I suppose this isn't exactly a new feature but it is new to me since I don't travel through tunnels very often. What exactly is the software using when it decides to switch over to night mode when in a tunnel? GPS Location? A light sensor? A combination of both?
Hard to say without knowing which device and firmware version we're talking about, so what follows is my speculation.
I have a Garmin Nuvi which continues navigating in a tunnel, turns and all, despite losing its satellite reception. Mine doesn't switch to night mode, but it clearly knows it is in a tunnel from the map data. Since loss of reception is a given in a tunnel, the device must be dead-reckoning while it's in there. (Re-broadcasting is out because it would affect the signal timing which is key to calculating a fix). In fact, I have tested this by entering a tunnel fast and decelerating once inside it. The device alerted that it had lost reception and continued to display my entry speed, "passing" turnouts and the tunnel exit before I did.
From that, I inferred that the firmware was using map information, possibly cross-checked with loss of satellite lock, to know when I've entered a tunnel.
Perhaps it uses the ambient light sensor in your phone and changes to night mode when there is less ambient light.
Easy solution to force night mode that I found.
- go to settings
- go to date and time
- disable automatic date and time.
- set your time to 7pm or later.
That should set the app to night mode you can change the time back when you're done traveling
(Galaxy S7. Version 7.0)