Gregory Clive Griffith, 60, allegedly turned in front of a motorcyclist on Southern Cross Drive, Holt, on July 7, 2005.

The collision knocked the rider, Tommy Willis, 32, of Hall, off his bike and trapped him under the vehicle. He died in hospital about 10 days later. Griffith, who was uninjured, was breathalysed at the scene and was found to be over the limit.

In February 2006 he was charged with one count of negligent driving causing death.

In the ACT Supreme Court yesterday, Justice Hilary Penfold found Griffith was unfit to plead due to alcohol-related dementia.

She found he would not be able to understand the nature of the charge or his rights in relation to the jury, and he would not be able to follow the trial proceedings or instruct his lawyers. Griffith, who now lives in the dementia ward of a Canberra nursing home, had previously been assessed as fit to plead, as the legislation says that memory loss alone is not a ground for a finding of unfitness to plead. However, a neuropsychological assessment found he was not fit.

In September 2007 ACT Health psychiatrist Graham George found Griffith was fit to plead, even though he had no memory of the accident. Since then, Griffith had been assessed by a neuropsychologist, Tania Lioulios, who found in February this year that Griffith was unfit to plead.

Dr Lioulios's examination addressed the criteria of the Crimes Act that relates to fitness to plead.

She found Griffith suffered from an amnesic syndrome as a result of frontal lobe degeneration found in alcohol-related dementia, which affected ''encoding of new information as well as retrieval of past information''.

Justice Penfold accepted this finding. No further orders were made.