Although Taming the Turing Machine is about avoiding confusion in the digital world, broken mental models exist in the physical world too. Here's a favourite example.
What do you think the UK road-sign on the left (the rectangular one) means? If you're like me, you have to think hard every time you see it. It actually means, "You have priority over on-coming vehicles." It's used at one end of a piece of road that's only wide enough for one vehicle. Its complement – used at the other end – is the round sign that means, "Give way [yield] to on-coming vehicles."
It seems I'm not alone in finding these signs hard to understand. In fact, they're often installed with an extra plate underneath explaining in words what the graphics are supposed to mean. That's tremendously helpful, but additional explanations are almost always the sign of a broken mental model. So what's going wrong with these signs? Let's take a look.
They do obey the rule that informational signs are rectangular, whilst signs giving orders are circular. Of course, a Give Way sign is normally an upside-down triangle with the words Give Way in it, but we'll let that go for the moment. They also obey the normal wayfinding convention that an arrow pointing upwards means ahead, whilst an arrow pointing downwards means the opposite. So far so good. That last point is so engrained that you might not even realise it is purely a convention. It's based on imagining the vertical sign as being laid on the floor. If we imagine the vertical sign being put on the ceiling instead, the arrows would need to be the other way around. That's why emergency exit signs above doorways usually have the ahead arrow pointing downwards. An upwards arrow in that position might cause people to turn around and run back from whence they came. It's subtle stuff. But I digress.
I think the main problem with these signs has to do with the size and colour of the arrows. In both signs, the big arrow has priority. That makes sense. Big is more important than little. Everyone knows that. But why is the other arrow red? To my mind that seems to over-rule its size. Red is more important than black or white, isn't it? And don't try to tell me "red means stop". That does actually work to explain the signs in this instance, but a red ARROW? When did that start meaning stop, or even give way? It just doesn't make sense.
Perhaps we should launch a competition to redesign these signs. Why not have a go yourself and share your thoughts in the comments section below. In the meantime, I think there's another important lesson for interaction design:
Yes, icons are great. Sometimes they're quicker to recognise than words, and sometimes they fit into smaller places. Oh, and they're multi (or even non) lingual. But sometimes you just can't beat those good old-fashioned icons we (nearly) all understand – words. Don't be afraid to use them.
[The other lesson here, these poor examples apart, is that there are people who've been designing great "user-interfaces" for years. They're called sign designers, and it's well worth understanding more about what they do, particularly wayfinding.]