this to prove to be some magic number. The answers are almost certainly qualitative, not quantitative. What do the places housing those 1,650 (or however many) people per square kilometer look like on the ground? How are those people living?

Patrick Kennedy of Walkable Dallas-Fort Worth observes that complex systems are counter-intuitive. They don't respond to inputs in simple, reductionist ways; they are not the sum of their parts. Policies that try to fix one factor in isolation are going to fail.

Pernicious side effects of well-intended policies are all manifestations of the underlying nature of complex systems.  When someone tries to change one part of a system, it pushes back in uncanny ways, first subtly and then ferociously.

One example [MIT professor Jay Forrester] cites in Urban Dynamics is the vicious cycle of poverty.  “The harder a policymaker tried to alleviate poverty, the more poverty increased.  Low-cost housing intended to revive inner cities (single use and concentrations of poverty), actually crowd out businesses that might have created jobs, while attracting underemployed people and concentrating them in decaying neighborhoods that made it harder to break out of the vicious cycle they were caught in (and unlivable, unlovable places).

By continually trying to alleviate poverty through further concentrations of low-income housing and more roads ‘to get to jobs,’ we’re simply making matters worse.


Our favorite champions of Strong Towns Math down in St. Louis, at NextSTL, found a rare opportunity to do what approximates a controlled experiment—at least as close to one as is possible in the complex realm of urban economics. Their latest post looks at two development patterns in the same neighborhood of South St. Louis: "Fatties" versus "Sveltes". These are single-family homes on relatively narrower or wider lots, as shown here (fatties on the left, sveltes on the right):

What is striking is the contrast between the two groups in assessed value per acre. The sveltes come in at $196,894 per acre while the fatties come in at $123,322 per acre. That’s a difference of $73,572. Had the entire study area been of the svelte variety the total assessed value would be $13,082,560, $1,878,330 or 16.8% more. That’s $142,471 in potential total property taxes each year. The city made a poor development decision when these were platted many decades ago, thus have come at a high opportunity cost.

This amounts to a hidden subsidy for the fatties. With more frontage they have more feet of street, more feet of water and sewer pipes, more feet of power lines, more streetlights, service providers have to travel further, etc. 

What are the consequences of repeating development decisions like this on the scale of an entire metro area? Or, to a large extent, an entire country?

Many places are grappling with this, and many are changing—some slowly, some quickly, most very belatedly. Dave Alden at North Bay Design Kit muses in two posts on the mindsets that drive people's willingness or unwillingness to accept change, and whether this is connected to the baby boomer generation's experience of social upheaval in the 1960s and 1970s.

Looking back, I can also see that my urbanism may be a distant reflection of the 60s and 70s.  With its tweaking of the nose of authority by challenging the shibboleths of suburbia, by pushing for an independent plan of the Sonoma Marin Fairgrounds, by advocating for block parties, and by many more acts questioning the status quo, urbanism is my way of finally arriving at the barricades, forty-five years late. 

One thing that unites a lot of Strong Towns supporters is the feeling that great places allow some spontaneity, some surprise. We're taken aback by dreary monotony, and we also recognize that dreary monotony can be extraordinarily fragile. Monocultures—whether biological, cultural, or in the built environment—cannot adapt incrementally. They resist change up to the point at which the pressure to change is simply too great, and then they give way cataclysmically. Alden's post recalls how the restrictive social environment in his high school in the late 1960s was a pressure cooker—draconian rules led to resentment and then a flood of dramatic change. So it is in our failing cities. What will happen to the places most steadfastly resistant to change when their unsustainability is, finally, completely evident?

Along these lines, Granola Shotgun looks at Bullhead City, Arizona—an astonishingly fragile place for multiple reasons. Johnny explains the mindset of its initial master planners:

The primary attraction in Bullhead City, other than nearby Laughlin casinos, is the river. In a desert environment waterfront property sells at a particular premium. In fact, the river is probably the only reason Bullhead City even exists in the first place. So it’s understandable that the development pattern that emerged involved carving the riverfront into hundreds of long narrow lots where each home would have its own private dock. But there are problems with this style of development.

The problem with Bullhead City is its single-minded attempt at "front-loading value;" its initial planners looked only at maximizing the property value of the private waterfront lots. But the rest of the city has suffered since from an almost complete lack of access to the city's only intrinsic amenity: the Colorado River. And property values drop off precipitously as soon as one is a block or two away from the river.

Steven Shultis at Rational Urbanism looks at the commercial, rather than residential, realm and also sees fragility and decline. The United States has way too much retail—in fact, we have over three times the retail square footage per capita of Germany, and that's another high-income economy!—and the "dying mall" phenomenon is the inevitable economic correction. What kind of retail spaces will survive in the long run?

Like the Zócalo in Oaxaca, the Puerta del Sol, or Faneuil Hall, these places will be social destinations which also provide whatever people want to buy in a space which isn’t from a neighborhood shop or the 21st century equivalent of mail order. Goodbye, “Power Center” (Man, I hate that term!), goodbye “Big Box”. Hello real places integrated into the fabric of real, walkable, multi-use spaces.


A public realm that people value and enjoy. Places that are meaningful and unique. Places that allow spontaneity and experimentation. It's not about adding up the value of individual lots. It's not about easy-to-quantify metrics.

There's certainly a place for quantitative thinking—we have to make the math work at the end of the day for our local governments to stay solvent. But there's a kind of humility involved in acknowledging that cities are far too complex for the solutions to these problems to be reduced to numbers.

You can check out the entire member blogroll on the Strong Towns member site. If you're a member with a blog and would like your work to show up there, please let us know about it.