Mr Chris Eng and his colleague were on their way to a car workshop yesterday morning when they came across an accident scene.
There were badly mangled vehicles, bloodied victims and metal debris scattered over the Ayer Rajah Expressway.
"It was so messy... There were so many (injured) people around," said Mr Eng, 31, a claims adviser, who was still in shock 10 hours after the accident, in an interview with The New Paper yesterday evening.
A private bus, a Malaysian registered van and a white BMW sedan had collided on the AYE, in the direction of the city before Clementi Avenue 6, at around 7.30am yesterday.
The front of the van was crushed and its windscreen was shattered.
The BMW had crashed into a railing and its rear was also badly damaged.
The private bus, which was parked a distance ahead of the van and the car, had a few scratches.
TNP understands that the car driver had stopped for a while near a bus stop to pick up two people when the accident happened.
A Singapore Civil Defence Force (SCDF) spokesman said it was alerted to the accident at 7.30am yesterday.
There were 12 casualties - 11 men and a woman in her 40s, the spokesman said.
Mr Eng, who said he was the first on the scene, stopped to help the casualties and call for emergency services.
He said: "All 12 people had blood on them. I was shocked that so many people were injured.
"The driver of the BMW looked very helpless and shaken (by the accident). He was conscious, but told me that he felt giddy."
He saw a man comforting the injured woman while waiting for help to arrive.
She was taken conscious to the National University Hospital (NUH) with a fractured left leg and head injuries, the SCDF spokesman said.
SCDF officers also used hydraulic rescue tools to extricate a man trapped in the van.
He was taken to NUH together with three other men, said the spokesman.
Seven other men, aged between 17 and 20 years old, were taken to Ng Teng Fong General Hospital with minor injuries.
Among those injured was a group from Malaysia who had rented a van for a day trip here, Lianhe Wanbao reported.
The 61-year-old private bus driver involved in the accident was not injured, but is still recovering from the trauma.
His son, Mr Sabri Ibrahim, 32, told TNP: "(My father) is still in shock... He has been a private bus driver for more than 10 years, but this is the first time he has been involved in such a big accident."
Mr Sabri, who works in the information technology industry, added that his father is assisting with police investigations. He declined to reveal any more about the accident.
Investigations are ongoing, a police spokesman said.