Self-Organizing Systems In The London Bridge Pret A Manger

I have been re-reading Philip Ball’s Critical Mass, his book about “social physics”, how the study of physics can lead us to understand aggregate human behaviour better. It’s very wide-ranging and interesting, with pretty obvious implications for my day job. One of Ball’s early chapters is about path-formation and “flocking” (eg. how a mass of people can most efficiently leave a room - vital to understand this when planning fire exits etc.). He doesn’t actually mention queue formation but it’s the same sort of thing, and it’s a problem that strikes me when I go into Pret A Manger for the occasional breakfast bacon and egg baguette of a morning (om nom nom).

The London Bridge Pret A Manger is managed by a gentleman of boundless enthusiasm, both for his job and for the rules it requires him to enforce. I often see him upbraiding staff - in a friendly way, naturally - for incorrect cap position, insufficient service speed, etc. His main role in the customers’ lives, though - a thankless one - is to inform us that there are six open tills and can we please form six queues, instead of, as invariably happens, one big long queue which fills individual tills from the front. Is he right?

I have drawn an scientific DIAGRAM of the Pret a Manger London Bridge layout which I reproduce here. The black dots represent queueing individuals in the formation that they commonly (without prior planning) adopt. The red dots represent the queuing layout that the manager wants them to adopt.

The manager’s version is clearly more efficient when it comes to minimising queue length and from a management perspective it allows him a fairer assessment of the performances of individual workers. But the longer queue is in the collective interests, not only of current customers in the shop but also of prospective shop entrants (I’m assuming, and my own experience tallies with this, that neither method has a positive impact on the time it takes you to get served, though see below for more on this). The six short queues favoured by the manager would clearly make access to the tables, and to one end of the sandwich cabinet, more difficult. (Very importantly, this end of the cabinet has the Pret Chocolate Bars in). Whereas the one long queue allows better access to and from table areas and cabinets, with a minimum of queue crossing even at busy times. The crowd here have organised themselves into the most mutually beneficial system possible in terms of using the shop space.

But is this system unstable? Every individual beyond a certain distance in the queue will find their own utility maximised (i.e. will get their nosh faster) if they leave the queue and go immediately to one of the more empty tills. We now have a kind of prisoner’s dilemma in action - Ball in his book talks about the p.d. and game theory but it’s in the bit I haven’t read yet. Some people who are left behind in the main queue or do not redistribute fast enough will then find their utility lowered (i.e. it takes longer to get the sammich).

Left to its own devices the queue doesn’t collapse into sub-queues - whether it’s a spirit of mutual co-operation that drives this, or a kind of selfishness (the benefit created by quicker sandwiches is not equal to the disbenefit created by breaking the social convention) doesn’t really matter. What matters is that the manager’s encouragement to move from long queue to sub-queues creates this collapse, and therefore invariably causes an increase in queueing time for some customers. He really should leave well alone.

1. FT's Alan on December 14th, 2007

   lovely diagram.

   oo, i read that on holiday earlier this year. i don’t think he ever touches on queues cos they are a too complicated case for his social physics - it’s more social chemistry - and there is a whole branch of http://en.wikipedia.org/wiki/Queueing_theory.

   the physics model only works when the forces between the people are relatively weak, his gas/liquid/solid/phase diagram analogies. once the organisation between people becomes complex enough based on other people, prior experience, PD stuff, then the physics analogy has to be abandoned.

   i see this one long queue in the pret here in the hammersmith broadway and that has the same layout. the other one in the high st has a different layout and doesn’t lead to this formation. it’s the ‘5 items or fewer’ discipline learned from Tesco metro i reckon, combined with everyone knowing were it many shorter queues there is the frustration of ‘my queue is the slowest’. social reserve AND the fact that an interloper will find the arrangement quite hard to exploit (trying to start a smaller queue with a large number of people clearly seeing you do this) might make this quite stable in this shop layout.

2. Andrew Farrell on December 14th, 2007

   Every individual beyond a certain distance in the queue will find their own utility maximised (i.e. will get their nosh faster) if they leave the queue and go immediately to one of the more empty tills.

   Aren’t you mixing your systems here? Certainly the manager is acting against his interests, as the one long queue minimises the amount of time that there’s an empty till.

3. FT's Tom on December 14th, 2007

   I don’t think I’m mixing my systems, just pointing out that individuals in the long queue system could collapse it by themselves (and from an individual p.o.v. maybe should) - but this doesn’t actually happen unless the manager pulls the trigger, so to speak.

4. Steve on December 14th, 2007

   what about having a separate COFFEE QUEUE? separate queues in somewhere like Pret should work in theory because it’s generally fast and everyone’s getting the same sorts of things - hot drinks is the only potential problem.

   i have a slightly different dilemma when i go to Nincomsoup. I only ever want their wraps and Fentimans cola but they position these behind the queue of people wanting soup so I either have to appear rude by cutting in to grab said food and then cut in again between the people receiving their soup and those finally paying for it.

5. Steve on December 14th, 2007

   i mean either do that or just wait in the queue with them until i can get to the stuff i want and THEN bypassing soup-wielders.

6. Steve on December 14th, 2007

   Old Street Pret (or Moorgate Pret North) has done well by making it impossible to form separate queues for their six tills because of the seating area being in front of them. Instead you uniqueue sideways between the seats and the tills and then do a 180 towards them when a cashier calls you over.

7. a logged-out pˆnk s lord whatnot on December 14th, 2007

   when it’s busy my pret has a separate food only queue

   i always get cross when OTHERS set up their “own” line — ONE QUEUE ONE VISION

8. FT's Tom on December 14th, 2007

   The concept of a separate food/drink line seems to be one the manager reaches for on occasion, sometimes he shouts “No hot drinks at this till” but he’s so hyperactive it’s never very clear which till he means anyway.

9. a logged-out pˆnk s lord whatnot on December 14th, 2007

   my pret is long and thin and has no free-standing tables, only ledges at the windows — the main tills are at the back, the food only till is right by the door (and only open when super-busy)

   i used to know some who edited the pret in-house magazine! MOVE OVER THE W!RE!

10. Marcello Carlin on December 14th, 2007

    my pret is long and thin

    *supply Eddie Brabenesque punchline of your choice*

11. FT's koganbot on December 14th, 2007

    I browsed this book at the library and decided I will want to read it when I get the time. You will definitely like Duncan J. Watts’ Six Degrees, the one I wrote about in Rules Of The Game #18.

12. Linked by: Post of the Week » Blog Archive » Shortlist for weekending 14th December 2007 on December 15th, 2007

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