Right, all you user interface designers out there: I've found probably the cleverest yet simplest user interface feature ever. And you know what? It's in the user interface of the car park at Johannesburg airport.

I was standing waiting for my lift, when I noticed that there were loads of red and green lights around the car park. Further investigation showed that there was one light on the ceiling above each parking space; if the space was occupied the light was red, and if it was free the light was green. Each light clearly had a little sensor looking downwards and detecting whether there was a lump of metal beneath.

So if you're driving around the car park, looking for a space, the answer's easy - don't peer down the rows of cars trying to figure out whether that gap is really a gap or just a small car tucked well into the space. Just look for the lights on the ceiling.

It's a classic piece of user interface design; it's simple, it's clever, and it works.