UK commuting flows (minimum 15%)
2.4 Comparisons with Other (city) Datasets
Having established the TTWA-city dataset, it is instructive to compare characteristics with some of the other main city-area definitions described in Table 2.1, to get an idea of the scale of differences both in terms of levels and growth rates.
TTWAs versus Functional Urban Areas (FUAs) and Metropolitan Regions
Both FUAs and Metropolitan Regions are constructs of the European Commission-OECD, so it makes sense to group them together. As mentioned above, data on FUAs are
somewhat limited. At best what can be compared is population size for those cities which are common to both definitions. Figure 2.1 provides such a comparison.
40 Figure 2.1: TTWA vs FUA Population for 2014
Clearly there is a wide range of differences among the 63 areas which coincide43, with (in all but 18 cases) a tendency for the TTWA to contain a larger population than the FUA. This is not too surprising, as the TTWA will typically encompass a wider area due to its commuting definition. The more tightly bounded a TTWA is around an urban centre (or centres), through its inability to attract commuters from further distances (this could be due to lack of economic weight, lack of transport infrastructure, and/or limitations due to geographical location) the more likely it is to resemble the functional urban area.
Metropolitan Regions are a broader definition of the Metropolitan Areas mentioned previously, namely they are urban agglomerations with more than 250,000 inhabitants and are constructed from NUTS3 regions44. As they are built from administrative building blocks, more economic data are available covering output (GVA), employment, population and other indicators45. Using this definition, there are 37 metropolitan regions for the UK, 35 of which correspond to the selected TTWA areas46. Figures 2.2 - 2.4 show some comparison across a range of different indicators.
43 For the purpose of comparison, the Warrington and Wigan FUAs have been added together to better match the corresponding TTWA.
44 Adjacent regions are added to the agglomeration if more than 50% of the population are included within its area.
45 See http://ec.europa.eu/eurostat/web/metropolitan-regions for more detail.
46 The TTWAs of Blackburn, Blackpool and Preston are combined to equate to the equivalent metro region. In addition, the West Midlands metro region is equated to the Birmingham TTWA. Depending on the indicator of comparison, not all the metropolitan regions are available from Eurostat.
41
Figure 2.2: TTWA vs Metro Region Population for 201147
Similar boundary issues are likely to be present for metro regions, although given the less-well defined spatial building block of NUTS3 regions it would be expected that this is less marked than for the FUAs. This seems to be broadly the case, and there are a greater proportion of areas where the TTWA has a lower population than the metro region.
Figure 2.3: TTWA vs Metro Region Population Density for 2011
47 2011 is chosen as the period of comparison for cross-sections because this is the base year of the TTWA construction and is therefore the most accurate year available.
42
Figure 2.4: TTWA vs Metro Region Employment Growth 2000-15
When a similar comparison is made for population density around half of the areas are within +/-30%, as shown in Figure 2.3. However, there are some notable outliers, particularly Newcastle and Doncaster. A marked deviation would be a sign that, in the case of Newcastle, a more rural hinterland is included in the definition of the metro region – which would seem to be the case as the total population ratio is not that large. For employment growth, there is a reasonable correspondence (a correlation of 0.43) although there are some outliers, as shown in Figure 2.4.
TTWAs versus Primary Urban Areas (PUAs)
Another useful comparison can be made between the TTWA city data and those for PUAs, particularly as the latter are often seen as the de-facto definition of cities in the UK and popularised by the Centre for Cities in their series of Cities Outlook reports48. As mentioned
48 See, for example, https://www.centreforcities.org/publication/cities-outlook-2019/.
43
previously, the PUA definition starts from the basis of the built-up area of a city, which is then approximated by the closest fit of LAD boundaries. There are 63 PUAs, representing the UK’s largest cities and towns. Of the 63 PUAs, 57 correspond to TTWA area names, allowing a reasonable sample for comparative purposes49.
Figure 2.5 shows the extent of difference between the two spatial definitions by ranking the ratios for population and population density50. Clearly there are a few areas (Slough and Heathrow being the extreme case) where the TTWA population vastly exceeds that of the PUA. In general, TTWA population is greater than that of the corresponding PUA – in only four cases (London, Huddersfield, Bournemouth and Birmingham) is the ratio below unity. In contrast, the population density of a TTWA is in general lower than that for the same PUA – in only three cases is it higher (Warrington and Wigan, Southend, and Barnsley). The findings confirm what would be expected – the TTWA is a broader area allowing for commuter zones, whereas the PUA focuses on approximating the urban zone as much as possible.
Figure 2.5: TTWA vs PUA Population Total and Density
In addition to looking at population, productivity differences between the TTWAs and PUA were also examined. These were based on my own calculations (i.e. using GVA and employment data and appropriate LAD combinations) because the data in the Cities Outlook
49 The Wigan and Warrington PUAs are combined together to allow comparison to the Wigan and Warrington TTWA.
50 The rankings do not necessarily represent the same area.
44
website is only in current prices, whereas the TTWA data are calculated in constant prices.
As the TTWA areas are typically larger and less dense, one might expect productivity levels to be lower in TTWAs when compared to PUAs. This is generally found to be the case, with approximately two thirds of PUAs having a productivity level higher than the equivalent TTWA. However, these productivity differences are not particularly large, with the range of the ratio from 1.06 to 0.81. In addition, there is a negative correspondence (correlation -0.48) between the ratios for productivity and for population, i.e. those TTWAs which are larger than the equivalent PUA also tend to have a lower productivity level (although the difference is not that great).