As my Final Year project at University, I created a GPS output device for pedal cyclists. It mounted on the handlebars of a bike and used a green laser to draw vector graphics onto the road ahead. It would draw left, right and straight ahead arrows, roundabout symbols with junction numbers in the middle, and a U-turn symbol.
The hardware was feature complete - and safe(!) but never polished. The power consisted of 4 12v SLA batteries which were heavy. The interface to a smartphone (I used an android phone at the time) was via Bluetooth - an app on the phone listened for text-to-speech log messages from Google Maps and parsed those to work out which symbol to draw. It could only really handle those things which it was specifically told to.
The code to the hardware is on my GitLab page for the cycleGPS and the code for the Android phone is...somewhere.
The hardware was a Raspberry Pi running debian plus a Microchip DtoA converter to drive the galvonometers. They were an off-the-shelf unit from eBay. Something like this but without the controller.
At some point I'll find the pictures and put those here too.
Edit: Here's the video I included as part of my final year project's presentation.