Concrete slabs were sent flying when a trailer truck fell on its side on the Pan-Island Expressway (PIE) yesterday afternoon.

One slab hit a van, crushing it, while another was flung across the centre divider to the other side of the highway.

The result: collisions on both sides, causing massive traffic snarls in both directions during the evening rush. Traffic on the highway eased only at about 11.30pm.

The trailer flipped sideways just before the Stevens Road exit in the direction of Changi Airport. The police were alerted to the accident at 4.55pm.

One of the five concrete slabs, each measuring about 2m by 2m, landed on the rear of a van in front of the truck.

A taxi, which was behind the truck, was hit by its trailer.

The van driver complained of back aches and was taken to the National University Hospital (NUH).

The slab that was thrown across to the opposite side of the expressway almost hit a car. Its driver, MrLawrence Ng, 47, swerved to avoid it.

'There was no time to think,' said the National University of Singapore professor. 'I just trusted my instincts and swerved.'

His car collided with another car and a taxi. A woman in the other car was taken to NUH after she complained of pain in her left elbow and left thigh. She also suffered an asthma attack due to the shock.

SMRT taxi driver Ng Tian Eng, 53, whose cab was behind the trailer, said he saw it swerving from left to right moments before the accident.

The rear of the trailer then hit the left side of his cab. MrNg braked hard, but his taxi still ran over the road divider.

'If I had not slammed on the brake, I would have died,' he said.

The four Indian tourists in his cab, who were on the way to the airport, were stunned. One suffered minor cuts on her arms from broken glass.

Mr Ng flagged down another cab for his passengers, who paid him the $8 fare before they left.

The driver of the trailer truck, who looked to be in his 20s, was in a daze, said MrNg.

'I scolded him, 'Are you trying to kill us?'' he said.

The driver was later seen puffing on a cigarette. There were cuts on his elbows.

The police did not release his name and said investigations are in progress.