Category Archives: Network Growth

Path dependence

If you don’t know where you’re going, any path will get you there.

Path dependence is the idea that where we are today depends critically on where we were yesterday. Some systems are path independent, those that have a single unique equilibrium. Finding the solutions to some math problems is independent of where you start, as long as you follow a particular algorithm.

However, most systems we deal with on a daily basis have some characteristics of path dependence. Where you live might depend on what job you took, which depends on what your previous job was and where you went to school, and a different decision anywhere along the way would change today’s position.

Nowhere is this more true than transportation. On the one hand, it is obvious that certain locations were destined to be important cities because of significant natural advantages. New York has a deep harbor at the confluence of major navigable river. Chicago is at the pivot point between vast agricultural lands to the Northwest and the shortest land path to the East Coast. It was natural railroads would flow through the point on the map we now call Chicago.

On the other hand, many city sites that were selected for natural advantages in one technological era (The Romans selected London and the Dutch and English chose New York in large part for their capability as ports), remain important even after that technology becomes obsolete. With the logistics revolution and the new dominance of container shipping, London’s shipping has moved northeast to Felixstowe as large container ships cannot easily ply the Thames, while New York’s shipping has migrated to the wide open spaces of New Jersey.

The one-time advantages result in a set of complementary investments and inter-related decisions that take on a life of their own. Because of local trading advantages, commodities markets, banks, insurers, and other related organizations located nearby. A critical mass of those institutions felt no need to migrate just because their initial raison d’être vanished. While a building is under construction, temporary framing will often be used until the more permanent structure is erected. Once the final building can stand on its own, the falsework is dismantled. In a sense, everything is falsework for what comes after.

This kind of mutual complementarity happens repeatedly in transportation. Airplanes are the perfect example of mobile capital. If Amalgamated Airlines no longer wants to serve a particular city pair, the airplane can easily be redeployed elsewhere. Yet 80 years into the commercial aviation industry, airlines today serve mostly the same hubs their predecessors did on the Airmail routes of the 1930s. American Airlines is still in Dallas, United in Chicago, Delta (Northwest) in Minneapolis, and so on.

While very few decisions are completely irreversible, transportation decisions come close. Where we place a right-of-way, or an airport will explain where that facility will be decades, or even centuries from now.

A slight deviation from the efficient path to solve a short term problem today will cost travelers time for years to come. It is important to get the design right for the long term. (Undoubtedly this has social costs, see e.g. I-94 through the Rondo in Saint Paul).

But a slight deviation from the path will also change what the long term is. Build a bridge “here” rather than “there”, and then you will adjust all of the roads feeding into the bridge to meet it “here” (instead of “there”). And then land will be developed along the road to “here” to take advantage of the newly created accessibility, properties will be platted, buildings will be built, travel and trade patterns established, and other critical dependencies will come to assume that the bridge is “here”. At some point, say 50 years in the future, the bridge will need to be replaced. Even if “there” was a better location than “here” initially, after five decades of adaptation, it is quite likely that “here” is better now. The whole may have been better were a different initial decision been made, given conditions at the time. Given current realities, that path must now be foregone.

In transportation we say build it right the first time, because there won’t be a second chance. And that is true. But also remember the world will adapt to whatever we do, and we cannot let the perfect be the enemy of the good.

ICA Workshop on Street Networks and Transport

ICAheader

I am on the program committee for the ICA Workshop on Street Networks and Transport:

“Street networks, as one of the oldest infrastructure of transport in the world, play a significant role in modernization, sustainable development, and human daily activities in both ancient and modern times. Although street networks have been well studied in a variety of engineering and scientific disciplines including for instance transport, geography, urban planning, economics and even physics, our understanding of street networks in terms of their structure and dynamics is still very limited to deal with real world problems such as traffic jams, pollution, and human evacuations in case of disaster management. Thanks to the rapid development of geographic information science and related technologies, abundant data of street networks have been collected for better understanding the networks’ behavior, and human activities constrained by the networks. This ICA workshop is intended to gather researchers together to present the state of the art research and studies, in an interdisciplinary setting, on street networks and transport. Suggested topics include, but not limited to as long as they address issues related to street networks and/or transport:

  • Spatial statistics and spatial analysis along networks
  • Topological analysis and space syntax
  • Pattern recognition with street networks
  • Map generalization on street networks Complexity measurement of street networks
  • Human evacuations and simulations
  • Transport modeling based on street networks
  • Geospatial analysis of the OpenStreetMap data

Submission
All manuscripts in a length of 6000-7000 words should be in English, single column, single-spaced with figures and tables within the text. The manuscripts in MS Word 2003 format should contain authors’ affiliation and email, abstract (no longer than 200 words), and up to five keywords. To submit, please use EasyChair at http://www.easychair.org/conferences/?conf=icaworkshop2013

The structure and evolution of a skyway network

skywayOverTime
Recently Published:

We study the structure and evolution of the downtown Minneapolis, Minnesota skyway network. Developed by private building-owners, the network evolved from tree-like to grid-like over the course of 50 years. We find that decentralized forces with the goal of maximizing individual buildings’ profitability shaped the network. Our analysis shows that a building with greater office size, a sign of greater accessibility, was more likely to be connected earlier. The distribution of existing skyway segments is found to follow a power-law function of the average degree, closeness, and eigenvector centralities of the vertices. We further explain and model the evolutionary process using an agent-based model. The simulation results suggest that the model replicates the network structure and its evolutionary process.

The Genius of Dirt Roads

In City Journal, Brandon Fuller writes: The Genius of Dirt Roads :

“Angel writes that governments in the developing world, whose financial capacity is often limited, should focus on what may sound unglamorous: establishing an arterial grid of dirt roads throughout each city’s future territory, much as the commissioners did in Manhattan. The grid should connect to the city’s existing network of roads, of course, and it should cover an area that the city expects its future population growth to require. These arteries will one day carry public transportation and private traffic, and such infrastructure as water mains, sewers, storm drains, and telecommunications networks will follow their routes.”

The grid has advantages and drawbacks. In Planning for Place and Plexus we write:

The morphology and queuing properties of the plexus (its supply and demand) ultimately determine both the efficiency of the network in moving people and the efficiency of the land use. Radial (hub-and-spoke) networks allow easy access to the center but create inconvenient sharply angled parcels. In contrast, 90-degree grids maximize travel times (for anyone traveling in a diagonal direction) but create efficient parcels. A major issue with network topology is the interconnectedness of the network. Interconnected networks, be they grid or radial in nature, enable and even encourage through traffic, while a tree-like network discourages that problem. The topology of the network, grid, radial, organic (curvilinear) or otherwise, affects its performance.
The regular grid (with occasional interruptions) is arguably the most common topology for cities. It has been employed in cities for millennia. In the United States, the most influential legislation affecting the morphology of roads was the Land Ordinance of 1785. In many respects, it laid the foundation for future land use-transportation policy by adopting the Public Land Survey System, creating townships and subdividing them into 36 sections of one square mile (259 hectares) and 144 quarter-sections of 0.25 square mile (65 hectares) each. Roads delineating each of the sections were referred to as “section roads.” Subsequently, many urbanizing areas continued to use the centerlines of those roads as the location of present day arterials; the arterial networks are often further broken down into a finer grid of blocks.
A key point that has not been generally considered is the flexibility that the uniform and undifferentiated mesh networks (termed “grids” here) provide to changes in land use. A uniform grid allows alternative spacing between activities, spacing that can change with economies of scale. For instance, consider retailing. As described in Chapter 9, many stores—especially grocery stores—have been getting larger, while their numbers have dropped. Many New Urbanists, who advocate small-scale neighborhood retail, bemoan this phenomenon. Suppose that economies of scale indicate that it is efficient for the average retail store of a certain kind to increase in size from 1,000 to 2,000 ft2 (93 to 186 m2). Previously there may have been one such store every 10 blocks (one for every 100 square blocks); now there can be one every 14 blocks (one for every 200 square blocks). A grid allows the flexibility for re- spacing while keeping nearly optimal size stores. …
A tree network, in contrast, fails to provide such flexibility; a store can locate either at the neighborhood center, at the community center, or at the regional center; it can serve perhaps 5,000 people, 15,000 people, or 60,000 people. A store optimally sized to serve 10,000 people cannot be located at a consistent node level—or, if it is, it cannot be efficient. A firm may need to locate stores in some neighborhood centers and not others, causing people to go into other neighborhoods in some places.
Recognizing that grid-based road networks might not lend themselves to locations that were not situated on flat, featureless plains, designers introduced several variations. To conform to the contours of the land, Frederick Law Olmstead employed curving streets in many of his designs (e.g. Roland Park, Maryland). Permutations continued to evolve over the years, and the “loop” and “lollipop” designs became the standard in suburban settings

I think the idea of a particular network topology (grid vs. tree) depends a lot on the topography. Getting this right is important. The idea of laying something out in advance (Angel’s main point), so that property rights and development can occur on that lattice, seems a very good one.

Track Record: Do Major Urban Subway Networks Evolve along Similar Patterns?: Scientific American

Susan Fecht @ SciAm: Track Record: Do Major Urban Subway Networks Evolve along Similar Patterns?:

“No two subway systems have the same design. New York City’s haphazard rail system differs markedly from the highly organized Moscow Metro (above), or the tangled spaghetti of Tokyo’s subway network. Each system’s design is the result of many factors, including local geography, the city’s layout and traffic distribution, politics, culture and degree of urban planning.”

Nice summary of recent research by Roth, Kang, Batty, and Barthelemy “A long-time limit for world subway networks” in Journal of the Royal Society: Interface (which might be behind a firewall if you don’t have library access)
I am interviewed in the SciAm article.

Seeing historical processes in road networks’ patterns

Tim de Chant on Strano et al.Seeing historical processes in road networks’ patterns:

“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.
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

A Short History of Intersection

Urbangram has A Short History of Intersection

“The Egyptian symbol for ‘city’, Richard Sennett points out in Conscience of the Eye (p. 46), consisted of a cross bounded by a circle, providing one of the earliest signs of the urban; intersection and enclosure — grid and boundary — come together in this hieroglyph. The notion of the urban as an intersection offers an insight into one of the longest-running themes in urbanism – centrality. Whilst Egyptians made a point of right-angled street intersections, the Roman grid plan (Hippodamus) was designed to explicitly generate centers at different scales, with the earliest phase of settlement locating itself at the intersection of two prominent axial streets, the cardo and the decumanus. To this day cores, hubs, and centers form ever-present concepts in urban discourse; all relate or allude to a set of slippery spatial phenomena we call ‘centrality’.”

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