SINGAPORE - A case of road aggression cost a motorcyclist his life and his female pillion rider her left leg.

The accident happened two years ago when the rider engaged a motorist travelling at high speeds along a road in Woodlands, a coroner's court heard.

On Tuesday (July 25), State Coroner Marvin Bay found Mr Jason Tan Jia Sheng's death to be a "tragic traffic misadventure'' after a two-day inquest into his death on Aug 23, 2015 .

"The evidence is unequivocal that both vehicles had travelled at paces in an aroused or agitated frame of mind," said Coroner Bay. "Notwithstanding this, it is plain that the collision was not a malicious or deliberate act on the part of either party."

Mr Tan, 22, was among a group of motorcyclists travelling together that night.

At the same time, Mr Ivan Chew Zong Xian, 29, was driving home in his Lexus car with his wife. He spotted the group of motorcyclists at the traffic light junction of Gambas Avenue and Sembawang Avenue.

Two motorcycles overtook him while the rest remained at the back of his car. All the vehicles turned left into Woodlands Avenue 12 towards the direction of Seletar Expressway.

Mr Chew said he was travelling at 80 to 90kmh when he decided to filter from the leftmost lane to the centre lane. Mr Tan overtook him at a much faster speed on the rightmost lane.

Both vehicles then made a right turn at the junction of Woodlands Avenue 5, and stopped at the red lights at the next junction.

While driving towards Woodlands Avenue 9, Mr Chew saw from the rear view mirror Mr Tan, who was initially on the first right-turn only lane, travel forward in his direction.

Mr Tan allegedly switched to the left lane, overtook him from the left side of his car, and then rode near the front of his car. Mr Chew claimed that the pillion rider, aged 23, turned to look at him and stuck out her palm, facing upwards. He interpreted this gesture to be a challenge, as if to ask "what are you doing?''

Mr Tan then applied the brakes, causing Mr Chew to also brake. As soon as Mr Tan moved to the right, Mr Chew picked up speed to break away from Mr Tan's motorcycle, purportedly to avoid engaging in any confrontation.

Mr Tan also picked up speed and overtook Mr Chew from the extreme right lane by veering to the left.

Mr Chew recalled the motorcycle moving towards the right side mirror of his car on two occasions.

He alleged that he was pressured by Mr Tan's actions to veer his car to the left to avoid a collision with Mr Tan.

While Mr Tan was near the car's right rear wheel, Mr Chew was purportedly concerned that his car was veering too much to the left. So he manoeuvred back to the right and released the accelerator pedal, without braking, to prevent his car from spinning.

After regaining control of his car and straightening it, he proceed at 60 to 70kmh. When he looked to his right mirror, he saw Mr Tan's headlamp less than a half car length behind, and heard a loud sound on his rear right bumper, but did not feel any significant impact.

One of the other motorcyclists, Mr Sim Beng Hui, who is Mr Tan's friend, had a differing account.

According to him, Mr Chew appeared to have been trying to force Mr Tan off the road.

He said he saw Mr Tan looking at Mr Chew when their vehicles were at the junction of Woodlands Avenue 12 and Avenue 5. When the light turned green, both Mr Chew and Mr Tan picked up speed and went straight towards Woodlands Avenue 9.

Mr Sim was initially puzzled at Mr Tan's course as he was supposed to turn right to head to a friend's place in Woodlands to "relax and chill''.

But he followed suit and rode straight. He saw Mr Tan, who was 100m ahead, moving from right to the centre and then to the leftmost lane, behind Mr Chew's car.

After both vehicles passed the intersection, Mr Sim saw the car swerve from the centre to the left lane at a fast speed, applied its brakes for a moment, and swerve back to the centre lane, while Mr Tan was behind the car. It looked like Mr Chew was trying to force Mr Tan off the road, said Mr Sim.

Immediately after the swerving action, Mr Tan's motorcycle flipped upwards into the air, Both Mr Tan and his pillion were flung off the machine. Mr Tan was subsequently pronounced dead in hospital at 4.16am while the pillion rider suffered a complete below knee amputation of her left lower limb. She also had multiple fractures and serious head injuries and was discharged on Jan 22, 2016.

Coroner Bay said in his findings that it would be apparent that Mr Chew and Mr Tan had "built up a significant level of animosity'' at the junction.

The collision occurred on the Lexus right rear when Mr Chew swerved to the leftmost lane before coming back to the centre lane.

The contact had propelled the motorcycle left, where it had skidded and toppled at the road kerb.

He said while there was no expert consensus on this aspect, it appears more likely that Mr Chew did in fact momentarily apply his brakes, before the collision.

"This case does call to attention the fact that road users should exercise courtesy, restraint and sound judgment even when they face challenging circumstances, including encounters with inept or inconsiderate motorists, on the road,'' he said.

"This sad case underscores the high cost of belligerence and aggression, which can escalate to a 'contest of wills' between assertive and competitive motorists, inspiring heedless and unrestrained behaviour that can yield a tragic outcome.''

The Attorney-General's Chambers will study the findings before deciding what course of action, if any, to take.