Micro- and Macro-structures of a More Sustainable City
3.3 Comparison of Areas and Dimensions of the City Models
So far few and rather vague assumptions have been made regarding the land required by each city model to accommodate the same number of people. The general consensus seems to be that the core city requires the least amount of land and therefore helps preserve the countryside, but no figures are available as to whether this is true in practice, and so I shall proceed to calculate the land required by the various city models. Excluded from this comparison is the galaxy of settlements, because of its high degree of dispersal, which cannot be the model for a sustainable form of city or city region (though the neighbourhood does play a part in the micro-structure of the city). Also excluded is the polycentric net, as it represents a composite of other urban forms; its performance may depend on its actual configuration but can be assumed to be similar to that of the city models of which it is composed. In the following a comparison is made of the dimensions and the overall city areas of the core city, the star and the satellite city and the linear city. In order to achieve
comparability, the study is based on the following conditions:
• The overall area of a city to include open country; the amount to be 40% of the total area, with 60% built-up area (including streets, squares, pocket parks, etc.). This proportion follows the Chinese city model though with the smallest percentage of farmland and forestry included in the city’s administrative area, but still able to render the city largely self-sufficient regarding the production of vegetables and meat as long as soil and climatic conditions are favourable.
In Glasgow, of all areas included within the administrative boundary the countryside is 22%, vacant land 10% and the built-up area 68% of the total city area; the latter includes public parks and gardens (information obtained from the City of Glasgow Council Planning Department). The overall relationship between built-up and open land may therefore be around 60% to 40%.
• The average gross population density (the number of persons per hectare (pph) of city area excluding open countryside) to be 60 pph. Again, the actual density could be higher or lower and does not really matter all that much in this calculation, as will become clear later, but for comparative purposes a density has been selected which achieves a sufficiently high population in neighbourhoods and districts to support their respective central function. This figure is also similar to Howard’s Garden City, which accommodates 32,000 people on 405 ha, the area designated for the Social City, and achieves a respectable gross population density of 79 pph (a fact very frequently ignored when Howard’s Garden City is falsely classified as garden suburb, which would have a gross population density only of somewhere between 10 and 40 pph). Some of the people of a social city may actually live in the surrounding country and the actual population of the city may be somewhat lower. A population density of 60 pph is also the kind of middle ground in a city like Glasgow—which varies between 10 to 120 pph with an overall gross population density of 35.09 pph over the total city area (Glasgow City Council, 1995). Assuming that about 40% of the city’s area is not built upon (open space, forests, etc.), then the net population density is around 58.5 pph, which is close to the 60 pph chosen as threshold value.
• Each model to accommodate a population of 250,000 and 500,000
respectively; this will allow an insight into the changing land requirements and dimensions of each city model as a result of doubling the population.
Other and larger population figures could have been chosen, but with fixed gross population density and a standard amount of open land included in the city area the overall dimensions of city models are likely not to vary
significantly with increasing or decreasing population; they only get larger or smaller. Therefore the actual population is of little significance, particularly when considering composite city models like the satellite, the galaxy of settlements, the polycentric net, which can grow to metropolitan size and several million inhabitants. A more limited population has been chosen to test the performance specifically of the core city. The upper population size of 500,000 is close to the populations of Copenhagen, Zürich, Glasgow, cities
already mentioned and of importance in this context, and the first figure (Lynch’s maximum population of a satellite city)
3–25. Comparison of required area and dimensions of the core city:
(a) population 250,000; (b) population 500,000
may prove a threshold for the viability of some models beyond which they may no longer work efficiently within the given conditions.
• For the dispersed city models—the star and the satellite city—the population accommodated in the central city to be about 23% of the total city population, with 77% in the ‘fingers’ or satellites. This distribution follows that of Howard’s Garden City and makes sure that the central city is only proportionally larger so that the satellites are at least semi-autonomous.
On the basis of these conditions the dimensions of each city model are calculated for the two population sizes. Figures 3.25–3.28 show the city configurations in diagrammatic form and the cities’ dimensions. Regarding the micro-structure of the city models, all have a hierarchical composition with neighbourhoods forming districts, districts forming towns and towns forming the city. But there are some differences. The central area of the core city (Fig. 3.25) cannot be expected to show clearly differentiated neighbourhoods, districts and towns as development is continuous and of the same or similar density and compactness. Neighbourhoods and districts may be there in the concentration of specific uses or a specific spatial or formal configuration, but they may overlap, and facilities as well as services may be distributed throughout the central area.
Only towards the edge of the city may neighbourhoods, districts and towns form a visible structure. The central towns of the star (Fig. 3.26) and satellite city (Fig. 3.27) have a similar structure to that of the core city and show no clearly visible hierarchy, though one may actually exist. But the fingers, and to a degree the satellites, may show groups of neighbourhoods and of districts. In the smaller star city the fingers are formed by districts along the concentric transport routes; those of the larger star city may be formed by towns. This differentiation is the result of the space provision around the smaller and larger central town.
The linear city (Fig. 3.28) too has to some degree a hierarchical structure in the form of a linear configuration of districts as agglomerations of neighbourhoods.
An arrangement of this city model as agglomeration of towns is excluded as this would require a complex secondary transport network, which would contradict the linearity of development along one major transport route. The consistency of hierarchical composition makes sure that all centres have the same or similar access and catchment areas, but in the core city this composition is not clearly visible. Each of the models is represented to the same scale to allow a comparison of their dimensions. Table 3.01 documents the areas and dimensions of the four cities under comparison.
At a population of 250,000, the core and satellite city and the linear city are identical in overall area required as well as the size of built-up area and open land; the
Areas and Dimensions of City Models with a Population of 250,000 star has the same built-up area but needs 5,143 ha more open land than the other cities if the depth of its fingers is 1,200 m. If the finger depth is increased to 2,400 m the star city’s overall area becomes identical with that of all the other city models.
Distances between the edge of the built-up area and the centre differ considerably as a result of the different geometry and fragmentation of the urban fabric; the best values are achieved by the linear city (1,200 m with the depth of development 2,400 m) and the core city (3,642 m), the worst by the star (6,203 m with the
3–26. Comparison of required area and dimensions of the star city:
(a) population 250,000 with finger width 1,200 and 2,400 m; (b) population 500,000 with finger width 2,400 m only
3–27. Comparison of required area and dimensions of the satellite city: (a) population 250,000; (b) population 500,000
Table 3–01. Comparison of land take and dimensions—matrix (built-up area 60%, open land 40%)
Population 250,000 Core city
Star city finger d=1,200m
Star city finger d=2,400 m
Satellite Linear city d=2,400m
Total land required (ha) 6,945 12,088 6,945 6,945 6,945
depth of development of the fingers 1,200 m). However, when maximum distances between any point in the city and the edge of the open country are compared, the ranking order is rather different: the linear city is best (1,200 m), followed closely by the star and satellite city (1,754 m), and the core city has the
(%) 100% 100% 100% 100% 100%
Built-up area (ha) 4,167 4,167 4,167 4,167 4,167
(%) 60% 34.5% 60% 60% 60%
Open land (ha) 2,778 7,921 2,778 2,778 2,778
(%) 40% 65.5% 40% 40% 40%
Distance centre to edge of built-up area
(m) 3,642 6,203 3,974 4,529 1,200
Distance centre to edge of city area
(m) 4,702 6,203 4,702 4,702 2,000
Tot. dimension city (diameter/length)
(m) d=9,404 d=12,406 d=9,404 d=9,404 d=17,363 Max. distance from
built-up area to open land
(m) 3,642 1,746 1,746 1,746 1,200
Population 500,000 Core city
Star city finger d=2,400 m
Satellite d=2,400 m
Linear city d=2,400 m Total land required (ha) 13,888 15,070 13,888 13,888
(%) 100% 100% 100% 100%
Built-up area (ha) 8,333 8,333 8,333 8,333
(%) 60% 55.3% 60% 60%
Open land (ha) 5,555 6,737 5,555 5,555
(%) 40% 44.7% 40% 40%
Distance centre to edge of built- up area
(m) 5,150 6,926 6,405 1,200
Distance centre to edge of city area
(m) 6,649 6,926 6,649 2,000
Tot. dimension city (diameter/length)
(m) d=
13,298
d= 13,852 d= 13,298 d= 34,721 Max. distance from built-up area
to open land
(m) 5,150 2,470 2,470 1,200
largest distance (3,642 m).
Despite the good scoring of the linear city, the picture changes when overall dimensions of the city areas are compared; the diameters of the core and satellite city and the galaxy of settlements are best (9,404 m), followed by the star (12,406m) with a finger depth of 1,200 m; with a finger depth of 2,400 m the star’s diameter becomes identical to that of the core and satellite city. The linear city has a length of 17,363 m, which clearly indicates that this form is not suitable for a large population.
Areas and Dimensions of City Models with a Population of 500,000 At a population of 500,000, the core, satellite city and the linear city are again identical in overall area as well as the size of built-up area and open land. The star has the same built-up area, but even with a finger depth of 2,400 m it would still require 1,182 ha more land, owing to the length of the fingers. This indicates that with extension the star includes between the fingers an
3–28. Comparison of the required area and the plan form of the linear city for a population of 250,000 and 500,000
exponentially increasing amount of open land, which can be prevented to a degree by increasing the depth of the fingers; but this in turn requires a secondary transport system in the ‘fingers’ to get people from neighbourhoods to district centres, and such a more complex transport system may be less feasible. This clearly indicates that the star form is suitable only if the length of the ‘fingers’ is kept reasonably short.
Distances between the edge of the built-up area and the centre again differ considerably as a result of the divergent geometry and fragmentation of the urban fabric; the best values are achieved by the linear city (1,200 m) and the core city (5,150 m), the worst by the star and the satellite (6,926 m and 6,649 m respectively).
However, when maximum distances between any point in the city and the edge of the open country are compared, the ranking order is rather different: the
linear city is best (1,200 m), followed closely by the star and satellite city (2,470 m); the core city has the largest distance (5,150 m). The diameters of the core, star and satellite city are equal (13,852 m). The linear city has a rather impracticable length of 34,722 m, which reinforces the argument that it is viable only when bridging short distances between different development centres.
The comparison shows that the linear city is not suitable on its own, but when linking core cities (like the fingers in a combination of stars, for instance) it may have its justification provided its length is limited. The star city is viable if the length of its fingers is kept reasonably short; up to a population of about 500,000 and with a finger depth of 2,400 m the star performs nearly as well as the satellite city. The cities differ with regard to maximum distance between edges of built-up areas and centre, which is understandably larger in the star and satellite city because of the decentralisation or fragmentation of urban fabric.
They also differ with regard to the maximum distance between any point of the built-up area and the edge of the open land, which is much shorter in the star and satellite city in comparison to the core city. The core city has shorter distances within the built-up area because it is compact, but has larger distances to open land because the open land is at the edge of development, not in between.
Regarding the compactness or fragmentation of the urban fabric in the core, star and satellite city, it becomes clear that it does not necessarily affect the amount of land required for the entire city as long as a stable percentage of open land is included in the city area. If this is the case, as was assumed in the comparison of city models, then the nature of the compact city debate changes considerably:
• With an identical percentage of open land included, the total area of the city models is identical or at least very similar; so is the percentage relationship between built-up and open areas. As a result of this and of identical average densities, the gross population density is therefore the same in all models.
• The decisive difference between the city models is then no longer the overall density of population but the degree of fragmentation or concentration of the built-up area and, as a consequence of that, better or worse access to, and more or less fragmentation of, the open land and a higher or lower degree of decentralisation of services and facilities.
If the conditions of the inclusion of the same amount of open land in the city area and the same average population density are fulfilled, the evaluation of city models can focus on performance values other than population densities, such as access to open land and degree of fragmentation of the open land. The question of the degree of compactness or fragmentation of the urban fabric too can be seen from another angle: the degree to which it facilitates mobility without congestion. And with these issues in mind, neither the core city with complete concentration of the urban fabric nor the galaxy of settlements with complete decentralisation of the urban fabric into something like neighbourhoods can any longer be considered as a preferred option. If these two city models are judged to perform worse than the other models—the compact city because of the
likelihood of congestion, lack of privacy and personal outdoor space and large distances to the open country, the galaxy of settlements because of the lack of core areas with services and facilities of a higher order than those of the neighbourhood and with increased distances between built-up areas—the remaining models all exemplify a moderate degree of decentralised concentration which provides similar functional, social and environmental qualities almost regardless of the overall size of population.
3.4 Comparison of the Potential Performance of the Six City