Thursday, July 29, 2010

Short Blocks Nurture Mixing and Diversity

Jane Jacobs looks at the circulation of people like a biologist or hydrologist might see the flow of water. She uses fluid imagery and sees the messy vitality of the delta rather than the simple order of a man-made ditch as her paradigm. Her point is easy:

Long Blocks Entail Monotony and Simplicity

Long blocks yield
endless stores and a depressing predominance of commercial standardization...there is geographically so little street frontage on which commerce can live, that it must be consolidated, regardless of its type or the scale of support it needs or the scale of convenience (distance from users) that is natural to it. Around about stretch the dismally long strips of monotony and darkness - the Great Blight of Dullness...this is a typical arrangement for areas of city failure.
Long blocks also mean people
are kept too much apart to permit them to form reasonably intricate pools of city cross-use...they sort people into paths that meet too infrequently, so that different uses very near each other geographically are, in practical effect, literally blocked off from one another.
Here she uses an image of fertility and explains why the generative power of city streets requires complexity and not simplicity:
Long blocks, in their nature, thwart the potential advantages that cities offer to incubation, experimentation, and many small or special enterprises...[they] also thwart the principle that if city mixtures of use are to be more than a fiction on maps, they must result in different people, bent on different purposes, appearing at different times, but using the same streets....

[I]t is fluidity of use, and the mixing of paths, not homogeneity of architecture, that ties together city neighborhoods into pools of city use, whether these neighborhoods are predominantly for work or predominantly for residence.

Jacobs observes that short blocks offer multiple paths to a given point, and that this ostensible inefficiency, the variety in route selection, is actually a profound source of generation, novelty, and creativity for a city district.

Salem's downtown has short, square blocks. This detail is from a 1917 USGS quad.

Street grids are surprisingly efficient at allocating traffic. Because there are multiple paths (see taxicab geometry) between points, each individual can make a set of easy decisions about where to turn and how to get there. They also accomodate different kinds of traffic: car, truck, bus, bike, foot traffic easily. The sum of these individual decisions tends to be an efficient allocation of traffic. The invisible hand lives!

Here's the approximate area of the Sunnyslope zoning map but from the 1917 map. It was open farm and orchard land back then.

But starting mid-century, the quest for planned efficiency led to a ramified (or dendrite) model of arterial, collector, and local streets. In theory, such planning looked good. In reality, this planned network of streets exacerbated congestion, degraded neighborhoods, created sprawl, and instantiated the kinds of planning and market failures we deride in planned economies.

Here's the current zoning map. It shows the suburban-style long blocks with loop and lollipop street networks.

Fortunately, Salem has escaped the worst effects of this kind of development. Still, Salem's suburban-style fringes are not well suited for neighborhood diversity, for economic incubation, and for a post-oil future. It is likely that in 100 years, the old gridded part of Salem will be the most vital and enduring.

At present, however, Salem's downtown doesn't have all the other ingredients, and short blocks alone cannot guarantee vitality. We will see that Salem does better with the mixture of old buildings, but not so good with mixed uses and concentration. In fact, in the longer term, the greatest assets of downtown are its mix of old buildings and its short blocks.

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