ABSTRACT
In other situations, flow researchers may need explicitly to recognize the existence of the channels (such as river systems, rail lines or ocean trade routes) that flows must follow in their path from origin to destination. Analysis of these constrained flows poses many special problems and their examination is, to date, found mostly in the urban traffic literature. In both the unconstrained and constrained cases, beyond a simple binary 'flowno flow' statement of linkages, the measurement of flow magnitudes involves a variety of approaches; most often these measurements are based upon value or quantity (weight, number of carloads, etc.).
