Driver cannot remember fatal WA crash

A man driving a ute that hit and killed a motorcyclist had an undiagnosed neurological condition, causing him to have a seizure and be unaware of what happened, his lawyer has told a Perth court.

Jonathon Charles Murphy, 34, was riding his motorbike on the Mitchell Freeway in Perth during peak-hour traffic in January 2014 when he was killed, while another driver also ploughed into the concrete retaining wall.

Michael Robert Donovan is accused of dangerous driving causing death, failing to report the crash and failing to stop to render assistance, dragging the victim's motorbike about 800 metres along the freeway.

Other drivers had stopped to help Mr Murphy, who was rushed to hospital, but he died a few hours later as a result of multiple injuries.

Prosecutor David Jubb told the West Australian District Court on Monday Donovan was travelling 20km above the 80km/h speed limit before he hit Mr Murphy.

Mr Jubb said one witness saw Mr Murphy fly into the air as his motorcycle became jammed under the ute and sped up to about 130km/h to follow Donovan before losing sight of him.

A second witness allegedly sped up to 120km/h to catch up to Donovan but neither he, nor his passenger, could get the driver's attention so they noted his licence plate.

Mr Jubb said another witness saw the motorcycle being "spat out" from the ute.

Donovan was arrested at his Innaloo home and later told officers 45 times during an interview that he had no memory of what happened.

Defence lawyer Gerard Cleveland said it was a "tragic accident rather than a criminal act".

Mr Cleveland said his client had a neurological condition called an "arteriovenous malformation", which caused a seizure and made his actions involuntary.

"He was unconscious and unaware and therefore not driving," Mr Cleveland said.

"He had an honest and reasonable, but mistaken, belief in his fitness to drive."

The trial, which is being heard by a judge alone, continues.