LED studded wands which, when waved back and forth, display eg the time have been around for a number of years (They are the ones that take advantage of the persistence of vision and seem to project an image in thin air).
I suggest making some practical use of these by attaching one to a windscreen wiper -which would generate the required movement. Ideally, the wipers could be manually offset from the screen to allow the display to continue working, even in dry weather.
This could be wired to an onboard GPS system to indicate the speed of the vehicle (57MPH -SLOW DOWN) or to highlight the location of eg a parking space or a particular feature in the street scene ahead.
