Here in New York City, my primary mode of public transportation is the subway - or "underground" as it is also known.
Where I live, the first (or last) twenty minutes of my trip is above ground, on an elevated track.
However, the next forty-plus minutes of the ride take place deep underground, with no possibility of achieving a cell-phone signal - with my phone being in a constant "Searching for Service" mode, which can be a drain on the battery.
I have become in the habit of switching to Airplane Mode as we descend below ground (or when I enter the subway for my return trip home) and it has proven to be a very useful tool in helping conserve battery life - especially on days when I'm out for twelve hours or more.
Yes - in the beginning there were a couple of times when I neglected to turn off the Airplane Mode upon exiting the subway -- but after a couple of very irate, "Why didn't you answer my texts ? Where wee you anyway?"s from my wife...that no longer happens.