Let us move to a second possible route structure, the one known as hub-and-spoke, very
common in air transport. A hub is a node that collects and distributes all flows, and usually
coincides with one that is origin and destination. Let us assume arbitrarily that the hub is in
node 2, and that only one fleet (i.e. one frequency) operates. Obviously, other hub-andspoke
structures could be considered as well, like a two fleet operation, one in 1–2 and the
other in 2–3. We have chosen to develop the one fleet structure because the others can be
constructed adequately using the two nodes system, as shown below. With our assumption
(see figure 4) a vehicle loads flows y12 and y13 in node 1, unloads y12 in 2 and loads y23,
then unloads y13 and y23 in 3, loading y32 and y31, goes back to 2 to unload y32 and load y21 in
order to go back to 1 to unload and begin the cycle again.