While I can’t prove my theory, I think that the best way to avoid clouded/fogged headlight lenses on modern cars is to simply garage them in order to shield them from excess UV rays. I say this because my cars–which have all been garaged since the late '80s–have never had this problem. That includes a Ford, a Honda, and 3 Subarus.
Back in my working days, the cars did get their share of scratches on the headlight lenses as a result of rocks/grit/gravel kicked up on them while commuting to work on the Interstate, but none of them ever suffered clouding/fogging of the lenses.
Then again, a friend of mine has a 7 year old Rav-4 that gets driven daily on the Interstate, and his headlight lenses still look good, and his car isn’t garaged. So, can somebody explain to me why some cars seem to be subject to this problem and others aren’t?