Just some other stuff that you didn't ask about, but are my rules: DD's share a car. The are required to work out who has the car when. I will intervene as a last resort. If both have a place to be (ex. DD1 has work, and DD2 has a clarinet lesson) at overlapping times DH will drive me to work and one girl will get my car for the day.
Rules: They must text/call me where they are when they get there. (Exceptions include getting to school or a five minute drive in our neighborhood.) During car rides their phones stay in their purses or the glove compartments. If they have a friend in the car the friend is allowed to hold onto the phone. We have had a lot of talks on car safety. DD1 has been the designated driver before. (With family, not friends
As for with my dd's: I simply sat them down once they were able to drive and told them. They can call anytime and someone
will come and get them. (DD2 used this once - for reasons that had nothing to do with friends. DD1 never has.) I also extend the same courtesy to her friends - if they are with her and they all need a ride she can call me and I'll come get them. No questions asked, although there may be a conversation or two thrown in there depending on why she was picked up. If their friends call them and they need rides - well, that we have to talk about depending on the situation. But I would rather DD's prevent someone from driving drunk then letting them get behind a wheel.
And yes, teens really do count on each other for rides. DD2 has had friends pick her up from work before, has driven friends home countless times. Honestly - it's easier for both the parents and the kids. DD2 and her friend have taken to having a seven person carpool for play practice, (it's a twenty minute drive one way.) and it is so much easier then every single kid either getting a ride from a parent or driving separately.
In our state you have to have your license for more then a year before you can drive more then one person who isn't family. That rule gets broken all the time (I know DD does it) but she didn't until she had been driving for what I deemed was a safe amount of time.
As a side note: almost four years ago three boys ran to Walgreens for some drinks and snacks. It was a three minute drive. A man running from the cops t-boned their car. The passenger was killed instantly, the boy in the back seat walked away with minimal broken bones and the driver was in a coma for three weeks and is still
slowly recovering. We teach defensive driving here and all of our kids know that even if it may not be them who is creating an accident they may still be involved in it. When you get around to having this talk, you need to remind your kids that even if they are driving safe they're not invincible.