I think the best guideline I ever heard was where the car had a curfew. Because more teens get into accidents after a certain hour (I can't remember the exact hour) my friend's parents wouldn't allow the car out after that hour. She could be out herself but not with the car.
I personally never got the car because we were pretty cash strapped & my Mom could not afford to replace/fix the car I had an accident. Plus the extra cost of insurance was outrageous.
Surviving sleep deprivation one day at a time with dd (Oct '11) & ds (Oct '08).
When I got my driver's license as a teen, my parents' car had a curfew! It seemed reasonable to me.
I was allowed to take the car if I asked in advance and if my having it didn't interfere with my parents' plans. I rarely drove myself to school, unless I had some huge item to transport, or it was exam time and I was only required to be there for half the day. I probably had the car, on average, once a week or so to go to friends' houses in the evenings.
Another condition on car access was that I had to run errands for my parents, on demand.
When I got my licence I was allowed to have the family car whenever I wanted to have it with all the family responsibilities that go with it. I had to wake up extra early to drive my dad to the train station. Had to take my siblings to their extra-curricular activities and if there were groceries or other similar errands they had to be done also. It was NO FUN having that car. Within 2 months I was back to the bus and being carless except on special occasions when I wanted to do something specific.
My father gave me a car when I turned 16 - after he taught me how to change tires, spark plugs, oil, etc. My mother really didn't care (I lived with my mom) but it just fell into place that I'd take my siblings to school and pick them up after. I mean, I was never asked to, but if I was going that way *anyway* then it was obvious.