Stopped traffic. A couple of decades ago, the only road from Silicon Valley to Skyline in the hills was blocked by construction equipment repairing a washout. My wife got out and walked up the line to see what was going on. She started talking to a man standing there. Turns out he was a freelance developer and my wife said, "My son knows how to program.", which lead to his firs job as a programmer in high school. In this case a connection made in stopped traffic.
Is that seriously it? Like people looking out the window? I spent 4 years in LA traffic and have never heard of that. It's definitely plausible though, now that I think of it.
Lived there for a decade, and recall a time that we literally turned off our cars and met random people in the jam. I think it was during the trend of people jumping off of overpasses. There's also the random attractive person that cuts you off, flips the bird, etc. you just can't get out of your mind.
I got punched by a guy at a stop light in Atlanta. Practically the same thing.
Context: I yelled at him for trying to turn left at the wrong time. He caught up to me a couple lights later, walked down the median and popped me through my open window, then ran away. I hadn't said anything bad, just "I have a green light," but I think he misinterpreted my pointing.