Last week, we were in Spain, enjoying the sunshine and using the motorways. And I was really impressed by their toll machines: attractive, easy to use, and even delightful. Even though they were taking our money. My very amateur video of the machine in action isn't great (sorry!), but it shows the key features of the system.
1) The instructions screen presents a clear representation of what to do next, independent of language. This isn't essential, but provides backup for those who might not be able to interpret the main interface.
2) Each action is clearly illuminated in a timely way: insert ticket; pay (card, notes / coins); get change; optionally, get receipt. Sure, the action sequence is simple, and there's limited scope for error, but the device leaves little room for doubt. [Contrast this with the story a colleague told me of observing someone buying a ticket from a UK rail ticket machine who could not locate a notes slot, so folded up a £5 note and fed it carefully into the coin slot.]
3) The coin slot, in particular, is well designed, opening and closing smoothly to accept coins at just the right time.
I know it's simple, but that's surely the point: it's only as complicated as it needs to be, and no more, and it's easy even for someone who speaks no Spanish to use without help.
Human–Computer Interaction specialists like me tend to notice poor features of interactive systems; it's delightful to celebrate a system that really seems to work well, come rain or shine.