I remember using Shop & Go at Safeway; seem to recall that worked slightly differently to current installations as you would scan a barcode upon entry to the Shop & Go area; one if everything scanned and another if you still had some items that you haven’t scanned. You had to have an ABC card (redundant acronym alert as ABC stood for Added Bonus Card!)
The screen would then tell you if you needed a rescan or whether you could proceed to pay. If you had to rescan then they would check all of your items. They had a normal checkout with conveyor for rescans, and then I think 2/3 stand-alone podium tills with no belt (this was long before the days of self-service checkouts of course).
I worked for Safeway when Shop'n'Go was introduced. The larger store where I started only had the system fitted after I'd transferred to another branch, so I didn't have any experience of using the system from a staff point of view. I used it twice as a customer, but was rescanned on both occasions. The idea was that once you'd had so many successful checks it would then only check you randomly. At the time it wasn't possible to use the Added Bonus Card (ABC) and staff discount together, so it wasn't really worth me using the system.
The ABC was brought in around 1995, shortly after Tesco introduced Clubcard and initially could only be used at the store where it was issued. It was later amended so that points could be earned in any store, but could only be redeemed at the "home" store, either in cash off your shopping (100 points = £1), or buying selected items with them which gave slightly more value.
Another initiative that ran alongside was the Greenbox. From memory these had to be purchased, but the idea was that the customer brought their boxes from home and scanned the shopping straight into them so that there was no packing required at the end. Special trolleys were introduced to carry them.
I think it was the takeover by Morrison's that killed off Shop'n'Go, but I wonder if other companies would have introduced self-scanning sooner had it continued.