Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A transportation network is a type of directed, weighted graph or network. Transportation networks are used to model the flow of commodity, information, or traffic (see transport network). Flows represent commodity flowing along arcs from the source to the sink. The amount of flow along each arc may not exceed the arc''s capacity, and none of the commodity may be ''lost'' along the way (that is, the total flow out of the source must equal the total flow into the sink).