Long-term effects of lowering the alcohol minimum purchasing age on traffic crash injury rates in New Zealand

Introduction and Aims: In December 1999, New Zealand lowered the alcohol minimum purchasing age from 20 to 18 years. We tested hypotheses that this change was associated with long-term increases in traffic injury attributable to alcohol-impaired driving among 18- to 19-year-olds (target age group) and 15- to 17-year-olds (affected by ‘trickle-down’). Design and Methods: We undertook a controlled before-and-after comparison of rates of fatal and non-fatal traffic injury to persons of any age attributable to impaired drivers aged 18–19 years and 15–17 years, versus 20- to 21-year-olds. Crash data including assessment of driver alcohol impairment were recorded by New Zealand Police. The pre-change period was 1996–1999. Post-change periods were 2000–2003, 2004–2007 and 2008–2010. Outcomes were population-based and vehicle travel-based rates. Results: Compared with the change in injury rates attributable to alcohol-impaired 20- to 21-year-old male drivers, injuries attributable to 18- to 19-year-old male drivers increased in all post-change periods and significantly so in the second post-change period (incidence rate ratio [IRR] 1.3, 95% confidence interval [CI] 1.1 to 1.5). For 15- to 17-year-old male drivers, rates increased in all post-change periods compared with 20- to 21-year-olds, and more so in the second (IRR 1.2, 95% CI 1.1 to 1.4) and third (IRR 1.2, 95% CI 1.1 to 1.4) periods. There was a short-term relative increase in harm attributable to 18- to 19-year-old female drivers (IRR 1.5; 1.1 to 2.0). Results were similar for vehicle travel-based rates. Discussion and Conclusions: Reducing the alcohol minimum purchasing age was followed by long-term increases in the incidence of traffic injury attributable to male 15- to 19-year-old alcohol-impaired drivers