Driving on the Highway.  

All the time. I know of at least 20 people that this has happened to. I also knew one lady where a rock cracked her windshield, she got it repaired and within a week she had another rock crack her windshield (from the same highway). I have a little crack in my windshield, but I'm not sure what caused it. Also, my parents regularly have to replace their windshields (it seems) due to rocks from big rigs (18 wheelers).

Here in my state (Massachusetts, USA) most insurance carriers have what is called Comprehensive, which includes glass coverage. If your windshield is cracked or broken due to vandalism, nature, etc they will replace it free of charge to you. It costs about $100 added to your coverage every 6 months (depending on carrier).

For a moment, forget about the car, Imagine you riding a bike & a stones of some sort flying toward you!! This happened with me many at-times & lucky that it didn't hit me on my head as i usually don't wear Helmets!!

I did have this happen to me years ago with my new to me, used car. I was on the freeway when sudennly, ping, broken spot on the windshield. The insurance company wanted those little rock cracks to be taken care of right away. They said it was cheaper to take care of it when it was little like that because they could just fix the spot and not have to replace the windshield. And it really worked too which I was amazed and thought that was really cool!