ABSTRACT
Here we extend the work described in Chapter 3 and consider more fully the effects of interventions in the system described by the model. In particular, we will consider the rather important example of the effects of lifting trade barriers. This problem of course bears some resemblance to the question of reducing transportation costs dramatically, or building a bridge or tunnel to link two previously separate regions. The point is that two previously stable urban hierarchies of centres are suddenly brought into contact with each other, and the resulting changes are clearly difficult to anticipate.
