Info about the Safeway scheme:
http://www.ralphallen.org/year 8/images/safeway.pdf
I remember using this ages ago and loved it!
Apparently it was dropped because of theft, either intentional or otherwise. Even with checks, people were known to be omitting some items. I believe that a lot of this is any more than people simply forgetting to scan an item or two when they go to put it in their trolley. But there were undoubtedly others who would use that excuse to get away with paying only a fraction of their overall cost.
Then there's issues with the purchase of alcohol; not everyone sees an attendant, so a 16 year old could easily purchase alcohol through this system. I'm pretty sure that's why Safeway used their "ABC" card as an age verifier , but as with credit cards, fraudulent use would have rampant.
Then there's the issues with rescanning mentioned in the above link. People expected to just pay and be on their way. They do not expect to have to go through a normal checkout again. At least without this scheme, everyone already knows what to expect.
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It's fantastic to see the way people fall over themselves to do the supermarkets jobs for them. I hate these things with a passion. I'd much rather wait a few extra minutes and help keep a human being in a job.
This is an interesting point. I will once again refer to Morrisons. There were no staff losses on the introduction of these machines in Morrisons supermarkets. Instead, the two "real" tills were replaced by four "self-scan" machines, with one operator required for the four machines. The displaced operator simply uses another till, . But in reality, the introduction saw two express tills close (leaving one open; and a second available if necessary), four new selfscan tills (to supplement the express tills) and an additional "normal" till open up. This is a net gain of three checkouts with no staff losses, and has helped cut queues for people who only want a handful of items. And, of course, there's an extra operator on the normal tills, which helps that as well.
This is an example of them being placed sensibly. And I'd imagine/hope that other supermarkets use a similar model. Sadly, I think my local Tesco has reduced till staff (probably be redeploying them in the store and not hiring replacements).