[Corrado]

>How hard would it be to make a GPS system that just assumed I might want to get gas or stop for a bite to eat if I took an unplanned exit off an interstate with hundreds of miles to go on my journey?

Just as a side note, I really like Google Maps' routing system. If I stray from the computed route it just silently recalculates and let's me know where to turn next. There is no "Recalculating" phrase at all. :)

[olex]

Same with the Mercedes COMAND navigation, it'll just announce the next turn whichever way you are going. I generally like the way Mercedes do things in this regard.

[Tloewald]

Mercedes invented the much imitated seat adjustment controls now found in many cars (control shape and layout matches seat, required motions mirror desired adjustment). Don Norman uses them as an example of excellent mapping in The Design of Everyday Things.

[morsch]

I think there's some value in communicating that the navigation system is confused by what you are doing. There are multiple reasons why it might be confused: I might have taken a wrong turn either on purpose or by accident. Or it might have an incorrect positional information or incorrect maps. The user is in a pretty good position to judge which is the case and to modify their trust into the navigation system appropriately, so it's useful when they are informed.

It doesn't have to be an obnoxious, repeated "Recalculating", I guess. A simple chime is pleasant -- but far less intuitive, you'd have to learn a new signal. Plenty of people are confused by navs making a chime sound when you're over the speed limit. An optical signal, like a big red X or a question mark on the display would be a good hint.