Relations between the Randstad urban area and spatial planning are complex. Spatial planning played a role in shaping the urban form of the Randstad, but in turn, the existence of a polycentric urban structure in the densely populated west of the Netherlands also shaped spatial planning. The implementation of urban growth centres peaked during the 1970s. Most of the growth centres were located in and around the Randstad area. Economic growth triggered migration from other parts of the country to the west of the Netherlands, and government feared that the population growth would result in uncontrolled urbanisation of the Randstad area. In 2006, the Netherlands Institute for Spatial Research published the results of an analysis of urban networks in the Randstad using indicators on the specialisation of urban regions, the relations between companies, commuting behaviour and shopping behaviour in the period 1992–2002.