The 50-year old mother of four has never been in trouble with the law, has never fallen from her bike, and thought it ridiculous she could not ride at 15km/h on a dedicated cycleway with an uncovered head.

A police video of the incident last year records the sergeant surmising "it's a hair thing", but Ms Abbott says it's nothing to do with caring for her exuberant hair.

Her objections to the law are based on her belief that wearing a helmet increases the risk of brain damage - and that forcing her to wear one is a breach of her civil liberties.

When she tried that argument in the Scone local court, the magistrate would have none of it. He found her guilty and fined her $50 plus costs.

But when she appealed in the District Court in March, she went a long way to convince the judge that, 19 years after the laws came into force, there is still no clear evidence of their public health benefit.

Ms Abbott argued that if she fell from her bike while wearing a helmet she would be at greater risk of brain damage from "diffuse external injury", an injury similar to shaken baby syndrome, than if she fell on her bare head.

It may seem ridiculous to suggest helmets could do anything other than improve one's chances in an accident and reduce brain injury levels in the community, but there is a serious debate under way on the subject in international medical and transport safety journals - and Judge Roy Ellis was happy to admit his own doubts about the laws.

He found Ms Abbott had "an honestly held and not unreasonable belief as to the danger associated with the use of a helmet by cyclists", and quashed her conviction, although still found her offence proved. SMH