ABSTRACT
The social worth of a road network is that it provides capacity to move from place to place so as to reduce the operating cost of movement. The stock of roads should be expanded while benefits measured in operating cost savings, exceed incremental capacity costs. Road networks have been built up over an historical time-scale and current decisions are made in terms of the balance of costs and benefits of additions to a massive existing structure. Suppose that additions to a road network can be taken to be independent of the existing system of roads and of each other in their outcomes. Methods of mathematical optimisation can be applied to compute the location and capacity of road improvements which will maximise some measure of social welfare, if some sacrifices of reality are made. Mathematical programming methods deal with discrete space and so the transport system must be represented as a graph of nodes and links.
