Just because you're traveling in a air-conditioned car, doesn't mean your protected from the sun's harmful UV rays.
Glass blocks UV-B rays, the kind that cause sunburn. However, unless it's specially coated or tinted, glass does not block UV-A rays, the rays that penetrate deeper and can cause skin damage, possibly even skin cancer. Since you don't burn through the glass you don't realize anything is going on. In short, you don't get burned, but the UV-A rays damage your skin without your knowing it for a long time. In fact, some specialists advise you to wear sunscreen if you're going to be driving in most of North America for more than about 20 minutes, and less than 20 minutes in the southern states.
So, next time you grab your car keys don't forget to grab the SPF-30.
"Vitamin D and Window Light" (Consumer Reports on Health)