What do you do if the drivers side windshield wiper moves off the windshield while the wipers are in use and car is moving more than 45 mph?

I'm going to add my answer above the original answer below because I think this question is regarding "wind lift" which is common with some vehicles and some types of wiper blades.

You may want to purchase wiper blades with airfoils that use the wind to push them down rather than lift them up. See related link below.

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Original answer:

What you do any time your vehicle becomes unsafe while driving. Slow down, pull over to the side of the road and stop and put on your hazard flashers.

To repair the problem, requires knowing the cause. The wiper has a hold down nut. If the nut is loose, you may be able to reposition the wiper arm and tighten the nut. If the nut isn't loose, then the wiper arm is probably stripped, and the fluted shaft doesn't grip it and the wiper arm will have to be replaced.

(BTW the above also means that you do NOT roll down the drivers window and bat at the moving wiper while driving in traffic half blinded by rain blowing in on your face. Apart from getting your clothes wet, the only result is to strip the wiper arm in case it wasn't already stripped so that you can skip diagnosing the problem, pull over and wait for the rain to stop, and then drive directly the auto parts store knowing exactly what you need. Your receipt may help you convince your boss/gf/wife that you really were late because you had car trouble, but don't tell them the part about getting wet, you'd sound like an idiot.)