At first, by the title alone of this question alone, I thought it was April 1.
But then I read the question, and although I'm not sure whether it's on-topic here, it's actually a very good question.
I can't give you an absolute numeric answer, but I can hopefully provide sufficient information to help.
Front windshields in most commercial vehicles allow very little UV light to pass through. That's why most eyeglasses that get dark when it's bright outside will not work within vehicles (there are new optical technologies that don't rely on UV light exposure that will work).
I'm not sure about side-windows, but light exposure through them is much less than front windshield due to the reduce area involved and the fact that they are more vertical than the front windshield. Anecdotally, I've traveled many hours in direct sunlight with all vehicle windows closed and I don't remember having any sunburn, so my "non-scientific single person study" indicates that side windows likey block most UV light transmission as well.
Of course, opening windows or a sunroof will expose you to more UV light.