Economies of agglomeration or agglomeration effects are cost savings arising from urban agglomeration, a major topic of urban economics. One aspect of agglomeration is that firms are often located near to each other. This concept relates to the idea of economies of scale and network effects. Diseconomies of agglomeration are the opposite. For example, spatially concentrated growth in automobile-oriented fields may create problems of crowding and traffic congestion. It is the tension between economies and diseconomies that allows cities to grow but keeps them from becoming too large.