The police never used to enforce the window tints years ago, you could have what you want fitted but they didn't like it. Visibility is a small part of it, as having 5% on your side windows is a little too dark, I used to roll with 5% all round & 12% on the windscreen & on country roads at night you couldn't see sign posts that well out the side windows but wasn't that bad to be honest. Maybe if your 50+ with bad eye sight but I was only 18/19 at the time.
It was one copper from what I got told, he got on one about the whole dark window tinting thing after the shootings & started studying the window tinting law to bring about the current situation we have now. Lets face it having 35% on your front door glass is no problem, you would have to be blind not to be able to see out of that & if that's truly what the Police are saying then majority of people shouldn't be driving at all with eye sight that bad.
It's all about control & the fact that they want to be able to see who's driving that car. Lot's of, lets say less than desirable people had there windows extremely dark, people with no driving license, criminals etc. I used to know of a group who would drive around, three of them with no licence with only one holding a driving licence. When they would get pulled over they would quickly swap seats & coppers couldn't prove who was driving as they couldn't see. Nearly every person I knew in the underworld had dark tints & then some, so the Police just wanted a way to stop it & that's the situation we have now.