Vehicle routing problem in real case application

The delivery of goods from a warehouse to local customers is important and practical problem of supply chain management. Current customer demand changes from day to day. If the total demand is greater than the whole capacity of the internal fleet, the use of the external carrier becomes necessary. Thus; it may be more economical to use an external carrier instead of an internal vehicle to serve one or very few customers. In this work we attempt to model the company problem. We first try to assign vehicles to warehouse and determine the private fleet size needed at each one by taking into account the interventions of preventive maintenance. Then we develop a second model that routes this limited truck from two central warehouses to customers' with the option of using an external carrier.