You may think that the sun can't harm you while riding in a car. Think again. While you won't get sunburned while riding in an enclosed vehicle, the sun can cause damage deep within the skin.
The glass found in automobiles blocks UV-B rays, and those are the ones that cause sunburn. However, unless it's specially coated or tinted, glass does not block UV-A rays; those are 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 twenty minutes, and even less than twenty in the southern states.
To really prevent this type of skin damage, it's best to apply sunscreen that blocks both UV-A AND UV-B for maximum protection while driving.