# One or Two? How Many Queues? DevFeed: [One or Two? How Many Queues?]() Original publisher: [Read original article]() Author: Marc Brooker Published: 2025-03-25T00:00:00Z Content type: article Language: en Sources: [Marc Brooker's Blog](), [Marc Brooker's Blog]() Topics: [systems]() ## AI overview The article examines when one queue is better than two using supermarket checkouts and restroom facilities as contrasting examples. It argues that precommitment is the crucial difference: a shared queue works well when customers can use any available service, but separate queues work better when people require different facility types. It also considers wait time and utilization as utilization approaches 100%. ## Source excerpt One or Two? How Many Queues? Very applied queue theory. There's a well-known rule of thumb that one queue is better than two. When you've got people waiting to check out at the supermarket, having a single shared queue improves utilization and reduces wait times. The reason for this is pretty simple: it avoids the case where somebody is waiting in a queue while there's a checker available to do the work. It also saves the sanity of the person standing behind a cheque writer or expired coupon negotiator. But one queue isn't always better than two. The other day, I was up at Crystal Mountain, and needed to visit the ahem facilities. In the base lodge there, the men's room includes a bank of urinals, and a row of stalls. As men's rooms often do. What it also features is a long queue at rush hour, and a sharp corner before its possible to see which of the facilities are occupied. The result is a queue with a mix of folks waiting for urinals and stalls, and often under-utilization of the urinals (because the person at the head of the queue is waiting on a stall). Single queue, lower utilization, longer waits. This situation got me thinking1: what's the crucial difference between these two cases? And are their bathroom usage patterns where one queue makes sense? As far as I can tell, the difference between the "two is better" and "one is better" situation is precommitment. Supermarket checkouts are fungible, and so work can be allocated to any one of them. There's no need to decide which you want to visit before joining the queue. When visiting the WC, on the other hand, one tends to have a strong opinion about what sort of facility is desired, up front. Somebody wanting to visit the urinal would likely take a stall, but the vice versa is seldom desirable. Does One Queue Ever Make Sense? Are there cases where one queue still makes sense, even in this precommitted world? Not that I could find. First, let's look at the results of wait time versus utilization (utilization $\