hmm, thinking a bit more about the tunnel exit problem even if speed is read from obd2.
The main difficulty is to detect position while driving freely without route.

If a route is set and displayed, why not simply assume the driver took the correct turn and happily display the position along the route until gps is ready again?

Yes, it might be "false info", but a good one to ensure the driver and continue the route without hickups.

If i could choose between exact representation of data that does give no info, and one that shows the most likely position but could be wrong. I am with the latter.
This could maybe shown by a blinking position marker to indicate: "Your exact position is unknown but along the route you should be here"

Even leaving out OBD/CAN, another way to detect speed change could be the phones sensors, or not? I mean isn't it possible to measure some kind of "g force" and conclude speed changes from last know speed?

Or maybe i am overthinking...

EDIT:
and heck yeah stepping it down a bit, i would also succumb a solution that simply assumes i am driving with constant speed just to continue the route display and give exit direction based on that vague assumption. Still better than no info.
I would know how to use it, drive with constant speed already 500 meter before the tunnel and be happy.
Wrong display = user error in that case