In urban planning, as in architecture, there has traditionally been a widespread use of the 'black box' approach to problem solving. Indeed, the origins of work in both of these fields has rested upon the ability of a

the high cost of. .. errors, particularly in the case of complex urban systems, is a strong incentive to externalize design thought, for only in this way can it be subjected to criticism and such testing as can be carried out, before expensive mistakes are made. The rapid search for new problem solving and design methods in all applied fields is a consequence of the realisation that problems have grown too big and complex to be left to the private judgement of even the most experienced designer. (Jones, 1970)

One of the consequences of the search for more systematic approaches to urban planning has been that planners have become increasingly aware of the inherent uncertainties in their task (Lynch, 1972). The problem has been put succinctly by Wedgewood-Oppenheim (1972). Plan makers are beset by the problems of uncertainty. These arise not only because they are concerned with the future which is difficult to control or to forecast, but because there is much in the present which they do not know or understand, which affects the success and the viability of their plans for the future.