A stoned P-plater was caught in dash-cam footage driving at 200km/h moments before he ploughed into another car and left an innocent woman to die.

Kayden James Lawson, 22, drove along a motorway on the NSW Central Coast so fast a truck he sped past shook, according to the truck driver, the Daily Mail reports.

"F**k me, he made the truck rock when he went past," the truck driver can be heard saying in dash-cam footage tendered at the Gosford District Court.

The footage then shows the green Holden Commodore Lawson was driving speed along the breakdown lane. Lawson then crashed into another vehicle and sent the innocent woman's car over a guardrail and 10 metres down an embankment.

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"He just killed someone I think," the truck driver could be heard saying to another truck driver further up the M1 on two-way radio.

The woman, Kristyn Rourke, suffered a permanent brain injury and a fractured neck. Her face was de-gloved, she lost her right ear, and she had a near "complete traumatic amputation" of her left arm, Daily Telegraph reported.

Rourke, then a 38-year-old senior property manager, was trapped in the wreckage of her Toyota Hilux for an hour in the November 2014 smash. She spent six months in hospital.

Lawson had tried to flee the scene on a skateboard, and stood about a kilometre up the road with his finger out to hitchhike.

The Toyota Hilux was shunted 10m off the side of a bridge on the M1 at Morisset. Photo / Twitter
The Toyota Hilux was shunted 10m off the side of a bridge on the M1 at Morisset. Photo / Twitter

Another motorist chased him, picked him up and did a U-turn to the crash site where others stood around the car to keep him from escaping.

The dash cam footage shows motorists leave their cars to assist and call emergency services.

Judge Mark Buscombe said Lawson 'giggled' when police spoke to him after the crash.

He told them, "It worked, it got rid of the car."

Lawson was taken to hospital suffering 'psychosis', the court heard on Friday.

He spent seven months as an involuntary mental health patient following the smash.

Judge Buscombe said Lawson had paranoid delusions.

Kayden Lawson has been jailed for two and a half years. Photo / Facebook
Kayden Lawson has been jailed for two and a half years. Photo / Facebook

Medical assessments found he had undiagnosed schizophrenia and was smoking three 'cones', also known as 'bongs', every day in the 12-months leading up to the crash.

Lawson was sentenced to four years in prison on Monday, with a non-parole period of two-and-a-half-years.

He had pleaded guilty to charges including dangerous driving occasioning grievous bodily harm with illicit drugs present in his system, and failing to stop to render assistance.

He will be eligible for parole in August 2019.

He was also disqualified from driving for five years.