On the supermarket trolley, a notice says that the trolley will stop if you try to take it outside the precincts of the shop.

Is it true?

Is so, how does it work?

Example sign from Hyperorg.com

enter image description here

  • Where? I have never seen or heard anything like that. Can you get a reference? Take a picture of it, explain WHICH supermarket, which chain and which area.
    – Wertilq
    Apr 17, 2013 at 14:07
  • 4
    @Wertilq I've seen this before in the United States. I'm not sure if it is a real device or just a sign to make people think there is a real device though.
    – rjzii
    Apr 17, 2013 at 14:15
  • 2
    I've used one, it does indeed lock the wheel when you pass the line.
    – vartec
    Apr 17, 2013 at 14:19
  • 2
    These are commonplace in the UK, at least in areas where there is a problem with loss of shopping trolleys (carts). They lock at least one wheel. Shopping trolleys are surprisingly expensive and also cause a nuisance when abandoned far from the store. Apr 17, 2013 at 17:18
  • Funny thing, culture. In several European countries, trolleys are chained together and you have to put money in to detach them, then you get your money back when you 'plug' them back in again. I remember working in a supermarket in the UK where they tried that. The result must have been fairly catastrophic, because they changed back PDQ.
    – Benjol
    Apr 18, 2013 at 9:17

2 Answers 2


Yes, there are actually a number of systems that can be used with electronic wheel locking systems apparently being the most common.

Sign in parking lot for electronic wheel locking system

While there are a number of different systems that might be used, they all operate on the same basic principle, to quote one of the manufactures:

A: The CartControl shopping cart retention systems are patented electronic systems that comprehensively prevent the removal of shopping carts from store premises. The CartControl shopping cart retention systems combine a digitally-encoded locking signal, embedded perimeter antenna, and our patented self-braking shopping cart wheel. A digital locking signal is transmitted via an embedded cable. The path of the cable establishes a perimeter boundary. An electronically-activated, self-braking wheel installed on each cart is designed to lock when it comes within range of the perimeter boundary signal.

Shopping cart wheel that can be locked

This is done primarily to prevent loss of the shopping carts through theft although it does have the added benefit of saving companies money since they can be fined for the "litter" if the shopping carts leave their property.

  • in other words they repurposed a invisible dog fence, (I wonder what they do about battery life) Apr 17, 2013 at 19:57
  • @ratchetfreak afaik the system is largely passive, drawing energy to lock down from the signal it receives, just like a passive RFID chip draws power from the signal that triggers it to send its response. Can't know for certain, but that's how I'd (try to) design such a system.
    – jwenting
    Apr 17, 2013 at 20:36
  • Its a right pain near me as some people take the trolleys as far as they can, then abandon them when the wheels lock. This results in a wall of immovable(but rotatable, only one wheel is locked) trolleys to weave past.
    – Nick
    Apr 18, 2013 at 9:31
  • @ratchetfreak - Details on the mechanism of these devices can be found in Gatekeeper's patents on them, such as US Patent 6,127,927, among others.
    – Compro01
    Jul 4, 2013 at 18:15

Here's the product page from the manufacturer of the CAPS system noted in the sign you posted:


Carttronics CAPS® is a highly effective cart and trolley loss prevention solution with the lowest lifetime cost of ownership. CAPS is operating on thousands of store sites in dozens of countries around the world, including independents, regional chains and sites owned and operated by 15 of the top 20 global retail chains.

The CAPS system consists of a small gauge antenna wire located in a narrow saw-cut around the site perimeter, a small electronic signal transmitter located in a secure location in the store, and one Carttronics’ locking CAPS caster on each cart. When someone tries to leave the store property the CAPS caster automatically releases a durable yellow braking shell that rotates down to separate the wheel from the ground and stop the cart. The braking shell is readily reset by store personnel using Carttronics’ hand held reset controller, placing the cart back in service.

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