Heart-stopping footage shows the buggy rolling down a sloped driveway during a windy spell in Hesperia, California, as its guardian unloads her car.
The pram with the baby boy inside was caught by a gust of desert wind and began rolling towards speeding traffic.
The woman, believed to be the baby’s great-aunt, was seen trying to stop the tragedy from unfolding but she soon stumbles and falls to the ground, as the pushchair careers toward an adjoining four-line highway.
In the rush to get back to her feet, she broke her right knee, forcing her to scream out helplessly.
The woman had been looking after her great-nephew when a gust of wind set the pram rolling towards a main road in the city of Hesperia, northeast of Los Angeles.
A man is seen dashing over and grabbing the pram just a few feet before it would have reached the highway.
He was sitting on a bench nearby when he saw the incident begin to unfold.
He was named by NBC Los Angeles as Ron Nessman, who happened to be walking back from a job interview at a nearby Applebee’s restaurant.
Mr Nessman told Live5news: ‘I knew I could get it and I got it and I’m thankful for that because I really wouldn’t want to see the end result if I wasn’t there.
‘She tried everything she possibly could to get up. Her knees were bleeding when I got up to her. She was still shocked and she was crying.’
His sister, Donna Gunderson, who witnessed the drama from a nearby patio, added: ‘I heard screaming to the right. And I look back and I see a stroller going down the driveway and my heart dropped. The cars do 50-55 and it was a busy time of day.
’The incident should serve as a lesson to lock the wheels on a pram, she added, particularly during high winds.
Mr Nessman, who had recently moved in with Ms Gunderson after being homeless for about eight years, said he is still looking for a job and thanked his sister for helping him ‘get right’.
