In logistics systems, there is a very clear trend towards balancing flows. The ideal becomes continuous replenishment or a single-piece flow system. The goals of such actions are obvious: reducing inventory, better utilization of handling equipment, etc. However, these aspirations have their limits. It is not always possible to centrally manage traffic throughout the entire logistics system, and it is not always worth eliminating every instance of waiting. To fully understand the issue of the cost-effectiveness of eliminating queues and downtime, one must refer to the concept of congestion. In doing so, one must set aside its commonly known meaning, in which it simply denotes overcrowding.
To strictly define congestion, it is necessary to determine the conditions for its occurrence in economic life. Congestion can be spoken of when:
• it is possible for such a number of buyers or users of a good (device) to appear at a given time that cannot be served simultaneously
• buyers or users of the good sacrifice certain resources (this can be time) to obtain it
• the amount of resources expended per unit of the good depends on the time of its acquisition or receipt.
• buyers or users of the good sacrifice certain resources (this can be time) to obtain it
• the amount of resources expended per unit of the good depends on the time of its acquisition or receipt.
It is, therefore, a universal phenomenon. It occurs in front of a coffee machine, in a store, on city streets, at airports, in production halls, etc., etc. It is always associated with the efficiency or throughput of the device to which the congestion applies. Observing queues, we ask questions: should we install a second machine, open another checkout, widen the street, increase the number of places where tools are issued in the production hall? The answer to these questions would be simple if not for the fact that congestion changes over time.
Excess capacity and excessive congestion
In systems such as: warehouses, terminals, ports, production halls, states of excess capacity and excessive congestion constantly intertwine. The period during which such cycles are observed can be a day, a week, a quarter, or a year.
The utilization distribution shown in Fig. 1 is most often associated with transport systems. Everyone can see that this is exactly the distribution of users of Św. Marcin Street in Poznań, or the A2 motorway. However, it must be remembered that this problem concerns most logistics systems.
The article comes from the magazine “Logistyka” 6/2006.