The goods lorry was on its side after crashing into a car, a blue Honda Stream.
Petroleum surveyor Arief Budiman, 28, and his friend saw two men trapped in its cab.
Fearing for the men's lives, they used their car to block oncoming traffic.
They then smashed the windscreen with an iron rod and pulled the driver and passenger to safety.
"It all happened so fast. I don't even know who passed me the iron rod," Mr Arief said.
The accident happened at the junction of Sengkang East and Buangkok East Drive at about 3pm yesterday.
Mr Arief was on his way to a friend's place when he saw the goods lorry and a car collide.
"We were several cars away from the traffic light. The lights had just turned green in our favour when bang," he told The New Paper.
The blue Honda spun, while the lorry landed on its side.
Mr Arief said: "My friend and I thought the people in the vehicles were done for. Seeing how badly the car was damaged, we wanted to help the people inside, but the driver of the car managed to come out with his son. They looked fine."
That was when the two men drove towards the lorry to try and get those in its cabin out.
"Someone handed us an iron bar, which we used to break the windscreen," he said.
Uber driver Adrien Wong, 44, who was nearby, said: "I saw a woman hugging a child in a yellow T-shirt, who looked shaken.
"From the looks of it, the woman was his mother and his father was the one who drove the car."
Police said they received a call about the incident at 3.03pm.
The Singapore Civil Defence Force sent a fire engine and two ambulances to the scene. Its spokesman said a boy was taken to the KK Women's and Children's Hospital, while a man was taken to Changi General Hospital. Both were conscious.
When the TNP team arrived at the scene at 4pm, the car and lorry were still there.
The driver of the car, who gave his name as Mr Chua, said he was travelling along Upper Serangoon Road.
"I stopped at the traffic junction. The lights had just turned green and when I started to drive, I felt a bang. I really didn't know what happened. The next thing I knew, the air bags were deployed and my car was damaged," he said.
The lorry was coming from the KPE towards Buangkok Drive, he added.
THREE-VEHICLE COLLISION AT UPPER SERANGOON ROAD
About an hour and a half after the accident at the junction of Sengkang East and Buangkok East Drive, there was another crash nearby.
It involved a car, a motorcycle and a lorry.
Mr Ronald Tay, who works in the IT industry, sent in pictures of the accident in front of St Paul's Church at Upper Serangoon Road.
Mr Tay told The New Paper that according to the driver and passengers of the lorry, both the car and motorcycle collided just when the lights turned green.
Police received a call at 4.27pm, informing them of the accident.
A Singapore Civil Defence Force spokesman said an ambulance was sent to the scene and the 27-year-old motorcyclist was taken to Tan Tock Seng Hospital.
He was conscious.
Police are investigating.
This article was first published on Dec 17, 2015.
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