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Lowering the alcohol purchase age has been linked to a long-term increase in the chance of drunk drivers aged 18 or 19 being involved in car crashes that cause death or injury.
The age for buying alcohol was lowered to 18, from 20, in 1999. There has been regular debate about it since, but in 2012, MPs voted 68 to 53 to keep the age at 18 - contrary to the Law Commission's recommendation to return the age to 20.
Short-term studies have linked the change to an increase in the serious-crash risk for intoxicated drivers directly affected - those aged 18 and 19 years old.
Now, in the first long-term study, Massey University researchers Dr Taisia Huckle and Karl Parker have found this increased risk has become the new normal.
In the years leading up to the change, drivers aged 18 or 19 had roughly the same chances as those aged 20 to 24 of having an "alcohol-involved" vehicle crash that caused injury or death.
That increased in the years following the change, putting the younger drivers at 15 per cent greater risk in the first six years, then at 21 per cent greater risk up to 2010.
Dr Huckle said that despite the appearance of a second increase after 2006, statistically that was not the case - the increase in risk, established in 2000-2006, had been "maintained" from 2006 to 2010.
The alcohol-involved serious-crash risk of even younger drivers - aged 14 to 17 - was significantly lower than for the 20-24 group for all three periods.
In their paper in the American Journal of Public Health, the researchers concluded: "Raising the minimum purchase age for alcohol in New Zealand would be an appropriate public health intervention.
"Several US studies have found that raising the minimum legal drinking age is related to medium- to long-term reductions in vehicle-related harms among those younger than the drinking age."
The Tumai whanau and the Blakes of North Waikato, and others such as the Taylors of Christchurch, have told of the pain of losing a child in a car crash caused by a drunk teenager.
"We had our beautiful boy taken from us, not only by a drunk driver but someone who was driving like a lunatic," Jane Taylor said after the death of son Alex, 18. He was in a car crashed by a heavily intoxicated friend in 2007.
Katie Blake, 17, and Liam Tumai, 18, suffered fatal injuries when they were flung from a car crashed near Huntly in 2010 by their friend Darren Siteine, 18 at the time. Siteine was sentenced to more than two years in prison after pleading guilty to dangerous driving causing injury and driving with excess breath-alcohol causing death.
Alcohol Healthwatch director Rebecca Williams said stronger regulation of liquor marketing and persuading the Government to force up the price of alcohol with tax increases and minimum prices were the keys to reducing the damage.
Increasing the purchase age was her fourth-ranked policy priority but it was still important as research evidence had proven its effectiveness, she said, although it was of little interest to MPs at present.
Justice Minister Judith Collins - who voted for the age to rise to 20 - said it was too soon to consider adding to the 2012 reforms, many of which came into effect last year.
"It's important that we allow enough time to understand the real effects of these reforms before we start looking further into initiatives such as raising the minimum alcohol purchase age."
Comparative crash risk
The study compared the alcohol versus non-alcohol serious-crash risk of drivers aged 18-19 with those 20-24.
It looked at three periods: before the alcohol purchase age was reduced to 18, immediately after the change, and 7 to 11 years after the change.
Before the change: the younger drivers had a zero to 22 per cent lower chance of being drunk and in a vehicle crash that caused injury or death.
Immediately after the change: the younger drivers had a 15 per cent higher chance than the older ones of being in such a crash.
Long-term: the younger drivers had a 21 per cent higher chance than the older ones of being in such a crash.
- Martin Johnston of the New Zealand Herald