Until we have an intelligent or touch-sensitive windshield

Often the sun is too low for sun-flaps and too bright for sunglasses.  It gets particularly difficult when it is shining right behind a suspended street-light.

I've been in this situation several times, at a particular traffic light, about 7:30 in the morning.  I can't block the sun with the sun-flaps and still see when the traffic light turns green.  I end up sticking my fist out to block the sun.

An easy solution would be to have a disk of thin tinted plastic, about fist-sized--10 cm diameter--that can attach electrostatically to the windshield, with a small handle to pull it off and place it where it needs to go or store in a dashboard rack.

The ideal size and shape of the plastic could be worked out with user studies.  It would make sense to have the tinting stronger at the center, and fade out towards the edges.  It would also make sense to have an alternate shape that had a flat edge cut from the circle, not a half-circle, but cut it from a point about halfway from the center--this would be to line it up with a traffic light edge.

A more symmetric shape would work when the problem is just that the sun is very low and bright while you're driving.  It could get dangerous trying to drive with your fist stuck out, as I sometimes try to do, driving home around sunset.

Often the traffic report sites "solar" trouble as the cause of massive backups in the morning.  If everyone had these disks, we might get to work a little sooner.

A better and much more expensive solution would be to have the tinting built into the windshield.  You could touch the spot you wanted to darken.  You could touch it again to clear, or have a "clear-all" button on the dash.

A more advanced solution would be for the windshield to track the sun and the location of the driver's eyes, and adjust its tinting accordingly.