One queue adds an overhead for the time it takes for the person at the front of the queue to notice a cashier is free and walk to that cashier. I have watched this system in action at the Post Office, Rail stations, and one supermarket and this overhead can be significant. One supermarket actually employs somebody at the head of the queue just to tell them which cashier to go to and when.
jfb - It also doesn't have any negative effect on "the geegaws sold in line" as they just put the "geegaw" displays along the side of the one queue. In fact it is better for the store in that respect as they can show more variety.
The best way I can think of to get rid of this overhead is to have a rule where each cashier has a queue depth 2 that is fed from a single queue. But how would you get people to stick to just 2 people? And it would be very annoying to be just behind a slow person.
In theory people should naturaly start moving forward when they notice that somebody is finishing up but at least in the Rail station and Post office the counter staff have a tendency to shut their counter without warning or start processing some other task before they are ready to serve the next person.
One queue adds an overhead for the time it takes for the person at the front of the queue to notice a cashier is free and walk to that cashier. I have watched this system in action at the Post Office, Rail stations, and one supermarket and this overhead can be significant. One supermarket actually employs somebody at the head of the queue just to tell them which cashier to go to and when.
Whole Foods in NYC has solved this problem by having one queue, but when you get towards the front, there are five separate, color-coded lines. At the front of the queue is a giant television that shows a bar of color for each lane. When a register opens up, the number for that register slides into the color bar for the next person in the sequence, and a voice announces "Register 12". It's fairly efficient at keeping the queue moving as well as managing people's psychological need for multiple lines. It also prevents cashiers from wasting time, because as soon as a transaction is finished, their register is automatically assigned to the next person in the queue. The only "wasted" time is the time walking from queue to register, and pleasantries.
I've seen the two level queue in use at some airports for security and immigration. But they do have a person splitting the head of the long queue into the short queues immediately in front of each x-ray machine/agent.
It works pretty well, but can still be a little frustrating when you see a person who was behind you in the main queue get through first because your sub-queue took longer.
People usually won't act to optimize for the best throughput, just minimize their relative wait times. The behaviour of drivers on congested roads tends to confirm this.
jfb - It also doesn't have any negative effect on "the geegaws sold in line" as they just put the "geegaw" displays along the side of the one queue. In fact it is better for the store in that respect as they can show more variety.
The best way I can think of to get rid of this overhead is to have a rule where each cashier has a queue depth 2 that is fed from a single queue. But how would you get people to stick to just 2 people? And it would be very annoying to be just behind a slow person.
In theory people should naturaly start moving forward when they notice that somebody is finishing up but at least in the Rail station and Post office the counter staff have a tendency to shut their counter without warning or start processing some other task before they are ready to serve the next person.