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consolidated. The need to provide theoretical cover for this situation has encouraged a new generation of polycentric theoretical models within the new urban economics (NUE). Also, central place theory and its hierarchised areas AbstractThe polycentric models of the new urban economics (NUE) predict that population density decreases while increasing the distance to employment centres. In contrast with this, some studies have calculated non-signifi cant gradients or even positive ones, which appear to threaten seriously the usefulness of these theoretical models. Does this mean that this theoretical framework should be given up in order to understand the decision-making processes of the actors in a polycentric city and their cumulative effects on urban structure? Or, rather, is it a matter of overcoming problems with the appropriate estimating techniques? This study has tested the effect of decentralised and integrated sub-centres in the Barcelona metropolitan region on population density in 1991 and 2001. From the preliminary results, it is clear that population density increases with distance in a considerable number of the sub-centres that have sprung up as employment has decentralised. It has been detected that this result is due not so much to the higher value of more distant residential land compared with that nearer the employment sub-centres, but to defi ciencies in the econometric model used. The problem is that the sub-centres belonging to this group are very close together. Once this is resolved, it is demonstrated that, although distance has less effect on decentralised sub-centres than integrated ones, in both cases the effect is negative; that is, when distance increases, population density is reduced. Therefore, the results obtained are not clearly contrary to the predictions of the theoretical models.

JEL classification: R R2 R3 R4 Keywords:Urban spatial structure Suburbanization Transportation infrastructure a b s t r a c t I investigate the effect of improvements to the transportation infrastructure on changes in location patterns of population in Barcelona, Spain between 1991 and 2006. At a census tract level, I verify and extend the finding of Baum-Snow (2007a) that transportation improvements cause suburbanization: (1) improvements to the highway and railroad systems foster population growth in suburban areas; (2) the transit system also affects the location of population inside the central business district (CBD). To estimate the causal relationship between the growth of population (density) and transportation improvements, I rely on an instrumental variables estimation that uses distances to the nearest Roman road, the nearest 19th century main road, and the nearest 19th century railroad network as instruments for the 1991-2001 changes in distance to the nearest highway ramp and the distance to the nearest railroad station.

At the present time, most large cities in the world are polycentric and, at the same time, they are undergoing processes of employment decentralisation and deconcentration. It has been argued that polycentricity is just an intermediate stage between monocentricity and a more unstructured, chaotic and amorphous location model, scatteration. For the case of the polycentric Barcelona, the aims of this study are to test: whether its employment is moving from polycentricity to scatteration; and, whether its employment location model is increasingly random and unstructured. The results show that, in spite of the decentralisation and deconcentration processes, employment concentrated in centres still represents a significant percentage of total employment and new sub-centres have emerged in the periphery. What is more, the results also show an increasing influence of employment sub-centres on employment location and density conditions. As a result, polycentricity has been reinforced.

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