Reason the DOT doesn't allow Lexan windshields, side glass, back window, etc. is that, unlike safety glass that has a ply construction with a plastic layer between inside/outside glass layers, Lexan does not shatter - it breaks. And when it breaks it makes for a very sharp edge that can slice most anything (especially our frail outer skin layers) like a scythe.
Safety glass is tempered to make it explode into many small pieces instead of big sharp, jagged pieces. I personally would not want to be in a Lexan windshield-equipped vehicle in the event of a frontal or side impact severe enough to stress the windshield pillars enough to break it. Granted, it might take alot more to break it, but when and if it breaks there is a significantly increased chance of it doing alot more damage than safety glass does.
Just something to think about.