Of A City Is Not A Tree, Clay Shirky says:
It’s a moment of disorientation I’ve had a couple of times — you find a great piece of writing, and think “Wow, this is really going to change things!”, only to discover that it is in fact decades old. The clash of historical vertigo with Internet Now is both wonderful and daunting.
Christopher Alexander’s A City Is Not A Tree, from 1965, argues that a natural city is not a tree — in the set-theory sense, with clearly separated hierarchies — but a semilattice, and that artificial cities oversimplify the structure of natural cities to the point where they don’t work anymore. For example:
Consider the separation of pedestrians from moving vehicles, a tree concept proposed by Le Corbusier, Louis Kahn and many others. At a very crude level of thought this is obviously a good idea. Yet the urban taxi can function only because pedestrians and vehicles are not strictly separated. The cruising taxi needs a fast stream of traffic so that it can cover a large area to be sure of finding a passenger. The pedestrian needs to be able to hail the taxi from any point in the pedestrian world, and to be able to get out to any part of the pedestrian world to which he wants to go. The system which contains the taxicabs needs to overlap both the fast vehicular traffic system and the system of pedestrian circulation. In Manhattan pedestrians and vehicles do share certain parts of the city, and the necessary overlap is guaranteed.