Some pages back (or was it in OSM thread?) i suggested to look into OBDfish for reliable speed data when gps is not available. It would be so cool if pure maps could pick up some data from a running obd fish instance and do magic no nav app has done so far(?).

Taking exits in tunnels is much more complicated though, i concour.
Sadly the turn indicator is not hooked to any ECU even in modern cars, so it is not accessible via obd2.

But it is mostly not necessary because i rarely remember having to take multiple exits in one continuous tunnel.

The most common situation are tunnels with some exits that leave out of the tunnel so the the signal would be picked up again when leaving and then correct the displayed position.
Most vital is the info when to exit/turn for the first time.

If you don't mind, i would love to ship an obdfish compatible ELM bt adapter to your door if you do not already have one. Just pm an adress in that case, i would direct an aliexpress shipment to you.

Another completely different thing that came up during my drive from frankfurt to cologne today.
Why does no nav app display the distance to a speed limit?
For "fuel conservation" it is quite nice to know there will be a speed limit around the corner and it thus makes no sense to give full throttle for an overtake you will not finish. Or going fast and not having to waste precious brakes but gently roll out to allowed limit some hundret meters earlier (e.g.).
I checked in all common android apps, no one is doing it. I would love to see info about speed limits ~1km before reaching them. Or depending on speed, some sane amount of seconds before.

Thanks for your verbosity! This is an awesome thread to follow.