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queueing theory. When we look at the math, we can see why the single most important factor that affects queue size is capacity utilization. The reason we care about queue size is because the bigger the queue, the longer things take.

Average Cycle Time = Average Work In Progress / Average Throughput If you have ever seen Little's Law before, you have probably seen it in the form of the above equation. What few Agile practitioners realize, however, is that Little's Law was originally stated in a slightly different form: Average Items In Queue = Average Arrival Rate * Average Wait Time

As we move from 60-80% utilization, the queue doubles. As we move from 80-90% utilization, the queue doubles again. And again from 90-95%.3 Once we get past 80% utilization, the queue size begins to increase almost exponentially, slowing things down to a grinding halt as it pushes 100% capacity utilization.