How about a computer monitor? Or overlay of information about your world (this bus will next arrive in 5 minutes; find out more about that statue; follow this blue line to your destination)?
So, your phone overlays a line in your field of vision indicating which direction you're going?
Or does it overlay an arrow on a digital representation of the street you're traveling, on a screen located away (and at a different focal length) from your actual view of the road?