In a previous study (Portnov and Erell, 1998a), an Index of Clustering was defined, which allowed an analysis of the combined effect of spatial isolation and remoteness of peripheral towns on the long-term patterns of their population growth. In the present paper, the analysis of the effect of clustering of the urban field on the patterns of population growth is extended to centrally located urban places, and the validity of this index is tested in two unevenly populated countries--Israel and Norway. In both countries, the effect of clustering of the urban field on the patterns of urban growth is twofold. In sparsely populated areas, the presence of neighboring towns appears to increase their chances of attracting potential migrants due to inter-urban exchanges, while in more densely populated areas, increasing clustering tends to reduce migration influx to a given locality due to inter-town competition. Following this conclusion, a strategy of "redirecting priorities" leading to the formation of urban clusters is proposed, which may enhance the potential of urban growth in geographical areas where this is desirable.