This is a public service announcement coming from your local prude.
Ladies, when you wear skirts that don't cover you knees while sitting down, you are exposing a lot more than you think. Those skirts almost always ride up and leave very, very little to the imagination. So little that anyone sitting across from you will know the color of your underwear. If that is the desired effect, then knock yourselves out. I would bet, however, that most of the women are clueless about this. They think that their skirt is not showing more than desired because it's not that short and covers the knee when standing up.
I could be wrong, of course, and in the age of bikinis no one cares about this. However, personally I would be petrified if I found out that half the F train car, filled with strangers with whom I ride pretty much every morning, has seen a whole lot more than I thought or cared to show. And would want someone to tell me.