Dispatching refers to a rule used to select a vehicle to execute a transportation demand. This problem exists already as long as people or goods have to be transported from one destination to another by, for example, public transport systems, 694 I.F.A. Vis / European Journal of Operational Research 170 (2006) 677–709 trucks, trains and airlines. The dispatching problem can be observed from different points of view. Firstly, a load is available for transport and needs to be assigned to an idle AGV. Secondly, a vehicle becomes idle and need to be assigned to a new task. Consequently, the dispatching problem is divided into two categories, namely workcentre initiated dispatching and vehicle initiated dispatching rules

According to Egbelu and Tanchoco (1984) the following heuristic rules can be applied in decentralised control systems for workcentre initiated dispatching:

• random vehicle rule: pick-up task is randomly assigned to any available vehicle regardless of the location of the vehicle and the load,
• nearest vehicle rule: the vehicle at the shortest distance of the load is assigned to the load,
• farthest vehicle rule: the vehicle at the greatest distance of the load is assigned to the new transportation request,
• longest idle vehicle rule: the vehicle that has remained idle for the longest time among all idle vehicles is dispatched to the load,
• least utilised vehicle rule: the vehicle with the minimum mean utilisation is assigned to the new job.

Some assignment rules for the vehicle initiated dispatching problem are:
• random workcentre rule: a workcentre with a transportation requests is randomly chosen and the vehicle is dispatched to the load at this workcentre,
• shortest travel times/distance rule: the vehicle is dispatched to the workcentre closest by. The objective of this rule is to minimise empty travel times of vehicles,
• maximum outgoing queue size rule: the vehicle is dispatched to the centre with the largest number of loads awaiting transport,
• modified first-come-first-served rule: vehicles are assigned to centres, in chronological order to their time of request for transport. A workcentre can have only one request at this list at a time.

Autonomous Guided Vehicle Systems