As has been seen, if reorganisation processes of metropolitan polarities are analysed by examining different space dimensions, they appear in all their complexity and ambivalence, bringing to light an articulate picture of risks and opportunities. It will be useful to return briefly to this picture in the conclusions, but first, it might be interesting to attempt to use some of the variables considered in the previous paragraph to try to produce an albeit schematic classification of polarities, at which we have only hinted up to now in the form of examples.
On the basis of which criteria, then, can the various types of pole that take shape on metropolitan territory be distinguished?
In this chapter, we intend to propose a classification based on four variables, to which recourse has partially been made in the analyses developed up to now.
The first (which, as has been seen, refers to the absolute space dimension) is the rarity of functions: here we intend to specify that recognition of this feature implies only the presence in the pole of one or more highly rare function and it is therefore highly specialised; whereas it is not supposed that completeness of supply exists there at various hierarchical levels, although in some types of polarity, as will be seen, it is presumed that activities exist to directly serve the surrounding residents.
The second variable (relative space) consists of potential generation of flows of users on the part of the polarity itself: this serves to distinguish basically between concentrations of functions that involve a strong influx of users and others that are directed only at highly selective bands of users.
The third and fourth variables refer, finally, to the relational space dimension.
In particular, the variable ‘generation of public space’ concerns the presence or not, in the sphere of spatial concentration of functions, of spaces for collective use open to public use and, therefore, territorial spheres that may also be frequented regardless of fruition of specific services. This factor may therefore be considered an indicator of the participation of a given polarity in the creation of proper urban spheres, equipped with potential for social aggregation and fit to mix the different populations of the city.
The variable ‘relations with the context’ concerns, on the other hand, what might be defined as the degree of permeability of the pole with regard to the (environ-mental, social, economic, symbolic) characteristics of the spatial context it belongs to. This serves to distinguish polarities with a low degree of permeability from oth-ers which, vice-voth-ersa, draw unusual elements from their environment and work on them, giving back a high level of local identity.
In Table 1, on the basis of these variables, a limited number of polarity types are obtained. To achieve this simplification, recourse has been made to a simple dichotomisation of the values that can be taken on by each variable (high-low).
As is obvious, the scheme could be further enriched using more complex scales or making use of different measures for each of the variables. Thus, for example, with regard to functions and their rareness, there could be a complex description of the functional mix present in a pole, while, with regard to flow generation, quantitative measurements could be resorted to. Assessment of public space generation and re-lations with the context, on the other hand, would lend themselves better to working out qualitative, more or less articulate types.
In any case, by limiting ourselves to a simple dichotomous evaluation, it is pos-sible to recognise the following six types of polarity.
By ‘technical pole’ is meant a concentration of highly rare functions which, however, do not generate flows nor public space, maintaining poor permeability with respect to their own spatial context. Its importance for the economic and cultural development of an urban area lies in the specialised nature and technolog-ical qualification of the functions (as happens, for example, in the case of research
Table 1. A classification of metropolitan polarities
Rareness Generation Generation of Relations with Type of Examples of functions of flows public space the context polarity
High Low Low Low Technical pole Highly
specialised research laboratories
High High Low Low Specialist pole Hospital zone
High High High Low Capsular pole Commercial
and Leisure Centre
High High High High Higher urban pole Multifunctional
Metropolitan Centre
Low High High Low Capsular sub-pole Hypermarket
Low High High High Urbansub-pole Multifunctional
Centre in local quarter
laboratories); nevertheless, the pole does not directly interact with the vast majority of the population and its localisation therefore obeys above all functional demands of the activities it accommodates and, if anything, complementarity criteria with other specialised structures (e.g. university teaching or with businesses involved in applying the results of research).
A ‘specialised pole’ is similar to the preceding type, but different from it because it implies a high generation of flows. A hospital pole could be an example of this type: its relations with the city are thus much stronger and more direct in that, though maintaining relatively low relations with the urban context (even though it may ac-tually influence the localisation of complementary activities, like the marketing of pharmaceutical products or health equipment), it generates strong flows, destined to affect the whole structure of metropolitan polarities.
A ‘capsular pole’ corresponds, on the other hand, to the type of specialised cen-tre of a high level discussed in paragraph 3. Shopping and leisure cencen-tres of large dimensions, theme parks and, generally speaking, zones of tourist attraction, for example, that do not promote specifically local resources, belong to this category.
Apart from generating flows, they produce a space of collective use in some ways similar to urban public space, even though they do not entirely possess its features (for example, they are mostly private property and subject to fruition restrictions, etc.).7Nevertheless, their permeability to the context remains limited: they try, so to speak, to artificially reproduce the forms of the city within themselves, but have no dialogue with the elements of the ‘real’ urban fabric that surrounds them.
This last aspect belongs, on the contrary, to those here defined ‘high urban poles’
par excellence. Although they have highly rare, specialised activities within them-selves, they not only produce flows and public space, but are also able to enter into relations with their immediate environment. This property is typical of poles of great metropolitan importance, the activities of which are diversified and based on
spatial spheres with strong stratification of meanings (for example, series of roads and squares of historic centres or also in more external but widely consolidated spaces).
The remaining two types belong to polarities at a lower level than the previous, whose activities therefore do not reach the same degree of rareness. They conse-quently have a more limited user basin, involving the zone or city quarter. Both are able to generate flows and public space, even if the importance of both these ele-ments is in proportion to the importance of the pole itself. However, in this case, too (as we have just seen for poles of a higher level), the two types are distinguished by the degree of relations with the context. If the latter is low, as happens, for example, for a hypermarket serving a complex of urban zones, the capsularity phenomenon will be reproduced on a lower scale; if, on the other hand, it is high, like in the centre with a multifunctional nature in a quarter, urban polarity rooted in its own environment will be the result.