Drink driving

Information on how blood alcohol concentration and age affect crash risk, legal limits and the impact of alcohol.

The best advice on drink driving is: if you’re driving, don’t drink at all.

Why? Alcohol is second only to speed as the biggest factor that contributes to road crashes. Alcohol/drugs were a factor in 30% of fatal crashes, 20% of serious injury crashes and 12% of minor crashes. That’s about 78 deaths and nearly 1700 people injured every year.

Drink driving and crashing seems to be a male problem. For example, in the 3 years 2011-2013, 85% of alcohol/drug affected drivers who died in crashes were male. Of all drivers involved in fatal crashes, the 16-19 age group is the most likely to be affected by alcohol/drugs, closely followed by the 20-24 age group.

Drink driving doesn’t just affect drivers. For every 100 alcohol/drug impaired drivers who died in road crashes, 47 passengers and 17 sober road users die with them.

A quarter of the total motor vehicle levies collected by ACC go towards the cost of claims for injuries from alcohol-relatedcrashes.

How blood alcohol concentration and age affect crash risk

Many studies show that the risk of being involved in a crash increases as a driver’s blood alcohol concentration (BAC) increases, and there is a clear increase in crash risk as blood alcohol levels increase. The effect is more pronounced for young drivers.

Comparisons can be made between the relative risks of being involved in a fatal crash when driving with zero blood alcohol and driving at the 50mg/100ml BAC legal limit by age (risk calculation made relative to that of a sober driver aged 30+):

  • persons 30+ years are 5.8 times more likely to have a fatal crash when driving with a 50mg/100ml BAC than if they were driving with a zero blood alcohol level
  • persons aged 20-29 years are 17.5 times more likely to have a fatal crash when driving with a 50mg/100ml BAC than a 30+ driver with a zero blood alcohol level. This age group is also 3 times more likely to be involved in a fatal crash when driving with zero BAC than a 30+ driver
  • persons aged 16-19 years driving with a zero blood alcohol level are 5.3 times more likely to be involved in a fatal crash than a 30+ driver with a zero blood alcohol level
  • people with a high blood alcohol level are more likely to be injured or die in a given crash than those who are sober.

Legal limits

The legal drink drive limits for drivers 20 years and over are a breath alcohol limit (BAC) of 250 micrograms (mcg) of alcohol per litre of breath and a blood alcohol limit of 50mg of alcohol per 100 millilitres of blood (50mg/100ml).

If you’re aged under 20 years, the legal drink drive limit for blood alcohol concentration (BAC) remains the same at zero.

Visit the NZ Transport Agency’s website for information about alcohol and drug limits, your rights and penalties (external link).

What happens when you drink?

Alcohol is a drug that slows down your brain function. It impairs your ability to make good decisions and slows down your reactions. Even small amounts of alcohol affect your driving for the worse.

How much you can drink and still be under the legal limit depends on a number of things, eg whether you are male or female, your size, and how much food you have eaten.

Once alcohol is in your blood there are only a few ways to get rid of it:

  • a little goes out through your kidneys in your urine
  • you may sweat a little bit out
  • you breathe some of it out through your lungs
  • but most of it (over 90%) is broken down and eliminated by your liver.

If your liver is healthy it takes about an hour to get rid of the alcohol in just one standard-size drink! If your liver isn’t healthy it takes even longer.

Here’s the problem – because your liver can only get rid of the alcohol from one drink per hour, when you drink more than one standard drink per hour, alcohol builds up in your bloodstream. Your blood alcohol level goes up.

This is also the reason you can still be way over the limit the morning after a heavy drinking session.

Visit the Health Promotion Agency’s alcohol and you (external link) pages to find out how much alcohol there is in a standard drink and how alcohol affects you (external link).

Last updated: 9 December 2014

Last reviewed: 9 December 2014