Christopher Alexander wrote a brilliant essay in 1965: “A City is not a Tree“. Long interested in Alexander’s work since graduate school in Berkeley, I recently re-read the piece which has been packaged in a 50th anniversary volume. The original article is available online free. There have been numerous other papers that have commented on various aspects Alexander’s piece, I can’t list them all, it has been cited over 1000 times.

Alexander criticizes new towns, notably my home town, Columbia, Maryland, for being treelike in its conception, rather than what he terms a semi-lattice, but we would more informally call a mesh-like network. The neighborhoods belong to villages, the villages are part of the city. The neighborhoods, following an element of the Radburn plan, are isolated, that is, one cannot go from one neighborhood to another without being on a village street. That’s not to say there is no through traffic, there is, because the neighborhood network has more than one outlet.

But it’s not simply the street network that is tree like, retail is also tree-like. The neighborhood center might have a convenience store (7-11 or Wawa), the village center would have a grocery and 10-20 smaller stores. Town Center had the Mall in Columbia with department stores and over 100 shops. Perhaps Walter Christaller would be proud that Central Place Theory was not merely descriptive, but also prescriptive.

Finally the schools were tree-like. The neighborhood elementary school fed the village-level Middle and High schools.

In practice it was not so tree-like. As a resident of the neighborhood of Bryant Woods in the Village of Wilde Lake, my mom could go shopping at a supermarket at Joseph Square shopping center in the Village of Harper’s Choice. It was only another mile down the road. Later as a resident of Longfellow neighborhood, I could open-enroll into Wilde Lake Middle School. And of course not every neighborhood got an elementary school, not every village got a Middle or High School, and the boundaries were overlapping. So while the stylized schematic drawing may have looked treelike from the perspective of an architect a few thousand miles away, it was not treelike in practice on the ground. See The Next America Revisited for my take.

I don’t believe the planners envisioned it would be so tidy — though better to start out tidy and let entropy emerge rather than start out chaotic and hopes it self-organizes into an aesthetically pleasing environment. Rather my impression from reading a lot of the documents and having lived there and hearing talks from Rouse company officials and so on is that they believed that treelike street networks reduced through traffic, just as planners favor traffic calming today. They believed shopping should be organized into centers, rather than sprawled out uncontrolled along streets, and they should be spaced to be closer to residents. They believed local government is somewhat hierarchical (national, state, county, city) and that village was an organization unit that had some value to regulate things at a more local level than the city (Columbia is not technically a city, it’s just a home owners association, though it is a Census-Defined Place and the second largest in Maryland, after Baltimore, just as was foretold in the 1960s). They believed kids should walk to their neighborhood school, so the neighborhood should be the right size to support the school, which should have X students for pedagogical and cost-efficiency reasons, and ideally students would walk to their middle and high school too, but that middle and high schools should be larger. So the hierarchy was a natural way of organizing that.

But even if Columbia is innocent of being as treelike as Alexander feared (and certainly some new towns were more treelike), the suburbs are certainly more treelike than cities. My students have measured the “treeness” of networks, introducing the metric in Xie and Levinson (2007) Measuring the Structure of Road Networks. For instance in Network structure and the journey to work: An intra-metropolitan analysis (under review) by Pavithra Parthasarathi and myself, we see that treeness is not surprisingly higher at the suburban edges of the metropolitan area than in the center, though it declines as we see rural areas, where the sparser network is also more mesh or grid-like. (See figure)

Still, sometimes the city is a tree, or at least aspects of it are. In particular, many networked physical infrastructures are better organized as trees, especially if they require a large capital investment (like a waste water treatment facility). Similarly, the stream and river valleys are naturally organized as hierarchies. Transit networks are also often more treelike or radial than roads, and while may eventually evolve into ring-radial system, don’t generally start out that way. See Roth et al. 2012.

Clearly social connections should not be assumed to flow in a way that maps directly to the physical layout of the network, all other things being equal, you are more likely to know your neighbor than a randomly selected person farther away. Yet, in a modern world with migration and telecommunication, you are likely to know someone specifically who is not on your block and to not  know everyone on your street. Growing up, my mom’s friends were scattered across Columbia, not just in her neighborhood. ‘Community without propinquity’ was first identified by Berkeley Planner Mel Webber, and certainly applied in Columbia as it does everywhere, where people could meet based on any kind of interest, not simply the desire to live on the same street. The physical form of the city does not represent how the city works, but more importantly the plans do not determine how the city works. People and their relationships are affected by their environment, and reshape it to suit their needs.