In my country, when we have to go somewhere it's not unusual to take a bike. During spring and summer some women will take a bike while wearing a skirt or a dress. Her legs will be moving up and down, the view will be obscured by other traffic, parked cars and even trees or bushes, and you have to stand at the right position, but sometimes you get to see a glimpse of her panties!
My ex-girlfriend did this. Even my more prudish current girlfriend does this. And they both seem to be oblivious of the fact that they're flashing their panties.
But I do think they're secretly aware of this and that it's a very innocent (and mild) form of exhibitionism (with plausible deniability). Either way, I think it's very sexy!