I was thinking of the headlights themselves, the primary source of glare.
But it hardly matters.

Photochromic lenses work via chemicals that ionize or otherwise change in
response to light. The glass variety responds to visible and UV, while
plastic photochromics require UV.

In order to selectively reduce highlights (to say nothing of amplifying
light in the shadows) the lens must anticipate the line of sight from the
pupil to the highlight source. Without that, you just get generalized
darkening, like the current electrochromic mirrors.

We can only speculate about the technology of the future but any solution
that works as you describe will require an addressable matrix in or on the
lens, capable of darkening in some places without darkening in others.
That's no big problem; a grayscale LCD TV could do that already.

But then it must selectively darken certain areas, so you will also need a
video receiver, perhaps mounted in one temple, and an image-processing
computer in the other, controlling an LCD matrix. The computer and the LCD
must respond quickly enough that the darkening effect can follow head
movements without lag.

When all of that comes together, there's still the problem of where to put
the battery pack.

And none of this does any good for shadows. For that you need amplification.
Get some night vision goggles. If you keep the lenses clean, headlights will
be well beyond exposure range and won't detract so much from shadows.

-MT