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> That's only true if the long queues are caused by some people having a difficult time at the booth. But that problem mostly goes away as soon as you have two

> That's only true if the long queues are caused by some people having a difficult time at the booth. But that problem mostly goes away as soon as you have two booths. It doesn't at all. Heathrow is the main airport discussed in this article. They had several booths the last time I went through, and it still took nearly two hours to get through the line.

thaumasiotes 8y ago
It's a result from queueing theory. Think of the queue of incoming work (such as immigrants to be processed) as the amount of work each takes on average plus some statistic representing a really large amount of time that most jobs will not require, but some will. First off, to prevent long waits, you need enough capacity to do the work at the average rate at which it comes in. If your bank gets an average of 20 customers an hour, you need to be able to process 20 customers per hour, or queues will not only form but grow steadily longer. But, even if you have a single worker doing work at exactly the average incoming rate, any outlier jobs that take more than the average amount of work (one unfortunate customer that takes the time of four ordinary ones) will cause the formation of a queue, because the only available worker is occupied. And any queues that do form will be permanent, because incoming jobs fill the queue just as quickly as your worker clears it. These queues have no good reason to exist -- you're doing the work just as fast as it arrives -- but they can't clear. A second worker will solve this problem almost entirely (on the assumption that outliers are rare) -- the expected queueing time from a single worker who is expected to process 10 customers an hour is vastly larger than the expected queueing time from two workers who are expected to process 5 customers an hour each. The improvement comes from the fact that while an outlier is jamming one worker, the queue behind that outlier can still be cleared by the other worker. Airport immigration doesn't work this way; they have short periods in which hundreds of people arrive simultaneously and long periods in which nobody arrives at all. So they provide some amount of staff and expect them to gradually clear the queues that form instantly whenever an airplane lands. They don't expect the entire airplane to be cleared simultaneously even though everyone on it arrived simultaneously. In this case, the queue time is just a function of how much work there is to do, and how much staff is available to do it, and your two-hour queue time is a problem of low staffing levels, not poor queue design.

thaumasiotes 8y ago

It's a result from queueing theory. Think of the queue of incoming work (such as immigrants to be processed) as the amount of work each takes on average plus some statistic representing a really large amount of time that most jobs will not require, but some will. First off, to prevent long waits, you need enough capacity to do the work at the average rate at which it comes in. If your bank gets an average of 20 customers an hour, you need to be able to process 20 customers per hour, or queues will not only form but grow steadily longer. But, even if you have a single worker doing work at exactly the average incoming rate, any outlier jobs that take more than the average amount of work (one unfortunate customer that takes the time of four ordinary ones) will cause the formation of a queue, because the only available worker is occupied. And any queues that do form will be permanent, because incoming jobs fill the queue just as quickly as your worker clears it. These queues have no good reason to exist -- you're doing the work just as fast as it arrives -- but they can't clear. A second worker will solve this problem almost entirely (on the assumption that outliers are rare) -- the expected queueing time from a single worker who is expected to process 10 customers an hour is vastly larger than the expected queueing time from two workers who are expected to process 5 customers an hour each. The improvement comes from the fact that while an outlier is jamming one worker, the queue behind that outlier can still be cleared by the other worker. Airport immigration doesn't work this way; they have short periods in which hundreds of people arrive simultaneously and long periods in which nobody arrives at all. So they provide some amount of staff and expect them to gradually clear the queues that form instantly whenever an airplane lands. They don't expect the entire airplane to be cleared simultaneously even though everyone on it arrived simultaneously. In this case, the queue time is just a function of how much work there is to do, and how much staff is available to do it, and your two-hour queue time is a problem of low staffing levels, not poor queue design.