Good idea. I like it. Not exactly sure how you'd do it.

My first thought is electrochromic glass ('light valves'), like houses have used recently. They way they work is they have two layers of glass with a conductor on each and then micro-balls (think sand) between the two layers and LCD fluid in between. The edges are sealed. You apply a voltage and the fluid turns dark.

By my description of this technology, I think you can see that trying to do this with curved glass would be difficult.

Photochromic glass on the other-hand, like auto-darkening sunglasses, might be a better technology for your idea, but they have temperature dependencies--on warmer days they won't get very dark. Maybe you're okay with that. They are not controllable at all, though. If you drive into a tunnel, it'll take fifteen minutes for full-light again.

Yes, I was thinking along the lines of the sunglass technology. Actually the glasswould lighten inside the tunnel and darken again once outside of the tunnel( if it's a sunny day)

I think this has already been done in some cars on the upper three inches of the windshield. (But maybe that was just permanently darkened--I'm not sure.)