Milan, 1913

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Once a road, always a road. That’s the gist of a recent paper that studied 14 different municipalities in the Groane region of Italy near Milan. In cataloging 174 years of road construction, the study’s authors discovered that nearly 90 percent of the regions 100 most vital routes today were already present in 1833.

The researchers also uncovered evidence that the layout and characteristics of road networks are indicative of the age in which they were built. This is nothing new. Take a look at any metro region surrounded by a postwar subdivision—stick straight roads of the late 19th and early 20th centuries give way to ever more writhing tangles of spaghetti. What’s new is that this study claims that top-down planning didn’t drive the changes. Rather, the researchers say Groane’s roads reflect broader societal changes, that the unique circumstances of each era—agricultural through modern—shape road networks more than central planning—or lack thereof.

To arrive at that conclusion, physicist Marc Barthélemy and his colleagues digitized roads using maps and aerial photographs from seven different dates between 1833 to 2007. They threw the resultant vectors into a geographic information system, or GIS, and then distilled primal graphs—simplified maps that show only roads (called “links” in graph theory) and intersections (“nodes”).

Between 1833 and 2007, the number of intersections grew proportionately with population. The number of intersections skyrocketed—there were only 255 in 1833 but over 5,000 in 2007—but the number of connections remained relatively constant at 2.7 on average. The number of roads also increased linearly with the number of intersections. It’s almost as if the expansion of Groane’s social network was mirrored in its transportation corridors.

How those roads interacted with each other changed through time. In the early days, many roads either intersected another mid-link—forming a T-junction—while the others simply petered out in a dead end. Main drags radiated out from town centers like spokes on a wheel. Congruent 4-cornered intersections were rare. Yet as time progressed and cities spread into the countryside, the previous radial expansion gave way to the grid. In other studies, the advent of the grid was attributed to the arrival of master planning, but here in Groane, Barthélemy and his colleagues note that urban planning was never the region’s strong suit. Groane, they write, “never witnessed any large scale planning whatsoever.”

It is because of Groane’s lack of central planning that Barthélemy and his colleagues are able to draw their conclusion, that road networks morphed not because of changes in our approach to planning but because of changes in society as a whole. In essence, they assert that changes to the network were not consciously done.

It’s not surprising, really. Roads are built to handle the traffic of their time. When navigating Cambridge’s labyrinthine streets, I’m constantly reminded that they were built for horse and carriage, not a horseless carriage. The demands of the automobile are sufficiently different from horse or foot traffic. Their greater speeds require straighter rights-of-way. Intersections need to be clear and predictable. Navigation also needs to be simplified—drivers moving at 10 miles per hour have more time to look for their next turn than those moving three times faster. The grid tackles these problems with aplomb.

Road networks are a product of the processes that created them, whether that be wagon traffic from farm fields plodding to town or taxi cabs streaming out from downtown. Discerning process from pattern is also the domain of another field—landscape ecology. Landscape ecologists sweat the details of spatial configuration to learn what ecological processes are at work. The laws of landscape ecology apply just as well in the city as they do in the natural world. The city is nothing but an anthropogenic ecosystem.

Ever since Geoffrey West and his colleagues uncovered the mathematics behind why big cities are economically successfully—but also crime ridden—it has been popular to search for formulae that describe urban processes and city development. This paper by Barthélemy and his colleagues is but the latest addition to a growing literature. By themselves, these discoveries are clever and insightful. But the interesting stuff will happen when urban planning completes the transition from an observation-based science to a mathematical one, much as ecology did in the recent past. Then we’ll have a real sense of how these models will change our understanding of cities.

Map scanned by University of Texas PCL Map Collection.


Strano, E., Nicosia, V., Latora, V., Porta, S., & Barthélemy, M. (2012). Elementary processes governing the evolution of road networks Scientific Reports, 2 DOI: 10.1038/srep00296

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The City 2.0

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TED is currently in full swing, and the program this year has an entire section devoted to the city. Fitting, given that this year’s TED Prize went to a city-centric project, one that hopes to crowdsource ideas to solve urban problems and reinvent cities. It’s predictably named The City 2.0. The site has a flashy splash page, but the innards still need some work—tapping in my current city of Cambridge, Massachusetts, sent me to a generic index page that encouraged me to “get connected” with other aspiring urban planners in my area, but responded to my clicks with little more than a broken Google Maps interface and some “COMING SOON” dialog boxes. For now, it’s long on pizzazz and short on details.

The TED Prize website fortunately has more on what The City 2.0 hopes to accomplish:

For phase I, the website (www.thecity2.org) will focus on helping individuals in forming cross-disciplinary groups to:

  1. determine the issue they want to tackle (i.e. traffic, lack of trees);
  2. determine a solution;
  3. develop an action plan;
  4. work to implement the solution;
  5. share the story of their success or failure with others.

Companies and organizations will be able to offer their tools to site users for use in executing their action plans. Ten micro grants of $10,000, coming out of the $100,000 TED Prize money, will be awarded in July 2012 to ten local projects that have the best hope of spurring the creation of their City 2.0.

To be clear, that’s $100,000 to be equally split among ten groups. Not a lot of money to tackle problems that probably need millions, even billions, of dollars thrown at them. Thankfully, there’s more:

As the site continues to grow and the overall platform grows we expect to:

  1. expand the functionality for individuals to connect and act;
  2. develop and design templates for knowledge sharing between new ideas formulated on the site and preexisting projects;
  3. build out our resource section with new local and global partners;
  4. introduce technology solutions for non-web based communities;
  5. expand our financial incentive program with larger grant offerings for active projects
  6. establish local and/or global gatherings on the City 2.0.

That’s a little better. This part of the project should have a longer-lasting impact than the small pot of grant money. Local civic groups often don’t have the skills or wherewithal to build a connected platform to publish their ideas and solicit feedback. The City 2.0 could provide that. But soliciting ideas is just the beginning. Many other hurdles stand in the way, and from what I can see The City 2.0 doesn’t propose how to address them.

The most obvious barrier is money. The City 2.0 acknowledges that to be successful it needs “companies and organizations willing to offer empowering resources” and “financial support”. It seems to me they are simply hoping companies and philanthropists will step forward and reward the best projects. That’s papering over a big problem.

The next issue is how to choose the best project. The City 2.0 says in its intro video that it will “combine the reach of the crowd with the power of the cloud”. Both crowdsourcing and the cloud are hot topics these days. Crowdsourcing in particular can give people a voice who otherwise may not have spoken up, and it leverages the law of big numbers to extract a handful of singular, stand-out ideas. But the real problem with crowdsourcing solutions for cities is more fundamental than that: Who decides which ideas to implement?

Lior Zoref, a crowdsourcing advocate, gave a TED talk this year about the wisdom of crowds in which he was joined on stage by an ox. After the gasps died down, he asked everyone to guess the weight of the animal and submit it to a website. At the end of his talk, he announced the average of the audience’s guesses: 1,792 pounds. The real weight of the cow? 1,795 pounds.

It is an impressive demonstration, but one that doesn’t sell me on the crowd’s ability to reinvent the city. That’s because crowd wisdom cannot apply to projects like The City 2.0. With the ox’s weight, there is one right answer. The crowd’s wisdom can be unambiguously verified. But with ideas and concepts like those solicited by The City 2.0, there is no right answer. And you certainly can’t distill an “average” idea from them all. Ultimately, a panel will have to pick the winners and losers. Those panelists will have enormous sway over the outcome of The City 2.0. If they are experts in their field, what’s to say the winners will be revolutionary, or even substantially different from their own work?

If winners are picked by popular vote—which I highly doubt—that, too, is no guarantee that the most promising proposals will be selected. People don’t always know what they want. “It’s really hard to design products by focus groups,” Steve Jobs once said. “A lot of times, people don’t know what they want until you show it to them.” There may be wisdom in crowds, but genius is usually confined to individuals.

I suspect it’ll take true genius to remake the city. We’ve been spinning our wheels in recent years, rehashing concepts of the city that have been around for decades, even centuries. Those ideas may have worked well in the past, but they didn’t have to contend with airports, globalization, or climate change. Today’s best solution may be unlike anything we have come to expect from cities.

I’m sure The City 2.0 will fund some great projects, but we won’t really know how they work until we really try them. Not small bits here and there, but big implementations. Trying on that scale takes money, and the only organizations with the money to do it are governments.

Does that mean it’s back to the old way, sitting through planning meetings and zoning boards? Maybe. Crowdsourcing is a great way to gather ideas, but implementing them takes community and persistence and enthusiasm. It’s possible that a website could create that community, but I’m skeptical—most social media tools piggyback on existing, real-world social bonds. I know I sound pessimistic about The City 2.0. I’m not entirely. I hope that the project will uncover a work of genius that would have otherwise been ignored, but I’m not holding my breath.

Photo from The City 2.0.

Red-eyed Vireo

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Earlier this week I pointed out that urban areas can actually increase tree cover over time, albeit with a caveat. The two studies I cited measured tree cover and only tree cover—they made no claims about ecological function. Luckily, other studies have done just that, including one that looked at migratory bird use of greenways in urban areas.

Migratory routes are important, though most research into migratory bird decline has focused on habitat loss in their breeding and wintering grounds. That has left a large piece of the puzzle unsolved—the habitat between point A and point B. Think of it this way: If snowbirds—you know, northern (human) retirees who flock to warmer climes in the winter—started disappearing and our best solution was to look for them at their apartment in New York or their rental in Boca Raton—ignoring rest stops and motels along I-95—we’d be doing a great disservice to our older generations. Ignoring flyways is similarly foolish.

There have been studies in more recent years that aim to fill this gap, and one published in 2009 by Salina Kohut, George Hess, and Christopher Moorman picks up the trail along, well, trails. They surveyed bird species abundance and richness—how many and how varied the itinerants were—in 47 greenways in and around Raleigh, North Carolina.

Greenways are a common and convenient way for cities to conserve natural habitat. Their linear form is well suited to urban areas, and they easily double as parks or recreational trails. They also can serve as stop-over habitat for migratory birds. Kohut, Hess, and Moorman were hoping to find the right type of corridor for migrating birds, where our feathered friends can take a load off and fatten up.

It turns out that most birds were not picky and would stop at just about any greenway, regardless of vegetation, adjacent land use, or corridor width. That’s not to say all greenways were entirely equal. Overall, birds favored corridors with taller trees and lots of native shrubs teeming with fruit. And among birds that live in forest interiors far away from human development and even open fields, greenways wider than 150 meters (about 500 feet) surrounded by low-intensity development were the most popular.

None of the greenways Kohut and her colleagues studied were as good as a regular forest, though. Still, with some tweaks—including widening corridors, siting them near low-intensity development, and planting with natives—greenways can make decent stand-ins for the real thing, at least as far as migratory birds are concerned. Residential neighborhoods can even make themselves into agreeable stopover habitat by mimicking vegetation found at popular stops along the flyway.

So greenways make for good bird habitat, but let’s not forget that they’re good neighbors, too. In addition to helping migrating fauna, they boost property values, add recreational opportunities, and work well as commuting corridors for cyclists. Five benefits from one land use. Not too shabby.

Photo by qmnonic.


Kohut, S., Hess, G., & Moorman, C. (2009). Avian use of suburban greenways as stopover habitat Urban Ecosystems, 12 (4), 487-502 DOI: 10.1007/s11252-009-0099-6

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Tree City

City tree silhouette

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Cities aren’t called “concrete jungles” for their leafy greenness. But perhaps it’s an inappropriate nickname. Several cities actually have more—not less—tree cover than what came before them. By way of example, take this from historian William Cronon: “There are more trees in southern Wisconsin now than at any point in the last 7,000 years.” That’s in part due to more than a century of fire suppression, but also the intense pace of urban development.

There’s ample scientific evidence to back up Cronon’s assertion. In the early 1990s, David Nowak, an urban forester with the U.S. Forest Service, found that tree cover in Oakland, California, between 1850 and 1989 rose sharply from 2 percent to 19 percent. Now, a new study by Adam Berland, a PhD student at the University of Minnesota, found a similar pattern in and around Minneapolis, Minnesota.

Oakland and Minneapolis—and many other metro areas, I suspect—were sparsely forested before urban development. As far back as 1500 BCE, what would become Oakland was regularly burned by the Coastanoan Indians to clear out the underbrush to simplify acorn gathering. What trees remained in the 1700s were logged for lumber and firewood by the missions. Then in 1848, what was left nearly vanished when gold was discovered in California. By the time Oakland incorporated in 1852, its namesake was nearly gone.

Fire likewise held forests in southern Minnesota at bay for thousands of years. Yet unlike in central California, a part of central Minnesota quickly afforested during a brief climate cooling 400 years ago. It wasn’t long lived, though—shortly after their arrival, European settlers swiftly knocked down most of the Big Woods for farming. The remaining flecks large enough to be called forests cover only 2 percent of the original area. In other words, forests near Oakland and Minneapolis had nowhere to go but up.

The arrival of dense settlement was something of a godsend for trees. Young neighborhoods and cities are often depauperate—it’s easier to build without big trees in your way—but they tend to accumulate tree cover as they age. And relative to the denuded landscape that came before Oakland and Minneapolis, those urban forests are more akin to a real jungle than a concrete one.

Urban forests are certainly an improvement from a tree’s perspective, but they’re not a panacea for habitat loss. Neither of these studies examined how those forests function ecologically. Just like 11 random people do not make a soccer team, a bunch of trees is not the ecological equivalent of a real forest. Not only is the understory substantially different in cities—houses are terrible forage for most insects and animals—but the types of trees are often radically different.

Still, these two studies should make abundantly clear that cities do function as ecosystems, albeit limited ones. And in some cases, they are more diverse and productive than what came before. This is especially true for metropolitan Minneapolis, where monocultures of wheat and corn were less diverse than the Big Woods they replaced and maybe less ecologically complex than the cities that replaced them. These two cases also underline the need for an urban ecology that doesn’t just study what systems cities create, but strives to shape those systems for greater ecological complexity and diversity.


Berland, A. (2012). Long-term urbanization effects on tree canopy cover along an urban–rural gradient Urban Ecosystems DOI: 10.1007/s11252-012-0224-9

Nowak, David J. (1993). Historical vegetation change in Oakland and its implications for urban forest management Journal of Arboriculture, 19 (5), 313-319

Photo by frozenchipmunk.

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Aerial view of Carrollton, Texas

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If you think American cities are sprawling now, just wait until 2025. In that time, the U.S. population will grow by 18 percent but the amount of developed land will increase 57 percent. Up to 9.2 percent of the lower 48 could be urbanized by then. And while that number includes cities and the infrastructure to support them—roads, rail, power lines, and so on—that number does not include land impacted by farming, logging, mining, or mineral extraction.

That 10 percent of the lower 48 could be crawling with people is a stark reminder that our nation—while immense—is not immune to the pressures of development. It’s also acknowledgement that despite years of hearing about the resurgence of American cities, sprawl is still king.

Today, it feels like much of what drove the suburbanization of America since World War II has changed. Incomes aren’t rising nearly as fast as they did in the 1950s and 1960s. In fact, when adjusted for inflation, incomes have stagnated or dropped in recent decades. Soaring gas prices and congested freeways have stolen some of the automobile’s glamor, too.

Yet two studies show that while the outlook in the U.S. may have changed, our desire for suburban living has not. The study’s results differ slightly—the 2004 paper says we’ll add 25.8 million hectares (64 million acres) by 2025, the 2009 manuscript says 22.4 million hectares (55 million acres) by 2030—but their conclusions are the same. American cities will continue to sprawl, adding more land per person than in the past.

In recent decades, the locus of suburbanization has shifted from the Northeast and Midwest to the South. With its warmer weather and lower costs of living, the South has grown faster than any other region in the U.S. since 1980. Development has been fueled by flat, cheap land and abundant freeways, which has pushed land demands well above the national average in some states.

That boom also meant the South was hit hard by the housing bust in 2008. But that doesn’t mean the market for suburban housing has disappeared. Living the burbs is still cheaper than the city, and since real incomes for most Americans have suffered in recent years, development will continue to chase lower land prices. The recession and housing slump may have put a damper on suburban development, but I’m guessing it’s just a temporary blip.

Another factor that should conspire against suburban development—higher gas prices—also doesn’t seem to have much of an influence. The 2009 study suggests development rates won’t take much of a hit from high fuel costs. To simulate rising gas prices, the study’s authors reduced the forecasted development rate in states where it was highest—primarily the car-centric South. Only 5 percent less land was converted from rural to urban uses.

It’s possible things could change—perhaps fuel costs will rise even higher, or maybe the home downsizing trend that’s in its infancy will mature. But I think we should prepare for a future filled with suburbs. In the South, where most of the development is happening, land continues to be cheap and easy to access. The same warm weather that drew many people there will also keep them in their cars. Nobody likes walking in the South’s sweltering summers, even if it’s just from the steamy parking lot to the over-air conditioned mall.

The question then is, how can we make the suburbs more environmentally friendly? Encouraging compactness would be a good start, even just at the subdivision level. Hopscotch development inflicts ecological damage well beyond its immediate footprint—there are many plants and animals that cannot survive surrounded by a sea of humanity. Dispersing job and commercial centers is another option, helping to reduce the number of miles people have to drive on a day-to-day basis.

In the end, though, we’ll have to push for more ecologically integrated development. We’ve seen small steps in that direction already—most new subdivisions must deal with run-off from rainstorms on-site rather than shunting it to an overburdened creek. It’s a start, but not enough to offset America’s suburban future.


Alig, R., Kline, J., & Lichtenstein, M. (2004). Urbanization on the US landscape: looking ahead in the 21st century Landscape and Urban Planning, 69 (2-3), 219-234 DOI: 10.1016/j.landurbplan.2003.07.004

White, E., Morzillo, A., & Alig, R. (2009). Past and projected rural land conversion in the US at state, regional, and national levels Landscape and Urban Planning, 89 (1-2), 37-48 DOI: 10.1016/j.landurbplan.2008.09.004

Photo by La Citta Vita.

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California as seen in Blue Marble, 2012

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NASA’s latest image of the entire Earth may not be the most detailed to date, but it captures the imagination in ways some earlier versions didn’t. Not only does it look more natural and less computer-generated, it also recalls the first Blue Marble image—one that changed the way people thought about the Earth. The image’s natural feel and nostalgic overtones are important both for NASA’s future and our own relationship with the planet. Continue Reading »

Map of photos taken in Minneapolis and St. Paul, Minnesota

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Cities were, for thousands of years, distinct and easily identifiable entities. You were either in the city or in the country. Medieval cities took this to the extreme, building walls to make explicit the distinction. Johann Heinrich von Thünen systematized the idea in 1826 when he sketched a hypothetical map that, when simplified, looked like a bow-and-arrow target. The city sat in the center and was surrounded by rings of successively less valuable farmland. It was all very orderly and very German. And for a while it did a good job describing the relationship between the city and the hinterland.

Then came the railroads and automobiles that shot holes through von Thünen’s well-organized bullseye. And in places where two cities were less than a few dozen miles apart, even the boundary between the two became blurred. Today, it’s not uncommon to find metropolitan areas with two, three, even four major cities anchoring them.

Von Thunen's model of land use

Multi-city metros would seem to be a many-headed monster, riddled with contrary opinions and paralyzed by indecision. But that doesn’t alway seem to be the case. As far as labor productivity is concerned, multi-city metros—or polycentric metros, as the literature calls them—may have a distinct advantage. A study of all metropolitan areas in the United States with populations above 250,000 by Evert Meijers and Martijn Burger shows that productivity is higher in metros with more than one city. The effect is especially pronounced among smaller metro areas.

Meijers and Burger speculate that’s because smaller cities tend to have smaller problems—less traffic, lower crime rates, and so on. By splitting the problems up among a few cities, polycentric metros can host a large population without experiencing the problems of a similarly sized, monocentric metro.

But the advantages of multi-city metros diminish as the entire area’s population grows. It’s as though the larger entity needs one place to focus its efforts. So a metro area with two cities, each one-half the size of London, wouldn’t necessarily be more productive than London itself.

Multi-city metros also fall short on other critical parts of city life—cultural and leisure opportunities. Cultural outposts like opera houses and art museums benefit greatly from larger populations, which typically contain more benefactors, both wealthy and otherwise. The same goes for sports teams. Every city would like one for themselves. Say Ft. Worth wants to build an art museum. It’s probably not going to attract some donors from Dallas, who would rather see one built in their city. Chicago doesn’t have such a problem. Monocentric metros don’t have to worry about sharing.

As cities’ borders swell, multi-city urban agglomerations are probably going to be more and more common. Even within existing metropolitan areas, smaller cities could rise to prominence. Minneapolis and St. Paul, for example, have had to contend with the rise of Bloomington. The key will be for leaders to learn to work together, coordinating efforts rather than stepping on each other’s toes.


Meijers, E. (2008). Summing Small Cities Does Not Make a Large City: Polycentric Urban Regions and the Provision of Cultural, Leisure and Sports Amenities Urban Studies, 45 (11), 2323-2342 DOI: 10.1177/0042098008095870

Meijers, E., & Burger, M. (2010). Spatial structure and productivity in US metropolitan areas Environment and Planning A, 42 (6), 1383-1402 DOI: 10.1068/a42151

Map of the Twin Cities by the inimitable Eric Fischer.

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