Queue Time

Queue time is a very specific form of waiting, one of the traditional seven wastes. It occurs when a person or item is in line behind something else and is waiting for the same resource.

Queue time occurs in two basic flavors. There is the time spent by the customer waiting to get into your value stream (i.e. the customer lined up at a fast food counter or on hold.) The other form of queuing occurs internal to the value stream. In this case, it can be people waiting to use a resource, orders backed up in a system, or packages stacked up waiting to be shipped.

Queue time is most commonly a result of overproduction, and in many cases accounts for the majority of the lead time in a value stream. The costs associated with excessive queuing are high. The people or items in line require space. If the line is long enough orders can change, parts can get lost, stolen, or go obsolete, and the queue might even need special management processes.

Queues tend to form around bottleneck processes, ones that constrain the capacity of the whole system, and are a great indicator of an improvement opportunity.

Batch processes also cause a form of queue time. Parts have to wait for a complete batch to go into the process, and then when they come out, they are queued up to go into the next process. Think of how a ferry system works. All the cars have to get in line to get onto the ship. Then they all load at once. When the boat docks at the other side, they are immediately queued up for the next process, namely driving off the boat.

Eliminating queues is one of the best ways to reduce lead time. Look for the root cause of the line, and attempt to remove it. Batch-induced queues are most commonly resolved by lowering setup times or switching to dedicated, in-line, right-sized pieces of equipment. Bottleneck queues are capacity related and are handled by either adding more capacity, or improving the process to be able to handle more throughput.