I think that each state may have they're own laws about tinted lights. They shouldn't be dangerous, I used to own an 89GT and I had the GTS taillight covers on and the lights shine through pretty brightly at night and during braking.
Here's a pic of a white stang with spray on tinted lenses...
I don't know about illegal, but i think they have to be able to be seen from soo far away. and i have seen several with blacked out tails that are really easy to see at night.
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Yes, they are very illegal. Rear stops lamps must be clean and obstruction free. Also, the red rear reflectors must be clearly visibly.
You can get a ticket for tinted/smoked taillights. Any covering, no matter how light, should cause the vehicle to fail inspection.
More importantly, if you are ever rear ended, you can be found partially responsible if the other person is savvy enough to point out your taillights and claim they couldn't see them. You would get a ticket for defective safety equipment and the accident could be partially your fault.
very illegal, changing how a lens will output light in anyway is illegal... with that said my HID's are illegal... but i can see and thats all i care about