This paper contributes to a theoretical basis for studying methods of transport network aggregation, and investigates the effects of various ad hoc aggregation procedures. A mathematical model of network aggregation is formulated. Some details are given of existing methodologies for aggregating networks, including link abstraction and reduction abstraction. The form of cost-flow function, required for consistent aggregation, is considered. Exact results are obtained for some simple network forms, including parallel and series connections between a single origin and destination. For more complex network structures, there are considerable mathematical difficulties in obtaining analytical results, but some general trends are indicated. Results are then presented for two examples: (1) an artificial circular city model; and (2) a model and its aggregates for a particular region of the city of Leicester, England. It is found that aggregate models of transport networks can usefully reduce both conceptual and computational complexity, although they cannot fully reproduce the behaviour of more detailed models. Thus they should be used with care, especially in relation to practical or policy interest in detailed spatial issues.
Samenvatting