Metropolitan Spatial Structure

Conceptualising the Importance of Centres

Every assessment of spatial structure starts off within determining the importance of centres.

As indicated by Preston (1971), one can make here a distinction between the absolute importance of a centre or its nodality and the relative importance of a centre or its centrality. In the light of employment and journey-to-work flows, the nodality of a centre can be expressed by the total number of jobs it provides, whereas the centrality of a centre can be defined by the number of jobs in excess of those demanded by the centre’s own inhabitants. The distinction between nodality and centrality can be traced back to the work by Christaller (1933), who argued that if the importance of a centre was determined solely on the basis of its size, then part of its importance must be ascribed to the settlement itself as an agglomeration and another part to the settlement as a provider of jobs, goods and services to its surrounding territory. Accordingly, it is possible separate the external

6 It should be noted that, over time, the monocentric model could also over-predict commuting distance as total commuting distance could drop when more people work and live in sub-centres (Small and Song, 1994). However, this would also mean that the predictions from the monocentric model are no longer reliable.

7 The use of polycentricity should not be confused with the concept of multicentricity. Multicentricity refers to the existence of multiple centres, while polycentricity emphasises a certain balance in importance of these centres.

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importance from the local importance of a city. The importance of a centre c in an open system (i.e., including linkages with other systems) can then be decomposed⁸ as follows:

Cci = Nc – Cco – Lc, where

Cci = the surplus of importance of a centre based on incoming flows from other places within the same spatial system; its internal centrality.

Nc = the absolute importance of a centre; its nodality

Cce = the surplus of importance of a centre based on incoming flows from other places outside the spatial system; its external centrality.

Lc = the local importance of a centre based on internal flows (people working and living in the same place).

For example, when examining the importance of a centre as source of jobs in a city-region, it can be argued that Nc represent total employment in part of the city-region c, Cci

represents the incoming commuting in part of the region c from other parts of the city-region, Cce represents the incoming commuting in part of the city-region c from places situated outside the city-region, and Lc represents the number of employees in the part of the city-region c that also live there.

Polycentricity, Network Density and Openness of City-Regions

Analogous to the distinction between nodality and centrality, there are two main approaches to assess the spatial structure of city-regions (Green, 2007; Meijers, 2008b).

The morphological approach is based on the attributes or internal characteristics of centres such as the number of jobs (see e.g., Spiekermann and Wegener, 2004; Meijers, 2008b).

The functional approach classifies metropolitan spatial structure based on the structure of flows within urban systems (see e.g., Green, 2007; De Goei et al., 2010). Both approaches look at the balance in the distribution of importance across centres, where polycentricity tends to be associated more with a balanced distribution of the importance of these centres (Kloosterman and Lambregts, 2001; Meijers, 2008b). Whereas the morphological approach assesses the balance in the size distribution or distribution of absolute importance of centres, the functional approach typically looks at the balance in the distribution of

8 See Burger and Meijers (2010) for a more elaborate discussion of these issues.

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functional linkages between centres (Burger and Meijers, 2011; Figure 3.1), where most often attention is paid to the distribution of intra-regional flows and, hence, internal centrality scores (e.g., Green, 2007).

Figure 3.1: Morphological Polycentricity versus Functional Polycentricity

For conceptual clarification and in line with graph theoretical conceptualisations of spatial structure (e.g., Limtanakool et al., 2008), we explicitly disentangle the spatial structure of city-regions from related aspects of the spatial organisation of city-regions such as the degree of network density (i.e., the extent to which the different parts of a city-region are functionally linked) and outward openness (i.e., the extent to which the city-region is connected to other city-regions). Not including network density in our measure of functional polycentricity was necessary because it is possible to encounter urban systems

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that are strongly networked in a hierarchical organisation and urban systems that are not networked at all, where centres are relatively equal in terms of their connectivity to other centres. If both primacy and network density scores are combined, we may find that urban systems with a highly unbalanced distribution of functional linkages but a high network density would receive a similar score as urban systems with a highly balanced distribution of functional linkages but a low degree of network density (Burger and Meijers, 2011). A similar argument can be made with regard to the openness of a city-region.

This does not mean that the degree of network density and openness are not important aspects of the spatial organisation of a city-region. In fact, it can be expected that the increased mobility of households and businesses will result in an increasing network density within city-regions and an increasing openness in city-regions. Moreover, a functional polycentric urban region is non-existent without a minimum degree of interaction between centres (Champion, 2001; Parr, 2004).

A Typology of City-Regions Based on Commuting

Based on the orientation of commuting patterns in city regions⁹, it is possible to distinguish between four prototypical city-regions, as shown in Figure 3.2 (Van der Laan, 1998;

Schwanen et al., 2003). We distinguish between three types of journey-to-work patterns between different parts of a city-region: traditional commuting (commuting from the surrounding territory to the principal city), exchange commuting (commuting from the principal city to the surrounding territory) and criss-cross commuting (commuting between the different parts of the surrounding territory).

The monocentric city-region is characterised by a low degree of exchange and criss-cross commuting because the majority of commuters living in the surrounding territory travel to the principal city, while the surrounding territory attracts very few commuters. In a polycentric exchange city-region, commuting has become reciprocal; commuting is no longer directed solely from the surrounding territory to the principal city but also from the principal city to the surrounding territory. In a polycentric criss-cross urban system, the different parts of the surrounding territory have become more dominant because they now attract commuters from other parts of the surrounding territory. In this, parts of the surrounding territory have become complementary to the principal city and are

9 It should again be stressed that we look here only at the directionality of the linkages between the different parts of the city-region and not at the strength of the linkages between the different parts of the city-region.

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increasingly important as centres within the city-region. Commuting flows decentralise as the number of workers commuting between the different parts of the surrounding territory and bypassing the (former) principal city increases. Yet, the degree of exchange commuting remains low. Finally, a decentralised polycentric city-region is characterised by a multi-oriented commuting pattern in which there is no longer a dominant centre. In a polycentric decentralised system, there is a large amount of both criss-cross and exchange commuting. Two types of regions fit this classification: (1) a formerly monocentric city-region in which employment has spread from the urban core to the urban fringe (‘edgeless cities’; Lang, 2003) and (2) city-regions characterised by the historical co-location of multiple smaller centres (i.e., conurbations). In such urban systems, the amount of traditional commuting may become very small.

Figure 3.2: Functional Typology of the Spatial Structure of City-Regions

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