SJS, as the thigh is definitely ervah, and since most humans, women included have their thighs connected to their knees, and since the knee itself is not one foot long (12″), but a mere 2″ or so, there is no way the knee can get uncovered and you can’t see part of the thigh. You might wish it’s not so, you might wish it’s invisivable for all men, but it’s not.
A skirt that doesn’t completely cover the knee while sitting or getting into a car automatically reveals part of the lower thigh as well.
That’s why we don’t do everything according to the letter of the law. We do MORE than the letter of the law. We make gedarim so that we cannot cross the boundries and come to violate the law itself.