watch queues form when the critical gap is high
Four approaches feed a single roundabout with no traffic signals of any kind. Every car waiting to enter continuously evaluates the live gap in circulating traffic and only merges once that gap exceeds a driver-chosen critical threshold — precisely how real give-way roundabouts work, and a fundamentally different control mechanism from a fixed- or adaptive-timer intersection. Circulating cars always keep priority and follow an Intelligent-Driver-Model car-following law around the ring; entering cars queue and wait exactly as long as their yield decision keeps them there. Push the critical gap or arrival rate up and watch a specific leg's queue grow while the ring itself still has spare capacity — the classic signature of gap-acceptance-limited, not capacity-limited, congestion.