Couldda just followed them (at a distance) and called the cops. Then you don't worry about getting yourself in trouble and drunk kid that hasn't had proper home training gets in trouble. Win win.
I don't know what I'd do. It also depends on what car I'm driving as to what I'd do. If I was in my zetec, I'd let it go, if I was in my svt, I'd have to size up the situation and decide, if I was in the escort, I'd jump in front of them and slam on the brakes (it needs some body work/paint job).
