The queue length refers to the length of a line of vehicles, bicycles, or persons waiting to be served by the system in which the flow rate from the front of the queue determines the average speed within the queue. Slowly moving vehicles or people joining the rear of the queue are usually considered part of the queue. The internal queue dynamics can involve starts and stops. A faster-moving line of vehicles is often referred to as a moving queue or a platoon.

Queue lengths are often reported in probabilistic terms; thus, the 95th-percentile queue is that queue length that is not expected, on a statistical basis, to be exceeded more than 5 percent of the analysis time period. Similarly, an average queue is the statistical average queue length that is expected to occur over the course of the analysis time period.