Multi-Period Routing in Hybrid Courier Operations Table 3. Alternative methods for allocating seeds Method M1 M2 M3 Description Allocation of seeds is performed by inserting the seeds in the first available route starting from period 2, i.e. a seed that expires in period 3 can be inserted in any available route of period 2 to 3. Allocation of seeds is allowed considering only the routes belonging to the expiring period of each seed, i.e. a seed that expires in period 3 can be inserted as a seed only in third period's routes. Method M3 differentiates from the M1 and M2 methods by allocating all available flexible customers in their respective expiration periods. Thus, steps 2a, 2b and 2c are not relevant in this case, but the procedure of Phase 1 is used for periods 2,...,P. ing routes one-by-one with a best insertion first technique, i.e. the customer that results to the lowest increase of the total routing cost is in- serted in the relevant route and the procedure repeats for the other remaining customers. It is clear that this is a greedy procedure that leads to a suboptimal solution. Phase 3 attempts to address the greedy aspect of steps 1 and 2c by improving the initial solution. This is performed by service call TEST RESULTS: ALLOCATION OF FLEXIBLE CALLS In order to test the effectiveness of the flexible call allocation method, we compared it against a simple scheduling procedure (S1) used in actual operations of courier companies. Typically, such procedures focus on one period at a time and allocate the flexible calls in the most effective manner, while ensuring that all expiring calls are allocated. Thus, S1 examines the routes of the