A road network is a large scale system with huge amounts of interdependent elements such as streets, junctions, traffic lights, traffic flows, and the like. Deep understanding of the nature of existing dependencies between the elements of the road network could offer decision makers and managers of different levels significant support in the transportation sphere. Thus, the Introduction chapter is devoted to a brief review on traffic theory development, and it illuminates important research directions in the field to be taken under additional careful investigation. The major attention is paid to problems concerning traffic assignment. The discussion tends to cover both practical and theoretical aspects. Essentially, the Introduction is committed to specify a general line of the book in a short way so as possible.