Queue Time is the specific portion of waiting time that a unit spends in a queue in front of a workstation, waiting for its turn to be processed. It is often the single largest component of lead time and one of the most directly addressable through batch size reduction and pull scheduling.
Queues form whenever the arrival rate at a station exceeds the processing rate. This happens for many reasons: upstream batch releases, unbalanced cycle times, bottleneck effects, arrival variability. The queue absorbs the mismatch, at the cost of lead time, working capital and hidden quality problems.
The mathematics of queues is Little's Law: average queue length = average arrival rate × average queue time. Rearranged: queue time = queue length / arrival rate. This means the fastest way to reduce queue time is to reduce queue length, by pulling less work into the system, by limiting WIP, by producing in smaller batches.
Pull systems with Kanban naturally limit WIP and therefore queue length. Small batches shrink the queue-length spike from each release. Balanced cycle times prevent any single station from becoming a chronic queue location. Cellular layouts remove entire between-department queues by putting sequential stations next to each other.
Queue time is where lead time actually hides. Attacking it, through pull, small batches, WIP limits and cellular flow, is one of the most direct paths to shorter lead time and lower inventory. Every hour removed from queue time is an hour returned to the customer's response time.
