is minimized, where dij is the length of link ij, and4 takes on the value of 1 if a link directly connecting i and j exists but is otherwise zero. Very simple algorithms have been developed for generating the MST of a set of points in both Euclidean and non-Euclidean space, and we consider their use again in Section 14.3 The importance of the MST is that (i) it is the simplest network configuration such that all nodes are linked and (ii) it provides a lower bound on total network length (i.e. all other connecting networks must have a length greater than that of the MST. Figure 3.15 shows the MSTjoining a system of cities in southern Ontario and Quebec. This was used by MacKinnon and Hodgson (1969) in a study of the relative efficiency of the multi-lane highway system connecting the 32 largest urban centres in that region. Their models successively added links to the MST, so as to maximize the total flow in the network (estimated by gravity models) within the constraints of the existing highway mileage 0. The model