Why Line Sorters?

In airports one often finds oneself standing in a central line that splits into many smaller lines.  For example, a single long line for security screening splits near its end into many small lines in front of each screening machine.

How should individuals in the central line be allocated to the many small lines? One obvious solution is to let individuals in the single long line choose the smaller line in which to continue.  Some people might choose badly, or be stressed by having to make a choice, but overall this approach requires little supervision, and competition between choosers should make lines roughly equally bothersome.  All else equal, if line A was consistently faster or more pleasant than line B, line A would attract more people until the added length on average cancelled A’s advantage.

Many airports assign employees to the task of sorting people into lines.  In some places the line sorter always tells people where to go, while in other places the line sorter usually lets people choose, and only sometimes intervenes to tell people where to go.  As far as I can tell, line sorters are never purely advisory; either then let you choose without offering advice, or they use their authority to command compliance.

On a recent trip, I had several occasions to observe myself being sorted from a single central line into a particular line, and then to track how long it would have taken me had I been free to choose a line for myself.  It seems to me that on average the lines I would have chosen completed faster than the lines to which I was assigned.  Now I didn’t collect formal data, and I could just be responding here to random luck, but I suspect not, because a theory occurred to me that would explain this pattern.

My theory: lines vary in speed due to variations in personnel, but line sorters prefer lines to take similar amounts of space.  Some employees just take more time when, for example, working a scanning machine.  When people notice that lines vary in speed, they naturally prefer the faster line, which will make that line longer in terms of the space it uses.  Line sorters then “correct” this imbalance by sending people to the spatially-shorter but temporally-longer line.  Sorted folks then regret being sorted, knowing that on average their trip would be faster if they could choose for themselves.

Now if there is limited space for final lines, it might make sense to assign someone to limit how fast people can move from the main line into final lines. I suspect, however, that informal custom would usually work fine – when the best final line got full, the main line would usually just stop and to wait for room in that best line.  If space were limited in the main line it could then make sense to assign someone to make sure all available space in final lines was used. But this should be pretty rare – it would usually be far cheaper to just have more space for the main line.

I suspect that what is really going on here is that orgs who manage lines are embarrassed by variations in personnel productivity.  It seems unseemly to egalitarian ex-foragers to let line-walkers publicly endorse some personnel as faster.  They’d rather pay to try to “hide” this variation, even though most folks in line will probably notice it anyway.

Another explanation is that variations in line length seem “chaotic,” suggesting that the line-managing org does not fully control the situation.  So “security theater” requires the illusion of control, via equal length lines.  This would better explain systematic differences between security and non-security lines. If true, this seems an interesting contrast to the “autonomy” literature, such as in medicine, where many say we should let people make decisions for themselves even when this hurts them a bit on average, so that they gain the benefits of feeling in control of their lives.

Added 1p: If some org personnel might conspire with folks in line, the org might reasonably want a random matching of folks with final lines. This doesn’t explain cases where folks can usually pick their line, but are assigned to lines when line lengths are especially mismatched.

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  • Zdeno

    Related: Always choose the grocery store line cashiered by the 16-year old girl, rather than the octogenarian or the heavily-pierced 25 year old guy.

    • adam

      My related theory is for ice cream parlors: if the server is of the opposite sex from you, you’ll get more ice cream than if they’re the same sex. It doesn’t seem to matter if it’s soft or hard ice cream.

    • Because they are prettier to look at?

      Those are often the slowest lines since the 16 year old girl hasn’t been at the store long enough to know the various produce codes and ends up having to look crap up or call for price checks.

  • my theory : I am guessing you travel mostly in the US ? I have frequently noticed that US airports employ extraordinarily large numbers of line sorters – a job that seems not to even exist in other countries, on in other environments (you don’t normally see line sorters at supermarket checkouts for intance). Perhaps it is a union thing?

    Anyway the line sorters need to do something….. so obviously they direct people a lot. My guess is that – on average – they do no better than random.

    It’s fruitless work – but the upside is they they provide some random data from which bored academics can identify false patterns.

    • gwern

      Thinking about it, I’m not sure line sorters *can* do anything other than change the variance in line-waiting-time.

      Thinking about it, it seems to me that moving people around doesn’t affect the bottle-neck: the head of the line. If we have 3 scanning stations, and each one is running flatout, then rearranging people before they get to the 3 stations is just rearranging deck chairs on the Titanic. Unless 1 of the stations at some point runs out of people to process, and idles, then they run equally fast no mater how many people are sorted and re-sorted. (It will take you the same amount of time to cut your grass if you go front-back or back-front, so long as you don’t stop or repeat sections.)

  • rosyatrandom

    Heh, I had assumed this topic would be about the speed of the people _in_ the lines themselves, e.g. basket-only queues in supermarkets. I have often thought other places (banks, post offices, etc.) would benefit from similar ‘express’ queues, as when you know you’ll be done in under a minute it is bloody annoying to wait behind a bunch of people with massive amounts to get done.

  • Some other possibilities:
    1. Your epiphany hasn’t occured to the line-sorting institutions. Perhaps all they need to do is instruct and remind line-sorters to factor line speed in to their sorting.
    2. Possibly, without conspiracy or coordination, when we position ourselves we position to maximize personal advantage, but when we position others (and maybe just objects in general) and have no personal stake we position for symmetrical aesthetics.

    Line sorting looks like a fun and easy experimental psych/econ project, with real world implications. I bet Prof. Ariely wishes he thought about experimenting with this, if he hasn’t done so already.

  • octavio

    I wonder why this is not performed like a bank queue.

    You have your central line. Users line up first in, first out.

    When you get to the front of the line, you wait until one of the scanning stations is ready for you (a light goes on, or a board indicates which screening lane to take, the guy standing there tells you, or the scanner shouts, next. You get the idea).

    Slow users and scanners do not hold up a line. Faster scanners can take more users. If a slow scanning line opens up, your scan may take a bit longer (assuming it’s the scanner’s fault their line is slow, as opposed to standing behind a group of people who wish to say, assert their rights), but you would be through in less time than if you were placed in a slow line.

    It’s worth noting that the US customs queue (when travelling to the USA) in most airports (I have been to) in Canada already do this, so it’s odd for me to understand why security does not as well. Some grocery stores in Europe do this as well, as do the “self checkout” lanes at North American stores (everyone lines up, and when a machine becomes available, you go to it).

    • Because the lines move faster when you put your laptop, shoes, etc.. in the bins before actually reaching the front of the line. If people had to select the screener at the last minute people would have to lug all their bins over to the screener and delay the process.

  • Sean C.

    I think it is largely an image thing. Examples of places that do fine without them:

    – Banks
    – Toll booths and car border crossings
    – Traffic merging in general

    Examples of places where they add value:

    – Ski hills. The line sorter groups people to make sure the chairs are all full. And preserves an open space in front of the lift so that people have room to maneuver more quickly

    – Restaurants. The individual queues for each waiter’s section of tables aren’t visible to the customer. So the hostess directs people appropriately

  • azmyth

    I can’t think of a good example outside airports, where the marginal cost of a line sorter is practically zero. You’re gonna have a bunch of security guards standing around doing nothing anyway, why not put them to work doing something that has a small but positive benefit, if only to enhance the “security theater” atmosphere.

  • Indy

    Compare civilian groceries with military commissaries. At civilian grocery stores, there is no “primary line” that divides into cashier lines. Individuals merely arrive when they are done shopping and sort themselves mostly according to speed, but also by space – if a particularly fast lane has a long line that back up behind the cashier area and into the store area, people will not usually pick it even if it were still faster overall.

    And to accommodate the demand for speed, these stores have also created special “express” lines based on number of items – a rough proxy for duration, and one that is easily, instantly visually verifiable for serious abuse. There are also self-checkout lanes everywhere now which are increasingly popular.

    Oh, if only there were some similar express services for airports. I’ve heard of a few “first class” lines in New York, but I’ve never seen them at the airports I frequent.

    Anyway, at military commissaries – there is, usually, a single, long, serpentine, primary line. The head of that line is barred from progressing to the cashier lines by a person or automated display unit. Whenever a cashier is done, she hits a bit “done!” button on her console, and the line nazi directs you to that lane (or the automated display unit says “next please!” and lights up the number of the lane that is now available.) They also have express and self-checkout lanes.

    I’ve found the commissary process to be very efficient. The cashiers are kept working at their constant individual rates, you wait the minimum amount of time possible in the primary line (where you spend the bulk of your time) and even a noticeable variance in individual cashier speeds seems not to matter very much in the “total time to completion” function.

    I think a similar process could work for airports in a kind of “one out, one more in” process. It automatically equalizes wait-times.

  • JS Allen

    I’ve experimented with this a lot at airports. I can almost always get the line I want, despite the presence of line sorters. I think the line sorters are just there to give some structure to the indecisive and anxious, to stop them from slowing things down

  • Robert Koslover

    Of some relevance may be the fact that in some airports, not all the scanning equipment is identical. In particular, you may have noticed that there may be an expensive millimeter-wave type scanner for just one line, and regular metal detectors for the others. Since they can’t run everybody through the mm-wave scanner, they may pick people randomly (rather than let passengers self-select for those). When given a choice, I generally prefer to avoid the mm-wave scanner lines, since they appear to go slower. But sometimes one isn’t given a choice.

  • Unnamed

    Does the airport care whether you go to the fastest line? They want to maximize throughput, which means keeping each screening station in use and not letting any line get too short. They also want to keep things orderly, which means not letting any line get too long. They don’t care if you wait a little longer and the next guy waits a little less.

    The people who assign you to lines don’t need to be better on average at picking a quicker line, they just need to be better at avoiding the extreme cases of lines that get too short or too long.

  • RJB

    The only places I can think of with line sorters are those in which space is important, as well as time. At an amusement part or at the ski lift (as a commenter above noted), a line sorter makes sure every spot is filled. In airports, crowdedness can be a security concern. I wouldn’t be surprised if TSA sorters are explicitly told to maintain clear line boundaries and uniform line densities for security reasons, with speed of processing a decided (if not distant) second place.

  • conchis

    To those advocating the “single main line / no smaller lines approach:

    This process increases average waiting times, because the time taken to get from the head of the main line to the cashier is wasted. When individuals are forced to join the smaller lines earlier, this time is not wasted, but instead is spent attending to the customer at the cashier.

    I would venture that the reason for line sorters is not so much to optimally allocate individuals between small lines, as to ensure they join small lines at all (rather than everyone waiting for a cashier to come free before moving, which would be optimal for risk averse individuals, provided there is sufficient random variation in line speeds.)

    • Khoth

      Some airports have a hybrid system which addresses this:

      There’s a single main line for most of the queue, but near the scanners it’s split into multiple lines each of which take about half a dozen people.That way you get the benefit of the single line for most of the queue, but you don’t get the inefficiency you describe.

  • My theory:

    Line sorting is not the type of profession one takes pride in and therefore the employees simply don’t care about getting individual people through the line faster. Further, to a boss who is just quickly glancing at the line, a uniform length (but non-speed optimized) line means that the sorter is doing their job adequately enough to not cause them to get yelled at/fired.

    Further thoughts:
    Any time a “main line” is breaks up into “smaller lines” OR there are just “smaller lines” in front of each resource there is an inefficiency compared to a “main line” being fed directly to the resources (i.e. small lines with a length of exactly 1). I notice this every day at the gas station – I want there to just be one main line so I don’t choose wrongly and get behind the person buying 100 lotto tickets, but there’s always a small line in front of each cashier and you have to guess whether or not each person is going to be slow.

  • david

    Airports receive people from a wide variety of cultures and backgrounds, and those people may have different norms to queuing. A sorter minimises unnecessary conflict and confusion from these differing norms.

  • John Maxwell IV

    >They’d rather pay to try to “hide” this variation, even though most folks in line will probably notice it anyway.

    I doubt most folks notice it.

  • My attitude in queues is usually not to worry. Since everyone else is trying to ensure they don’t get in the longest line, the lines will be approximately equal and it doesn’t matter which I take. Mathematical and psychological studies show that most people will feel like they’re in the slowest line if there is any variation over time in processing speed, so I try not to bothered when it appears that I got the long line. I don’t actually worry less than anyone else, but I feel superior because I know the worry is wasted.

    The airport security people don’t care about fairness. None of them, from the sorters and screeners at the front lines to the high bureaucrats at the top benefits from satisfied customers. In the grocery store and the bank, someone wants the customers to be as happy as possible, even though most people will be unsatisfied whatever algorithm is chosen.

    Using a throughput metric, it doesn’t matter at all which algorithm is used, as long as no queue is allowed to be empty when a machine/screener is available. The delays will be allocated differently and may be more or less fair, but the same number of passengers will be processed per unit time regardless.

    Airport screening lines are different from most other queues because there’s processing to be done as you approach the head of the line. The last 15 feet of the line people have to spend unpacking laptops, emptying pockets, bagging liquids, and removing shoes. So the processing part of the line starts 15 feet before the head, and it’s important that people be moved into those lines before the last minute. The main variation in processing time at screening is really the amount of time the different passengers in front of you take, and there’s unfortunately no way to avoid subjecting passengers to that variation. If one of the 6 passengers in front of you is slow, you’ll experience a period of standing still for a long time, and think that means you got the slow line. The psychological studies I mentioned earlier show that people discount the time when they’re moving in comparing speeds with neighboring lines. What you most notice is times when the person in the place you could have chosen surges ahead, and not the times when you slowly catch up.

  • Ed

    Also note that sometimes one or two of the sublines cannot be seen from the main line, so line sorters must direct people to them. And occasionally one line closes and a new line opens and this could lead to some confusion without line sorters. So there are cases where line sorters are useful. And maybe it is not easy to have employees shift between sorting (when sorting is useful) and doing other tasks when it is not, so there’s a lot of line sorters standing around trying ineffectually to be helpful.

  • Sorted folks then regret being sorted, knowing that on average their trip would be faster if they could choose for themselves.

    I don’t buy this. Ignoring the edge-conditions where some lines are empty, the average time through the line depends on the throughput of the individual lines.

    Choosing a lane is a zero-sum game. You can only make yourself faster by making someone else slower.

    • And since “making someone else slower” is an unpriced externality, that means without a line sorter we would spend too much mental effort trying to pick the fastest line (relative to the social optimum).

      • NotBraveEnoughToUseMyRealName

        I want to vote these comments up but there is no voting here :-/

        It astonishes me that trained economists don’t notice something fundamental like this, and instead there is a pile-on to lob gripes about not having been allowed to seize the best outcome – and damn the costs of rent seeking!

        Only one or two comments that I can see are paying attention to the nominal point of the entire enterprise – to filter the public as efficiently as possible so that most people are safe from a tiny, criminally insane, handful of people who might try to kill random strangers in the absence of security procedures.

        Hasn’t anyone noticed the differences from line to line? Some lines have that new radar thing, some lines are closer to chemical sniffers, and so on. Plus I mean, coordination is hard and I bet its good to just rotate people into an easier task every so often so they don’t get too bored looking for a gun on a video monitor. If you can have your papers all set, smile, and say thank you for their help then maybe they’d hate their job less.

        It seems to me that managing the safety process is the primary thing that matters here… not ridiculous quibbles over autonomy and saving that extra 30 seconds. Especially when I remember the likely inferential distance between myself and the many people who can be safely assumed to be optimizing the process.

        If someone can see a significant improvement to make to airport security processes which would produce an obvious aggregate benefit, that would be amazing, but no one seems to be offering that kind of suggestion.

        Wasn’t the point of this blog to “overcome bias”? And yet so much of the content here seems incredibly biased to me… and I don’t think I am substantially crazier than I was two years ago when I thought Robin’s content here was pretty good.

        Maybe there’s something obvious that I’m not noticing? But more than that hypothesis, I find myself wondering whether the process that generates content here is well enough designed that’s its worth paying attention to the blog anymore. Did Robin jump the shark at some point? Would Robin notice if he had? Does he have a line of retreat that would make it safe for him to stop posting if that were the right thing to do?

  • Proper Dave

    “It seems to me that on average the lines I would have chosen completed faster than the lines to which I was assigned.”

    Of course it does (rolls eyes). If you choose for yourself you will instead tell us how “the other lanes always move faster”.

    So “security theater” requires the illusion of control, via equal length lines. This would better explain systematic differences between security and non-security lines.

    This is even beyond speculative…

  • I don’t remember seeing line sorters before 9/11. Maybe it’s a failure of my memory but quite possibly the line sorters are there to serve some supposed security purpose.

    An earlier comment pointed out that certain lines may have higher quality equipment which would be totally wasted if terrorists could simply choose the other lines. However, I think the line sorters often serve another purpose.

    The TSA frequently insists they have screeners doing behavioral profiling and they don’t reveal who they select for additional observation/checks so terrorists can’t reverse engineer the system. This means they need someone standing around watching the queue and somehow selecting people for additional scrutiny. A line sorter is the perfect way to do this and the reason some lines are moving slower may well be that line is exposed to additional scrutiny.

  • Alex Tabarrok

    Don’t forget that by random variation alone more people are guaranteed to show up in the longer line for the same reason that more people find themselves on crowded airplanes even when most airplanes are not crowded.

    • Aron

      More people will wind up in the line with the highest *throughput*, much like planes.

  • Aron

    Security Theater sounds plausible. The appearance of a well-thought out and greased machine is important, and the appearance can be more important than reality.

    Covering up individual worker productivity variations seems weak. Passengers care generally about getting through quickly, particularly in comparison to those around them. They don’t care if they do this via a slow person but short line, or fast person and longer line.

  • A variation on security theatre is security ritual.

  • sam

    Line sorters at airports perform a function of debated utility: They make it more difficult for a security screener and a security evader to cooperate. Of course there’s nothing preventing the security screener, security evader, and line sorter form all cooperating together, I suppose.

  • Off-topic, but your fire-the-CEO markets and other posts have rested on the notion that there is a shortcoming in corporate governance that results in CEOs being overpaid. I wasn’t aware of anyone who seriously disputed that, but Megan McArdle recently indicated she had changed her mind on it, citing Ed Carr’s comparison of public vs privately owned companies. Any thoughts?

  • Robert Wiblin

    How would people in these lines effectively anticipate ahead of time which line is moving faster?

    Seems more likely these security staff are just conspiring to have extra trivially easy work.

  • It’s security psychology not security theater.

    If someone tells you to do something and you do it, you experience a loss of control. It is a mild form of hypnosis.

  • Robin, your observation that by being sorted your delay is longer on average than the line you would have chosen is bogus.

    Consider these two possibilities

    1) Lines are sorted by a mechanism we don’t know, with the constraint that all scanning stations are offered traffic constantly.

    2) Lines are self-sorted, with the constraint that all scanning stations are offered traffic constantly.

    If traffic is offered constantly, the total throughput in 1) and 2) is identical. Then for you to have an average shorter wait time in situation 2) requires that everybody else in the line with you has a slightly longer wait time.

    You could only get through the self-chosen line faster by beating out your fellow line-mates.

    As to the rest of it, I think the likely ideas have all been hit in these comments. Mostly, make sure people are prepping up to keep offered traffic at the bottleneck steady. Secondarily, I have seen partially hidden screeners that needed traffic directed to them to keep their offered traffic steady. Finally, I have seen these sorters participate in shutting down offered traffic to a station that was closing, and in reconfiguring the line physically to adjust for opening and closing of stations. There real value could be primarily episodic, unrevealed by a cursory glance.

  • Patrick McCann

    The line sorter can create an incentive for people to take less time at the screener if all the fast looking people get to go together. You may have noticed the elite traveler lines sometimes that people can self select into. In a recent trip to Costa Rica, there was a persons travelling without children line that went incredibly faster than the persons with children line. Some line sorters may be serving this function.