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 sentenced to a year's jail on Monday (Nov 25).

Businessman Lim Chai Heng, 56, who was expressionless on hearing the sentence, was also banned from driving for 12 years.

Judicial Commissioner Vincent Hoong said Lim's culpability was reduced because he was suffering from acute psychosis at the time, which impaired his judgment and caused him to discount the risks of his actions.

Psychosis is a mental condition in which one loses touch with reality.

The judge noted that while Lim appeared to be in control of his car, his actions were irrational - he was sending his son to work but instead exceeded the destination by 23km after telling his son that "God today won't let you go to work".

Deterrence is of limited weight in this case, the judge said, noting that a person with impaired judgment is not able to weigh consequences before committing an offence.

The prosecution, which had asked for at least two years' jail, sought a stay on Lim's sentence, pending a decision on whether it would be appealing.

Lim pleaded guilty last month to a charge of causing death by a rash act, which carries a maximum sentence of five years' jail and a fine.

Another four charges - three for causing grievous hurt and one for causing hurt - were considered in sentencing.

Lim was originally charged with causing death by dangerous driving. The charge was upgraded to culpable homicide before it was amended to the current charge.

On the morning of Dec 19, 2016, Lim was driving his son from their Hougang home to his new workplace in Depot Road.

Along the Central Expressway, 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 into 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, which became too narrow for his car to pass through.

He then made a three-point turn and drove against the flow of traffic as oncoming vehicles avoided him.

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

As Lim did so, motorist Tan Han Boon, now 37, swerved, causing his Mazda 6 to collide with a bus before hitting a concrete wall.

Lim's car then collided 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 slamming against the wall.

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

The Mercedes finally came to a 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.

A psychiatric assessment found that Lim was suffering from acute psychosis at the time, but the underlying cause remains unclear.

In July, changes were made to the Road Traffic Act to enhance penalties for reckless driving and careless driving.