A tram passes by a bicyclist and pedestrians in the beautifully livable town of Freiburg, Germany.
“What is required is a new definition of the city as a contact system, as a set of interactions and flows that define the kinds of network that enable creativity and innovation to thrive and grow.”
— Mike Batty and Peter Ferguson
Both Jacobs and Alexander pointed to the capacities of cities to promote a more sustainable way of life, with greater resource efficiency and much lower ecological damage. Both of them also linked these benefits to other benefits of cities, including quality of life for human beings, and opportunities for human and economic development.
However, neither Jacobs nor Alexander examined in detail the actual structural characteristics of cities that lead to these benefits. There are certainly strong hints in their discussion of cities as networks of overlapping and diverse interaction, and in Jacobs’ idea of a “spillover” — which I will discuss below. However, in the years since Jacobs and Alexander first expressed their insights, research has continued into the ecological characteristics of cities — with some surprising findings.
For those who do research in the sustainability of cities, one tantalizing puzzle looms large. On a number of metrics, higher-density cities are measurably “greener” than other places — that is, in many respects their residents have a much smaller ecological footprint. But in our current models, we can explain only about half of that difference, perhaps even less. Like the “dark matter” problem of physics, this “dark greenness” is both somewhat embarrassing, and very intriguing.
To be sure, some of the greenness of cities is not so hard to explain. For example, people drive less in bigger cities, because it’s harder to drive, and because it’s easier to get around without a car. (In fact, driving per person has actually gone down in recent years, even before the economic decline — and evidence suggests it’s because more people are living in compact, walkable cities and suburban neighborhoods.)
Other factors are small in themselves, but do add up: the closer spacing of buildings results in lower transmission losses and pumping energy; there is less embodied energy in roads and other infrastructure, and less operating and maintenance energy; urban residences tend to be more compact and energy-efficient; and more rural and natural areas are preserved, which provide important “ecosystem services.”
But the most intriguing reason may be the one we understand the least: people in cities actually interact and use resources in a more efficient kind of pattern — specifically, a network pattern. When we look at individual factors in isolation, we miss the synergetic effects of this network pattern, which may well explain why we can’t account for the observed magnitude of efficiency.
But there are other models that might help us — especially those that explain the dynamics of networks. In particular, there is a phenomenon in economics that’s known as a “Knowledge Spillover” (also known as a “Jacobs Spillover,” reflecting work on this idea by Jane Jacobs). It’s one of the reasons that cities are such powerful economic engines — and very likely one of the reasons we make cities at all. Simply put, it’s the idea that within a city, if you are making x, and I am making y, then our combined knowledge might allow us to make z together — but only if we are physically close enough that our knowledge can “spill over” from one sort of enterprise to another.
In practice, many such “spillovers” gradually connect and reinforce each other, creating a kind of virtuous circle of economic activity, and over time, spawning whole new industries. (Think of the automotive industry centered in Detroit, or the personal computer industry centered around Palo Alto.) This pattern is a classic kind of “interactive network,” familiar to many other disciplines.
The Jacobs Spillover is one form of an “externality” — an effect of an economic transaction that is not agreed to by the parties of that transaction, or factored into the accounting of such an economic transaction, but does have a consequence for one or more of the parties, whether positive or negative. An example of a large-scale negative externality is the depletion of resources like oil, or the accumulation of greenhouse gases that cause climate change. These phenomena will eventually impose a real cost, whether or not it is factored into the transactions. (Thus, one promising line of research is in ways to “monetize” these externalities, so that they can provide more direct economic benefits or penalties, and thus work to incentivize the sustainable use of resources.)
But the key observation of interest to us is that the spillover is an output of one system, which becomes an input into another — and this “virtuous circle” has a synergetic effect, increasing the economic efficiency of the city. This seems to be the case particularly when multiple variables are involved, and the synergies can compound through a kind of network effect. For Glaeser et al. (1992, 2009), this synergy from spatial proximity represents a core advantage of cities, and may even explain why cities are economically attractive places at all. This was also a theme of Jacobs’ own book The Economy of Cities (1969).
Nor is the Jacobs Spillover the only such externality that seems to be operating in cities. We may also consider the physical proximity of the consumption of resources, which may account for similar kinds of urban synergies. For example, Stockholm and other cities use “combined heat and power” to utilize the waste heat from power generation to heat homes. Many businesses use the waste output of one process as the input of another — for example, wood manufacturing businesses may use waste wood chips as a fuel source.
Such resource “spillovers” seem to operate in other realms too. If my daily activities are grouped close together, I may use the same trip by streetcar to stop along the way to meet another need that I might otherwise accomplish with a separate trip. These “transit spillovers” might account for a significant part of transit reduction in urban areas, beyond simply reduced distances between destinations.
It seems likely that all of these spillovers might in fact have additional interactive spillover characteristics — that I might, for example, also find a new job near my home on the way to another destination, and then share a creative bit of knowledge with a colleague, and then perhaps purchase some wood pellets at the local furniture store, and so on. That is, such spillovers might in fact form a kind of “spillover network” that has additional synergies. As this network grows within the city, its capacity to generate additional unforeseen spillovers also grows exponentially with its number of nodes, following the compounding power of networks.
A spillover is also an example of a process in which the output of one system becomes the input of another system, which is then able to transform it in some important way. A close analogy is the “metabolic pathways” of biological systems. Such processes typically do not operate in isolation, but as part of complex “metabolic networks.” This allows a complex interactive sequence of processing, with the efficient assembly of very complex outputs as a result.
In ecological systems, we can see a corollary when the excess chemical output or “waste” of one system becomes the input or resource of another. When these processes are part of a larger ecological network, the productivity of the system in biological terms can be greatly accelerated, while the “waste” of the system — the outputs that are not re-used — can be greatly reduced, or even brought to essentially zero. This network of “spillovers” — that is, of outputs becoming inputs — therefore represents a dramatic increase in the “efficiency” of the system — its ability to achieve high states of order with low waste.
Is a similar kind of process perhaps going on in cities? Can we use this model to understand and perhaps enhance the puzzling but remarkable resource efficiency of cities? Can we use it to confront the daunting problem of climate change, and, as suggested by the empirical observations, dramatically reduce the emissions of people within cities? This problem takes on special relevance as we see new patterns of urbanization around the globe, and the indications are that these new urban areas will follow the US model of very high consumption of resources with high levels of waste, and very high emissions per capita — a disturbing implication for the prospects of catastrophic climate change.
This concept seems to present a number of fruitful lines of investigation.
One revolves around the subject of urban density, or persons per unit of land — a controversial and problematic topic to be sure. Is urban density the only identifiable factor that affects this externality, or can we measure and model other factors? What about certain patterns of spatial distribution — for example, of uses, or of street networks, or of transit modes? What about Jacobs’ own hypothesis that walkable street-based urbanism, with its close stitching of private and public realms, was a critical mode of human connection, not reproduced in other forms of transportation?
What, too, is the role of human experience and choice, within the environment of a neighborhood or a city? As I discussed in the previous section on climate change, does the “choice architecture” of a neighborhood substantially affect the consumption patterns of people who live there? It seems perfectly plausible: if the walking paths within my neighborhood are unattractive, or conceptually unavailable to me when I am choosing to travel, I am less likely to choose to walk there. On the other hand, once I am in a car, if the neighborhood environment makes it easier for me to drive through, consume fast food, use disposable packaging, eat meat and so on, then I am more likely to make those choices — as indeed people do, on the evidence.
This structure of the neighborhood and its “choice architecture” seems to extend down to the scale of the sidewalk and its “intricate ballet,” in Jacobs’ memorable phrase. The networks of movement, interaction, experience and choice also include the smaller-scale structures of building fronts, entries, yards, forecourts, sidewalk seating areas, gates, gardens, and other myriad structures in the critical transition realm between public and private. This system of “place networks,” as I call it, is an overlooked but, I think, critical realm of urbanism, deeply connected to all of the other issues we have been discussing. (I will have more to say about what I am calling “place network theory” in a forthcoming volume.)
There is another interesting question to explore. Why is it that some suburban areas seem to have comparable levels of economic activity from spillovers, without the close proximity of people in cities? Why do they also have high emissions rates, which might suggest here is no correlation with economic spillovers as we suggest here? We hypothesize that this is because they are reproducing the conditions of proximity that facilitate spillover within cities, but doing so artificially, through relatively high-resource consuming activities — particularly, they are driving many trips of long distances in single-occupant automobiles. They are also performing other high-consumption, low-efficiency activities — living in isolated large homes, consuming poor-durability goods, living a “drive-through” lifestyle. They are able to maintain the economic spillovers after a fashion, using automobiles, cell phone contact, email, and other more artificial forms of contact — but only with a lifestyle that requires prodigious (and very likely unsustainable, it appears) consumption of resources. (This is the model that I previously referred to as the “crack cocaine” of economic development. It is highly effective in producing a quick economic “high” — but only at the expense of a planetary-scale hangover.)
If this phenomenon can be substantiated, how would a useful model be developed? What would be the metrics, variables, dynamic elements?
Another, more distant but ultimately most important line of inquiry is to explore strategies to exploit this phenomenon, assuming it can be substantiated. Can planners use strategies of “self-organization” to catalyze the spontaneous growth of such urban “metabolic networks”? Can they be facilitated with economic tools, such as pricing signals and other mechanisms? What is the role of “top-down” planning, such as the placement of initial elements of infrastructure and the like? My hunch is that a mix of all these strategies could be powerful and effective.
This, I suggest, indicates a promising research agenda for further investigation. The tangible goal might be to develop a “spillover network” or “metabolic network” model that might explain some of the more significant synergies and metabolic network-like aspects of urban systems, with the capacity to make differential comparisons based upon morphology. Such a model would ideally be capable of making useful predictions about choices in urban and infrastructure design, and urban and economic policy. Such a model might also facilitate the ability to monetize the currently unpaid costs of unsustainable urban development, and the long-term economic benefits of more sustainable development.
Photo essay for Chapter V.4
The remarkable example of Freiburg, Germany
Impressive achievements on sustainability metrics using urban form in combination with other technologies.
Figure V.4.1. The small German city of Freiburg. Photo: Thomas Maier, Wikimedia Commons.
Freiburg is a remarkable case of a relatively small town (220,000) that, like Portland and other cities, has experienced an urban renaissance in the last several decades. In the case of Freiburg, the city has retrofitted its historic neighborhoods with convenient forms of transit and energy-saving technologies, and it has built new urban extensions to accommodate population growth. These new neighborhoods feature a number of impressive innovations, including shared ownership, affordable housing, community gardening, convenient multi-modal transit, renewable energy, and other efficient, low-cost resource systems.
Within the city, the public transit system has been upgraded to offer convenient transportation by tram, rail, bus, walking, biking, or driving. Cars are accommodated in “traffic-calmed” areas, including so-called “shared space” areas that are safe for pedestrians.
Figure V.4.2. Bikes, pedestrians and rail users mix freely in the livable core of Freiburg. The tram offers an easy ride to the city’s new urban extensions.
As in Portland, regional planning has focused development within the city boundaries, limiting sprawl. Strict ecological standards protect the area’s natural ecology as well as its vineyards, orchards and farms, which provide a significant export economy as well as goods for the city itself.
Figure V.4.3. In the city’s center, beautiful historic buildings are retrofitted with renewable energy and conservation technologies, providing the sustainability benefits of both.
Figure V.4.4. Vendors prepare for a busy market day in the city’s central square, offering produce and other goods from the region. The market operates every day, and provides an important outlet for area farmers and producers — and healthy food for the city’s residents.
Most of the historic buildings in the city center were destroyed by Allied bombing during World War II. The city decided to rebuild on the original medieval pattern, not reproducing the previous buildings exactly, but producing new buildings in the same architectural form language. In this way the city really did experience a kind of renaissance, of livable and human-scaled architecture.
Figure V.4.5. Konviktstrasse, an infill development area completed in the 1980s.
Figure V.4.6. New apartment homes line community gardens and playgrounds in the new urban extension of Vauban. Roofs are made from solar panels. Photo: Arnold Plesse, Wikimedia Commons.
The city prepared twelve guiding principles for its “Charter of Sustainable Urbanism” to be used in shaping its new development. The twelve principles are grouped under three headings: Spatial, Content, and Process.
Figure V.4.7. The new urban extension of Rieselfeld includes a walkable grid of streets, gardens, playgrounds, public buildings, and homes in a compact, ecologically efficient form. None of the buildings are over six stories, and many of them feature balconies and terraces overlooking adjacent gardens and playgrounds. Photo: Volatus, Wikimedia Commons.
Principles of the City of Freiburg’s “Charter of Sustainable Urbanism”:
Figure V.4.8. Paving stones from the city’s recently re-paved center, using natural stones from the area in traditional patterns.
In the development of the city’s new urban extension, Reiselfeld, the plan followed seven crucial principles: