My area only has a few short tunnels (<1 mile) but if the client knows when it's entering a tunnel, it could calculate average speed by comparing the entry and exit times by using the GPS signal.
Unfortunately our tunnels are too short to really affect routing, so I can't offer any more insight. If there's a wreck or stop in the tunnel, which is very common, people back up out of the tunnels in just a minute or two.


