Preventing a rock chip from turning into a crack in you windshield is a matter of taking care of the damage fast.
If you hear a rock hit your windshield and don't see any damage, inspect the edges of you windshield for any missing glass or impact point. These types of damage requires immediate attetion if you want to save you windshield. The edge of the windshield is a high stress area and has no place to go except to the less stressful area of the windshield.
That is why the damage becomes a crack! Most people don't realize they have the damage and suddenly they see a long crack!
