> That's when they're needed most; my eyes have already adjusted to the other car's headlights
On a purely practical note from someone who is very light-sensitive, a combination of partially closing the eye closest to the light and fixing your gaze on the the outer edge of your lane (such as lane marker or eode of road) almost eliminates this problem, even for modern stupid-bright headlights.
Added benefit of letting you see more of your own lane in spite of the oncoming lights.
On a purely practical note from someone who is very light-sensitive, a combination of partially closing the eye closest to the light and fixing your gaze on the the outer edge of your lane (such as lane marker or eode of road) almost eliminates this problem, even for modern stupid-bright headlights.
Added benefit of letting you see more of your own lane in spite of the oncoming lights.