Transport
Just one example of the difficulties of a totally cash free society. There is currently a surge towards card payment of (eg) train fares. OK for London with fixed fares and zones; but if they tried to make it national is thee not a danger that using a card (or equivalent) will get one charged an expensive fare rather than the cheapest. They could rationalise the current fare structure but somehow I doubt it.
Biting the hand that feeds IT
