The similarity between this conditional cost function and the one obtained for the two
nodes system (equation 12) is evident. First, we obtain again the distance-flow term with
the same meaning, namely the capacity of the system, as the flows involved are those that
define frequency. Second, the pure flow term is generated by loading/unloading activities.
Finally, note that the obtained function reduces to that of the two nodes system if the four
new flows are set to zero and d23+d31 is defined as d21.