Empirical Setting and Methodology

Previous Research on Commuting and Spatial Structure in England and Wales

Despite a growing amount of theoretical literature on changing urban systems, only a small number of recent empirical studies have quantitatively assessed the development toward polycentric city-regions in England and Wales. Studies by Coombes et al. (2006), Nielsen and Hovgesen (2008), Green (2008) and De Goei et al. (2008; 2010) have analysed the dynamics of spatial structure in England and Wales empirically by using commuting data, where most work has been done on English and Welsh urban networks at the inter-urban and inter-regional scale (i.e. between city-regions).

Analysing commuting data for England and Wales, Green (2008) found that England and Wales became more connected at the inter-regional scale between 1981 and 2001. In addition, Green (2008) concluded that there is an increasing tendency for people to live and work at the urban periphery, where travel patterns have become increasingly diffused. On a similar note, Nielsen and Hovgesen (2008) examined the development of nation-wide commuting flows in England and Wales between 1991 and 2001. They found an increase of the average commuting distance during this period, which is exemplified by an increasing connectivity between rural areas and main centres.

Focusing on commuting network in the Greater South East, De Goei et al. (2008;

2010) concluded that the mega-city region could not be regarded as a functional polycentric urban region. However, they find some evidence for spatial integration at the intra-urban scale (i.e., the city-region), as well as a decentralisation of the urban system at the inter-urban scale in the sense that the hub function of London is decreasing in importance. On a similar note, Coombes et al. (2006) found some evidence for increasing linkage formation between the principal cities of the city-regions in the East Midlands urban network.

However, the empirical literature on spatial structure in general, and those studies in particular, pay limited attention to spatial heterogeneity present between different city-regions. The remainder of this chapter contributes to the existing empirical literature on

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urban systems by jointly addressing the dynamics, spatial heterogeneity and the specific polycentric pattern of city-regions in England and Wales.

Commuting and City-Regions in England and Wales

The present study used journey-to-work data between local authority districts form 1981 and 2001 to analyse the development of metropolitan spatial structures in England and Wales. These data were obtained from the Special Workplace Statistics (Set C) in the British census.¹⁰ The Census Interaction Data Service (CIDS) 1991/2001 common geography was used to avoid potential problems with the changes of district-boundaries over the past twenty years (Boyle and Feng, 2002). A subdivision of city-regions was made using the common geography. In this, we used the classification by Coombes (2000), which comprises 43 British city-regions. We defined city-regions by functional linkages and areal associations based on the 2001 Census. In this, ‘the city-regions are delineated on the basis of commuting data through an algorithm that optimises the boundaries on the basis of a size of employment criterion and a minimum threshold of self-containment of flows to workplaces’ (Robson et al., 2006, p. 8). Accordingly, the classified city-regions are to a large extent self-contained in that most people (>80%) who work in city-regions also live there.

The delineation of Coombes (2000) is non-nodal and exhaustive since there may be several employment centres within each city-region, and every area in Great Britain is allocated to a city-region.Compared to other classifications of city-regions in England and Wales (see Robson et al., 2006; Davoudi, 2008 for overviews), an obvious advantage of the Coombes classification is that it does not assume a priori the existence of only one main centre within a region. Hence, the bottom-up approach of delineating city-regions is useful for exploring polycentric patterns in a wider region (Davoudi, 2008).

Based on the classification of city-regions by Coombes (2000), we selected the 22 largest English and Welsh city-regions, whereby the more rural areas are excluded because these

10 Census output is Crown copyright and is reproduced with the permission of the Controller of HMSO and the Queen's Printer for Scotland. Sources: 1981 Census: Special Workplace Statistics (Set C) and 2001 Census:

Special Workplace Statistics (Level 1)

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areas are typically not metropolitan (for a similar selection, see Champion and Coombes, 2007). An overview of the city-regions included in the analysis is presented in Figure 3.3.

Figure 3.3: Selected 22 English and Welsh City-Regions

Quantifying the Spatial Structure of City-Regions

The degree to which incoming commuting and employment is centralised within city-regions is estimated to assess the development of metropolitan spatial structure of English and Welsh city-regions between 1981 and 2001. To assess the degree of morphological and functional polycentricity in a city-region, we used a primacy index (Van der Laan, 1998; Adolphson,

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2009). The morphological primacy index is calculated as the ratio of employment in the largest centre (i.e., the principal city) and the total employment the city-region and is thus based on the balance in the distribution of nodality scores. The functional primacy index is calculated as the ratio of incoming commuting into the largest centre originating from the city-region and the total incoming commuting originating from the city-region and is thus based on the balance in the distribution of internal centrality scores. A city-region is considered morphological monocentric if its employment is highly concentrated in one (principal) city. A city-region is considered functional monocentric if most commuting flows originating from other parts of the city-region are directed at the principal city (and no flows are directed from the principal city to the surrounding territory). The larger the degree of morphological and functional primacy is in a city-region, the lower the degree of morphological and functional polycentricity in a city-region.

In addition to the summary measures of polycentricity, we look at the specific patterns of polycentricity, which correspond to the typology of polycentric spatial structures introduced in the previous section. In this, we make a distinction between the monocentric and three types of polycentric region: the exchange, the criss-cross, and the decentralised city-region. This typology is based on (1) the degree of exchange commuting, or the degree to which commuters living in the principal city are oriented toward the surrounding territory and (2) the degree of criss-cross commuting, or the degree of commuting between different parts of the surrounding territory. We compared the degree of exchange and criss-cross commuting with the degree of traditional commuting, where commuting is defined as journey-to-work travels between the area of residence and another area of employment.

Hence, we exclude here journey-to-work travel of people who work and live in the same area. A more technical description of these measures is provided in Table 3.1.

Building on the discussion in the previous sections, we describe two additional features of city-regions. The degree of network density reflects the extent to which different parts of the city-region are networked or functionally interdependent and can be measured as the ratio of the actual connections between the different parts of the region compared to the total potential connections between the different parts of the city-region (Green, 2007). Here, the total potential connections between the different parts of a city-region are defined as the total number of employees in a city-region. A generally low ratio of the sum of internal centrality scores by the different parts of the city-region compared to the number of employees within a city-region indicates a low level of network

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density. Likewise, the openness of a city-region can be defined as the ratio of the number of employees in a region residing in other regions and total employment in a city-region (Patuelli et al., 2009).¹¹

Table 3.1: Orientation of Commuting and Four Types of Urban Systems