Shopping-trolley game theory

Shopping trolleys, or carts, are quite expensive. So supermarkets, and other shops that provide carts, prefer their customers to refrain from taking the carts home.

But if you're a shopper who doesn't have a car, or money for a taxi, then you either have to buy few enough groceries that you can tote them home in some bag or backpack or old-lady-trolley you bring with you, or you just take the bleeding cart home.

Only if you're angling for some sort of Kindness to Corporations Award would you then return the cart to the supermarket, of course. So forlorn stray shopping carts can be found by the side of the road in many nations. Supermarkets have to hire cart-collectors, and create hotlines you can call to report abandoned carts and enter the draw for some sort of prize.

Some shopping trolleys have security devices that, one way or another, lock a wheel if you attempt to abscond with the trolley. Now the trolleys cost more and the supermarket still has to hire people to go and unlock and return immobilised carts, but at least the trolley-wranglers won't have to go very far.

I think a more common attack on the wandering-carts problem is carts that require a deposit. They're commonplace in Europe and at airports and are slowly becoming more popular elsewhere, like Australia, where I live.

Each cart has a very solid coin-activated lock...

Shopping-cart lock

...with a short chain danging from it. There's a doodad on the end of the chain that's a cross between a key and a seat-belt latch, which clicks into the lock of the next cart in line.

Numerous shopping trolleys
(source: Flickr user Polycart)

To unlock a cart from the next one's chain, you have to insert a coin in the lock. The lock captures the coin when it releases the chain, and the coin is of sufficient value (here in Australia, a one- or two-dollar coin) that you'll probably want it back. Unless you've got some pretty heavy-duty tools handy, the only way to recover your coin is by plugging the key thingy on the end of another cart's chain into the lock of your cart.

You can, of course, use any metal object the size and shape of the appropriate coin to operate one of these locks. The supermarkets themselves often sell metal discs with a key-ring attachment that make sure you can always get a trolley, even if you haven't got any change. But the coin-lock system seems to work quite well, overall. It greatly reduces the number of completely escaped trolleys, and also reduces the number of trolleys abandoned in the rain out in the supermarket parking lot.

Aldi opened a store here in Katoomba about a year ago, and they have coin-lock carts, which I almost never see abandoned.

Except one time, I did.

First there was one cart sitting in the undergrowth by the side of the road, intact, with a coin (or a washer) in its lock. Then, as the days passed, there was another. And another. Connected together, just as they would be back at Aldi.

This taught me a bit of shopping-trolley game theory. If you're taking a coin-lock supermarket trolley home, you indeed do lose your coin (or washer), that first time. But subsequent trolleys taken to the same place can be connected to the first one, and those ones will give you back your money.

All the supermarket can do to stop this is count their trolleys frequently and send out a patrol whenever even a few of them have gone astray, and thereby lose one of the reasons why they opted for the more expensive, lockable trolleys in the first place.

And they clearly can't stop these dudes at all.

21 Responses to “Shopping-trolley game theory”

  1. Zarquon Says:

    The trolleys aren't expensive for the supermarkets, what is expensive are the fines for abandoned trolleys being charged by local councils.

  2. Jax184 Says:

    You might enjoy a documentary that was filmed here in Vancouver. It's called "Carts of Darkness," and is about homeless people who ride shopping carts for fun or as part of collecting bottles to return for the deposit. The film maker used to be a snowboarder until an accident cost him the use of his legs.

  3. wds Says:

    Over here in Belgium these locks were all the rage for a while. Then supermarkets figured out that they were paying more for the purchase and maintenance of the locks than they used to pay for the extra lost shopping carts, so they did away with them.

    I see surprisingly few shopping carts left by the side of the road though. The occasional student party getting out of hand, but other than that most people that take them home bring them back. This might have something to do with the fact the weels on them are a bit sucky (on purpose?) so you can't really travel large distances with them.

  4. MorganGT Says:

    Hmmm. I wondered why there was a bunch of locked-together trolleys outside a house down the road - I assumed someone wheeled the lot of them locked together the few streets up from the local Safeway for some unknown reason, and was therefore an idiot. Now I know they are a clever master criminal, and I am now very afraid......

  5. Max Monroe Says:

    You forgot to mention the small minority of people who get (and keep) their hands on a cart only to do horribly, horribly perverted stuff with it, like this: ;)

  6. Fallingwater Says:

    Actually, shopping carts are fairly expensive - I seem to remember their price being somewhere around 100 to 150 euro each in large quantities, though I could be wrong. They're conceptually very simple, but you have to consider they're built very sturdily, as they have to take years of abuse by distracted customers slamming them around while loaded.

    As for use in self-moving hack jobs, they're less than ideal - their construction doesn't give them good handling capabilities. I think you could improve things noticeably by welding the nose to the forward part of the wheel base.

    By the way, I've always wondered if the police could feasibly have a theft case against you if they caught you driving around in a parking lot with a trolley, assuming you'd be at least smart enough to remove the logo of the supermarket you swiped it from...

  7. evilspoons Says:

    Here in Canada, virtually every larger-than-tiny supermarket has had coin locks on their carts for as long as I can remember (I'm 25). I'm surprised when I see a place that doesn't have them.

    (The only large store I can think of with carts that don't require a coin is Ikea. They have the best carts too - all four wheels are on swivel casters, which is incredibly fun on their smooth floors.)

    The automated wheel-locks are fairly new here and the ones implemented at local supermarkets aren't particularly effective in the winter. You just drag the locked wheel along the snowy/icy sidewalk anyway.

    Fortunately the majority of carts ripped off from the supermarket nearest my parents' place don't make it very far - typically to the LRT station about a block away - so the job of those collectors is pretty simple.

  8. Says:

    1.) I love Aldi, $0.27 mac-and-cheese is a thing of beauty.

    2.) Aldi is the only supermarket I can think of that has locked carts (I live in Chicago) Every other store just has employees that collect them from the parking lot. Some even have little cart-pushing tractors to help the poor guys push 37 carts in at once.

  9. Mohonri Says:

    Could you just plug a cart's key into its own lock to get your coin back, or is it too short for such a loopback?

    Regarding the carts with locks that activate when leaving the parking lot, I recall reading somewhere about a guy who wandered around the edges of the premises with an antenna, sniffing out the wireless signal being transmitted on the buried cable. He then went home and built a clone, wrapping the antenna about his torso (!). And then he took his contraption back to the store and caused minor mayhem by activating the locks on the carts of innocent shoppers _inside_ the store.

  10. Fallingwater Says:

    The chain is, quite obviously, too short for the key to be fed to its own lock.

  11. n17ikh Says:

    Mohonri: hackaday has an article on said shopping-cart hackery.

    Here in the American South we usually call them "buggies". I've never personally seen one with a lock, except in airports, where they frequently have "Smart Carte" or similar systems. These actually cost money to use, in that they don't return all of your money when you return them to the corral. Hopefully supermarkets won't follow that trend..
    However, I could certainly see them doing so, as a set of carts is a decently large investment for a store - they cost anywhere from 100-200USD apiece in quantity, and a very large big-box store may have hundreds. Lots of labor-intensive spot welds coupled with the metal being either stainless-steel or the whole thing chrome-plated, designed to last for years in all sorts of outdoor conditions, while being abused by cars and shoppers alike - it's a wonder they don't cost ten times as much.

  12. hypocratus Says:

    There is a small supermarket nearby that has a simple, though labor-intensive, way to avoid people absconding with carts. They simple have store employees load the cars for the customers, thus not allowing the carts out of their sight.

  13. phrantic Says:

    I have memories of shopping trolleys costing something in the order of $400 each when I used to work for Coles.

  14. xuth Says:

    You should be able to get your original dollar back if you can get enough carts to make a circle. Something like this:

  15. corinoco Says:

    See at Baulkham Hills Aldi on Saturday:

    A shining example of humanity gleefully encouraging her ghastly spawn to smash open the lock of their Aldi trolley to get their $2 back; with a hammer they had just bought in Aldi. They were maybe 20m from the store, it would have taken all of about 15 seconds to simply go and reconnect it.

    Also seen; people have nicked the free-end chains from the Aldi trolley racks, so I lost my little metal disk token. :-( but it was 99c some I'm not that worried.

    Trolley technique - buy the stuff-you-can't-get-in-Aldi at Woolies, then take your Woolies trolley into Aldi. Take that, corporate control!

    Trolley blues - newer trolleys lack the bar across the back near the castors that let certain individuals ride the trolley back to the racks.

    Wireless trolley-locking? Linkage please! Trolley equivalent of TV-B-Gone or mobile jammers! A great use for my growing collection of old phones...

    Trolley memories - St.Ives shopping centre, 'Memorial St Level' on Sunday afternoons, circa 1980. Loose trolleys would be raced down the nicely-curving ramps; with or without passengers...

  16. Itsacon Says:

    A nasty trick with the auto-locking trolleys: If you lift them high enough over the `red line', they won't lock.

    They will then lock when the store employees try to wheel it back inside...

  17. cms108 Says:

    or you could just use one of these and save yourself the trouble...

    or use it to walk round the inside of the supermarket and steal the quid coins out of everyone's trolleys while they're not looking...

  18. cms108 Says:

    wait a min... that's the wrong one... that just lets you steal trolleys without using a coin... but there is another one that looks like an open ended version of the key on the end of the chain, that lets you open the coin slot and because it's open ended lets you withdraw the key too.

  19. Jonadab Says:

    Huh. I've never seen those here, but I can think of a couple of reasons why I wouldn't...

    1. The most valuable coin anybody ever has on their person here is a quarter. There *are* fifty-cent and dollar coins, but people collect them. They're so uncommon in circulation that most cash registers don't even have a drawer slot for them.

    2. I've never seen a shopping cart abandoned by the side of the road, either. All over the parking lot, yes. (You're *supposed* to take the cart to one of the special places in the lot designated for carts. Most people do. A few cretins don't.)

    Maybe the key difference is that American stores all have sufficiently large parking lots, so that nobody has to go beyond the lot to get to their car?

    Wait, strike that. I bet the real reason is that America has very few pedestrians, and we're not the ones who are too lazy to carry our groceries: the people who would be inclined to steal a cart are unwilling to go to the store without a vehicle in the first place. If their own vehicle is unavailable for some reason, they wait and go another day or else beg a ride off somebody. Yeah, now that I think of it, I'm pretty sure that's it. A typical American is astonished and appalled at the idea of walking to the grocery store. I should know. My own mom has been known to look at me like I'm some kind of alien freak upon finding out that I've walked to the grocery on occasion. Without the willingness to go on foot, there's little incentive to steal a cart. It's an unwieldy item to take home in your vehicle, even if you've got a pickup.

  20. frasera Says:

    I remember seeing those once or twice a decade ago, these days they just don't bother in the bay area in california anymore. I guess the fumbling over the lock thing is too much of a pain.

    There are some new target carts that are nicely redesigned though, no lock either. You can google it up:)

    They also have redesigned shopping baskets built with more ergonomic thinking in mind..not bad.

    But yes as the last guy said, the only people to really steal carts are homeless.

  21. mcp Says:

    There is some mild but effective social pressure to return carts to the cart storage. There even seems to be an understanding that if you are a long way from a cart corral it is okay to start a new one in next to your car since they should have put one there anyway. So pushing one cart into another relieves you of any social stigma: "There I'm doing my bit". But leaving one 5 meters from the corral is the same as littering in public.

    By turning the whole thing into a financial transaction, the store would loose the social pressure on its behalf. Freakonomics describes a similar result for daycare late fees. "Now I'm paying for it, I'll do whatever the hell I want."

    Neither solution is going to have much impact on people who take them home though.

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