Friday, 01 April 2016 11:26

Grid-based plans for urban extensions

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The value of a grid as an organising structure for new development is demonstrated through historic and contemporary examples from different continents. UN-Habitat's report concludes that "The main virtue of the grid is its simplicity; the rational organization of streets in a geometrical network simplifies the complex process of city construction and allows the urbanization of extensive areas with a single identity and a coherent image."

A grid layout works on many different levels. It provides a functional basis for transport and infrastrucuture provision. It gives legibility to a city or neighbourhood. It can be the basis for the provision of open space. It is a flexible means of accommodating new growth. Above all, and especially for today's rapidly urbanising cities, it provides a pattern for the organisation of growth that is easy to understand.

Geddes' Plan for Tel Aviv

The street and the block are fundamental elements for planning and design of urban development.  One of the report's case study examples that illustrate the application of a grid-and-block design is the plan for Tel Aviv done by Patrick Geddes, which I blogged about on a visit there in 2015. The orientation of Tel Aviv's orthogonal grid is north-south, with the main commercial streets paralleling the coast. There is a hierarchy of streets, while the blocks each have their own internal character with gardens accessed from lower order streets. Buildings are set back 4 meters from plot lines, with vegetation often occupying part of that space.

It's a modular structure, and this is the point that the report is making: a modular approach to development can create a common, replicable structure that can take locally distinct variations in form and detail. Mixed uses can be accommodated, while the basic flexibility of the structure allows for modifications through time, as happened in Tel Aviv.

Ouagadogou

Another of the ten case studies is a less familiar one. Ouagadogou is a city of about a million people in Burkina Faso. Most streets "are compacted earth surfaces that are shared by vehicles and pedestrians. Infrastructure is little or inexistent! and there are "no storm water drainage, water or electricity supply networks". After land nationalisation in 1984, there was a major programme of land reparcellisation in the informal areas around the city centre. The urban extensions have then been packaged in concentric rings round the centre, with each sector basically following an orthogonal grid structure.

While blocks typically have open space in the centre, the predominant development form within a block is self-built single story housing, making for low household densities. However, this is not informal housing; the development follows a plan and the state provides the land on which to build (but nor the freehold), making urban housing affordable in this very poor country.

The report highlights some of the weaknesses of the development in Ouagadogou. The individual plots are quite large, but the buildings are single story. Consequently, densities are low, so public transport is difficult to provide, and there are few local commercial services or small businesses. Self-builders on low and insecure incomes, especially those who are new arrivals from the countryside, are unlikely to construct the kind of multi-storey buildings that create the densities to make cities compact. The density in Ouagadogou is given as 63.5 persons per hectare, whereas in predominantly 4-storey (and much more affluent) Tel Aviv it is 150.

Street widths and Block areas

The report concludes that although the tencase study cities vary in size and density, "the basic units of block and street have comparable dimensions that relate to the basic housing units and human scale." The minimum street widths in the 10 cities range from 20 meters in Barcelona to 4.5 meters in Aranya (India), while the range for maximum street width is from 70 meters in Villa el Salvador (Peru) to 20 meters in Ouagadogou and the upgraded informal settlement of Mariano Melgar in Peru. Whether these are indeed "comparable dimensions" might be debated. Similarly, the median block area is given as 800 square meters in Aranya but 14,700 in the Back Bay area of Boston. 

"Generous" street widths and block sizes are recommended, so that cities can evolve and densify. However, "If street, plot and block sizes are too large, the city loses its relation to the human scale and is less feasible economically." It is rather frustrating that just what might be "too large" is not defined.

Regulation

It is important to stress that the use of the grid to plan urban extensions is seen as creating flexibility, not rigidity. The traditional, highly detailed Master Plan is explicitly rejected. A "General Plan" backed by a set of regulations is endorsed as an evolutionary approach to urban extension. The General Plan can be a "very simple document"  that sets out a hierarchy of streets and public spaces. The regulations then reflect the culture of a particular city and evolve through time. They cover "construction, vicinity, public ornament and hygiene".

Comments

The analysis is well focused and commendable in terms of its geographic spread and the type of cities covered - the three not already mentioned are Manhattan, Savannah (both USA) and Bahir Dar (Ethiopia) - though under-representation of Asian examples is a weakness. Similarly, the selection critera required cities with "layout in a grid or in a physically rational plan; mixed-use development; clear distinction between public and private space; street life and street frontage; endurance through time and adaptation to changing needs; adequate proportion of street, open space and built up areas; and well dimensioned streets and blocks." These preconditionalities leave the report open to the criticism that the results preceded the research. 

Tha approach is similar to that advocated by Pedro Ortiz, the World Bank's Senior Urban consultant, in a book I reviewed in another blog.  Ortiz also sees the grid as an efficient and equitable way of organising urban space, but stresses the need roll it out at a larger scale. Ortiz was Deputy Mayor of Madrid; Joan Clos, who heads UN-Habitat was Mayor of Barcelona. It's not just the football that these two cities are dominating!

 

 

 

 

 

 

 

 

 

 

 

Read 1002 times Last modified on Friday, 01 April 2016 15:56

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