Op - I'm so sorry you had to witness this - regardless of who was at fault.
When I was 17 DH and I (he was my BF then - and 20 yo at the time) were driving home one night (Oct 6, 1996) and the road split - they had altered traffic in order to rebuild a bridge. Traffic went off to the left and curved around - in between jersey walls and with little lighting in the area. There was what looked like a trashbag on the ground. I slowed down, but being between jersey walls there was no where to go - and there was a car behind me. With hardly any lighting we didn't realize what it was until we were right up on it - no, him. It was a man laying in the middle of the road - big guy with a black leather jacket on (that looked like a trash bag from a distance). We hit him then the guy behind us hit him, and then hit us. When we got out. The guy behind us had no idea why we stopped. DH said "We just hit a guy - he's under your truck." The other guy just turned around and looked and sat back in his truck trying to figure out what had happened.
Turns out the guy was on a motorcycle - drunk. He was seen by a witness leaving a bar and doing tricks on the bike - feet on the handle bars, etc. When he fell off, the bike kept going due to the jersey walls. A witness in front of him saw him fall off, but couldn't stop because the bike kept coming towards him - bouncing off the jersey walls. Once the bike finally came to a stop against the guard rail, the guy pulled over, and was walking back. That's when we came along.
DH went back to check on him and he was still breathing. He said the alcohol smell was very strong. When the ambulance came and brought the guy past my truck to get to the ambulance, DH forced my head down so I wouldn't see. Thank God he did. He still says he wishes he hadn't gone back there. The guy died on the operating table.
We later found out that he had his truck taken by the police the day before for drunk driving. I think about that accident often. The only thing that kept me sane was knowing he was drunk. He did this to me - I didn't do it to him. And thank God no one else was hurt or even worse - killed.
So another example of an accident with a teenager and a motorcyle. In this case the motorcyclist was at fault. I'm sure there are plenty of accidents where the teen is at fault. There are plenty where the motorcyclist is at fault. Regardless, the lives of these teens and the kids that belong to the couple that were killed have been altered forever and my thoughts and prayers go out to them, as well as the Op who had to witness this tragic event.