Seattle garbage truck driver Jeff Blackburn saw the whole thing unfold in seconds.
He was behind the wheel of a CleanScapes truck on his Queen Anne route May 31, 2012 when he saw a woman jogging with a baby in a stroller.
"I noticed as I was coming around the corner that a lady was just walking away from her stroller," Blackburn told KOMO news in Seattle.
The mother turned the stroller sideways at the top of Bigelow Avenue, and Blackburn saw her start walking toward a group of other women around the corner, but the stroller - with baby still on board - started rolling down the hill.
Blackburn acted quickly and intelligently to the unfolding crisis.
"So I started honking the horn and speeding up so I could catch up to it before it got to the intersection, because at the bottom of the hill was a busy intersection with stop signs."
Blackburn stopped his truck in the middle of the intersection, to block traffic, then jumped from the truck, and ran to save the baby.
"Luckily when I was honking the horn, a FedEx truck was going through the intersection and stopped because he heard the horn honking."
Blackburn and another woman stopped the stroller. He says the little boy was smiling when he got to the him. But the mother, who ran up seconds later, was not amused.
"She came running in -- she was freaked out and was shaking and... I don't even think she said anything, she just grabbed the kid and ran away," Blackburn said. "She was just really, really shook up."
The whole incident was captured by a video camera in Blackburn's truck.