Navigation by vibration
As an alternative to navigation by voice instructions or display, I suggest a simple (less detailed) navigation by vibration mode. This would be suitable for drivers who have the phone in their pants pocket. Use three vibrations patters that are elicited about 30m prior to the required action, e.g.
* long vibration - turn right
* 2 x short vibration - turn left
* 3 x long vibration - going the wrong way!
This has the advantage over display navigation that it consumes much less battery. And voice navigation is often not well audible in busy cities, or (if it is loud) can be annoying for other people.