A drugged motorist who ran a red light and struck a schoolboy has been cleared of dangerous driving by an Adelaide judge, after two expert witnesses suggested methylamphetamine can have a positive effect on driver ability.

In 2012, 39-year-old mother of three Leah Lenarczyk was on ice and other drugs when she ran a red light at Salisbury Heights in Adelaide's north, hitting a schoolboy.

The 12-year-old suffered multiple injuries, including a collapsed lung and a fractured skull.

Lenarczyk was acquitted of driving dangerously and convicted of the lesser charge of driving without due care.

Judge Barry Beazley said the prosecution had not proven methylamphetamine, also known as ice, had affected her driving.

"I am positively satisfied that the accused was not so affected by methylamphetamine that she was incapable of effective control of her motor vehicle," Judge Beazley said.

"There was no evidence in the driving leading up to the collision of any overtly poor driving, veering across the road, high speed or otherwise.

"She had none of the indicia or symptoms of someone affected by methylamphetamine."

South Australian Police and the Motor Accident Commission called a joint press conference in response to the case, reissuing their message that drug driving is dangerous.

Assistant Police Commissioner Linda Williams said driving under the influence of drugs is illegal and dangerous and there may be a need to strengthen drug driving laws.

The Motor Accident Commission's Michael Cornish said he was concerned by the prevalence of drug driving.

"Last year 24 per cent of people killed on our roads had some type of illegal drug in their system at the time of their death and that is a serious concern to us," he said.

"We just want to reaffirm today that drug driving is illegal, drug driving does affect your behaviour on the road and increases the risk not only to yourself but to the community."

The court heard Lenarczyk's three children, aged six, eight and 10, were in the car at the time as she had just picked them up from school.

She testified the light was green at the time of the collision and the cars around her were travelling at the same speed.

"I was picking up my kids from school, taking them home and driving normal speed and it was just there," she told the court.

"The lights were green, [he] just stepped out in front of the car that was next to me.

"I had no time to brake, nothing … the young boy stepped out in front of the car next to me and I didn't have any time to brake at all."

But Judge Beazley concluded the light had just turned red, given evidence from other witnesses.

Accused showed no negative side effects of drugs

In making his decision, Judge Beazley also took into account two expert witnesses who claimed ice, while illegal, could have some positive effects on driver alertness, depending on its concentration levels.

Professor Jason Mark White from the University of South Australia told the court the effects of ice included physiological changes such as increased heart rate, body temperature and blood pressure.

But he also noted that fatigue was diminished by ice and that both mental and physical performance may be improved as a result of taking the drug.

The second expert witness, Dr Michael Robertson, said when tested for alcohol following the accident the accused was found to be negative.

Dr Robertson agreed that ice's "positive" effects, when consumed, included improved alertness and concentration, as contrasted with the effects of alcohol.

He explained that the "negative" effects included sweating, dilated pupils, elevated blood pressure, nausea, restlessness and confusion, and even psychosis at high doses.

He said that there were no indications from either the police or lay witnesses of any signs consistent with ice intoxication.

Judge Beazley said as such he could not rule Lenarczyk's driving was affected by methylamphetamines.

Lenarczyk was tried in the District Court by judge alone.

She will be sentenced next month.