Rule won't hit 'moderate' drinkers

"Moderate drinkers" will not be affected by proposed drink-driving rules, an expert says.

However, the new rules will substantially cut the amount that can be drunk while people stayed below the legal limit.

The Government yesterday announced plans to lower the legal limit from 80 to 50 milligrams of alcohol per 100 millilitres of blood for drivers aged over 20.

The changes, which have the tentative support of Labour, could become law from mid-2014.

Allan Stowell, a forensic chemist and toxicologist who frequently serves as an expert witness in court cases related to drink-driving, said the new limit could still allow alcohol to be consumed on social occasions before driving.

"For example, a male of average build drinking two to three glasses of wine during a leisurely dinner, or three to four small bottles of beer during a social gathering lasting about two hours, is highly unlikely to exceed 50mg to 100ml at the end of the drinking period," he said.

"Women need to be a little more careful, but a woman of average build would be highly unlikely to exceed [the limit] after consumption of two to three standard drinks over a two-hour period."

The calculations are based on males weighing 85 kilograms and being 1.8 metres tall and females weighing 70kg and 1.65m tall.

Stowell calculated that under the existing limit of 80mg of alcohol to 100ml of blood, the "average" male could have five to eight standard drinks over two hours, with a meal, and still be under the legal limit.

Women could safely drink 3.4 to 5.4 standard drinks over the same period.

The calculations for individuals would vary, Stowell said.

"People who drink moderately really have nothing to worry about," he said, adding that this depended on what was considered "moderate".

Fairfax Media