SINGAPORE - The driver of a Mercedes-Benz that barrelled down the Ayer Rajah Expressway (AYE) against the flow of traffic nearly three years ago, leaving one dead and four injured, was suffering from acute psychosis at the time, the High Court heard on Monday (Oct 14).

The mental condition "significantly impaired" the judgment of businessman Lim Chai Heng, 56, leading him to discount the risks of his actions, despite signs of danger and warnings from his son, who was in the car with him, stated an Institute of Mental Health (IMH) report.

Lim told an IMH psychiatrist that when he saw cars and motorcycles coming towards him, he "knew finished already" and that "if not others hit (him), (he) will hit others".

The underlying cause of the psychosis remains unclear.

Lim pleaded guilty on Monday to a charge of causing death by a rash act, an offence that carries a maximum sentence of five years' jail and a fine.

Prosecutors sought at least two years' jail and a 12-year driving ban, while the defence asked for five to seven months' jail and an eight-year ban.

Another four charges - three for causing grievous hurt and one for causing hurt - will be taken into consideration when he is sentenced at a later date.

Lim was originally charged with causing death by dangerous driving.

The charge was upgraded to culpable homicide in January 2017 before it was amended to the current charge.

The court heard that on the morning of Dec 19, 2016, Lim was driving his son from their Hougang home to drop him off at his new workplace in Depot Road.

Along the Central Expressway (CTE), when Lim drove past the Braddell Road exit, his son asked why he did not take the exit to avoid paying Electronic Road Pricing charges.

Lim replied: "Do not be afraid, I know the way."

But instead of getting out at the Jalan Bukit Merah exit, which was the closest to the destination, Lim continued driving onto the AYE towards Tuas.

Ignoring his son's repeated requests to exit the AYE, Lim skipped another 14 exits and accused the younger man of not trusting him.

After travelling about 23km, Lim reached the Tuas Checkpoint and drove into the motorcycle lane.

After stopping for two minutes, he made a three-point turn before driving back against the flow of traffic. He merged into the car lane and continued driving as oncoming vehicles avoided him.

When he entered the AYE, he accelerated to a speed of 126kmh to 147kmh, speeding down the slope towards the Tuas West underpass.

Meanwhile, Mazda 6 driver Tan Han Boon, now 37, on seeing a vehicle in front of him swerve to avoid Lim's car, also did the same.

Mr Tan's car collided with a bus before hitting a concrete wall.

Lim's car, which was going at more than 137kmh, then collided head-on with a Toyota Vios driven by actor Jackie Liong Kuo Hwa, 38, whose wife Venny Oliver, now 40, was in the front passenger seat.

The impact sent the Toyota tilting to a vertical position and slamming against the wall.

Lim's car then crashed into a scooter, causing rider Teh Tze Yong and his pillion-riding wife Choo Yat Chiam, both now 37, to be flung from the vehicle.

The Mercedes finally came to halt after hitting the wall.

Mr Liong died at the scene from multiple injuries. Ms Oliver, Mr Teh and Ms Choo suffered multiple fractures, while Mr Tan suffered multiple abrasions.

No mechanical defects were found on Lim's car.

The bus cost more than $27,000 to repair, while the other three vehicles were scrapped. The damaged wall cladding on the AYE was repaired at a cost of more than $1,300.

Referring to sentencing guidelines for the offence, Deputy Public Prosecutor Kumaresan Gohulabalan argued that but for Lim's mental condition, he would be facing a starting point jail term of at least 4½ years.

Lim displayed an extremely high degree of rashness, amounting to blatant disregard for human life, caused more harm than would ordinarily have been expected of the offence, and breached multiple traffic regulations, the DPP argued.

The DPP said Lim should be jailed at least two years, after adjusting the starting point downwards to take into account his mental condition and applying a discount for his guilty plea.

Defence counsel Yusfiyanto Yatiman disagreed with the DPP's approach in arriving at the appropriate sentence.

He argued that Lim was remorseful and a first offender, and that his acute psychosis had diluted his culpability.