As salty pointed out and I forgot to mention (besides the advancements in plastics and vinyl), a majority of UV rays are already blocked by the glass used in automobiles today. Obviously we can't see UV rays, so how can you tell if the glass is actually blocking UV? The next time you run into someone that wears eyeglasses with Transitions lenses, ask them how well it works in their car. The answer you will get is DIDDLY SQUAT.
Many people, including myself, who bought into the hype of Transitions lenses were misled by the marketing of the product. Glasses can get expensive and cumbersome so if it can replace having an extra pair of prescription sunglasses in the car specifically for driving, it's worth paying for the premium right? Wrong! Transitions lenses do not work based on how bright or light it is how most people are led to believe. The dyes within the lense that makes it darken reacts to UV light. It works as advertised when you're outdoors, but when you're driving with your windows rolled up, your lenses will never get dark at all.
In other words, your car's glass is already blocking a majority (if not all) of UV rays.