We don't have a curfew or a car curfew, but I completely get wanting the car home at a particular time. It's a lot of money and stress to offer car use to a young adult. I don't feel like its my place to put restrictions on a young adult's free time, but I certainly don't have to put my car at increased risk by having it out in DC at a club driving home at 3am.

Drivers are far more likely to be alcohol impaired during the night than during the day. In 2007, 36% of drivers involved in nighttime fatal crashes were legally drunk versus just 9% during daytime collisions.

Approximately 75% of fatal crashes occurring between midnight and 3 a.m. involve alcohol.

I'm the PP that said drunk drivers.

Statistics show MORE drunk drivers are out at night vs the daytime. Sleepy drivers, too.

I have a rule the car has to be parked at 2AM...here, there, anywhere. No driving after 2. Its partly to protect my asset (the car) and her safety.

http://www.forbes.com/2009/01/21/car-accident-times-forbeslife-cx_he_0121driving.html

Dont know if it was the same PP that said drunk drivers in response to the question about the car curfew, That still doesnt make much sense. You are going with assumption that drunk drivers are only out after midnight.
You are ok with your au pair being out in someone elses car after midnight,or walking home from the metro as long as your car is safe at home in the garage?

Pp-one more thing- it's easier to lighten your rules later on then put more in place.

We require our ap to be home 8 hours before she works the next day. No curfew if she's not working, but we ask that she tell us if she is not coming home (we are clear that if this decision is made at 2am a text is fine). We don't need to know where she is or what she is doing but do want to make sure she's safe.