I would not allow them to drive the car. You drive the car and then at whatever time they are finished go pick them up. If they call at 4 a.m. so be it. It is so much better to be a bit inconvenienced then to have them drive. I am not saying that either one would even drink, but you just never know who will be these days. Just do the driving. Otherwise I am sure that you raised a very nice daughter and she will be able to handle herself.
My niece is 17, and that's sort of what they did. Her female friend's father has a restored antique car (even by my standards, a pre-WW-II vehicle) The two couples were driven to and from the prom in that vehicle, by the friend's father. Even though I live about 800 miles from them, I've met the friend, as she came down with my brother's family (and my mother) to attend my daughter's wedding last summer, I took the girls and my nephew to Six Flags in Arlington Texas.