Tapping the screen turns the feature on, while a second tap disables it. It's another useful feature offered by Android's virtual personal assistant. The screen shot of the feature which accompanies this article, was taken in the Netherlands.
Interestingly, for this feature, Google Now does not seem to tie into the alarm function of an Android phone. But what it does do is make sure that you are up and alert and ready to exit the bus or train at the right stop. Otherwise, it could be a long night as you retrace all of those extra exits you slept through.