Disciplining Other People's Children

Last week, as I was coming down our street towards the stop sign, I saw a boy, maybe about 10 or 11, riding down the sidewalk on his skateboard.  He had no helmet, no arm or knee pads and was on his cell phone.  He was going really fast and I had a feeling he didn't see me in our van.  Sure enough, when I was about 20 feet or so from the stop sign, he flew down a driveway and right in front of my car.  He never looked back, he just went right into the middle of the street, in front of me, to the left and then back in front of my car to the right.  When he went past my car a second time, he hit the curb and fell down.  The whole time, the boy never stopped talking on his cell phone.  After I made sure he was okay, I said, "do you know you rode your bike right in front of my car?"  He was like, "I'm sorry."  I thought he might cuss me out or tell me, "hey, you're not my mother!"  He knew what he did was wrong, I guess.  It made me think back to when I used to live on my old street in Maryland.  The street only had 24 houses and it was a dead-end street.  Right at the very end of our street, there was a family with three little kids.  The oldest was a boy and I don't know if he was ever told the word "no."  He rode his bike one time in front of my car one too many times, so I finally talked to his mom.  I told her, "you really should watch him better, he rides his bike in front of cars!"  I don't think the mom liked that too much, for pretty much the rest of the time they lived on our street, if she saw me, she would turn her back to me and pretend like I didn't exist.  I guess she didn't appreciate the fact I was attempting to discipline him.  I was actually just looking out for his safety.  When Lauren was little, like 2-4 years old, I think my unofficial nickname was the "playground police."  If I saw someone cutting Lauren off at the slide or felt she was being treated unfairly, I was right up in the kid's face. I was such a "helicopter" mom in those early years!!  When do you feel it's appropriate to discipline another parent's kid?  never? sometimes? always? I would love to hear!