Limiting blood alcohol levels for drivers, both young and old, is unquestionably a sensible policy, but setting different limits for different drivers - with zero tolerance for young people - complicates the issue and will be fundamentally unworkable.

Countries that set varying limits for different ages of drivers (including Australia, Austria, Canada, New Zealand and the United States) don’t tend to use zero tolerance but generally set their blood alcohol content (BAC) limits low to prevent other variables corrupting results. Otherwise there is the risk that young people will be charged or fined for using mouthwash or eating an alcoholic chocolate.

Zero tolerance targets people with comparatively safe levels of alcohol in their blood, rather than punishing those who represent a genuine threat to their fellow drivers.

Studies conducted in the US and Canada have concluded that the majority of fatal road accidents are caused by a hard core of drinkers. One widely cited study found that more than 65 per cent of drinking driver fatalities were caused by drivers with more than 150mg of alcohol per 100 ml of blood – almost double the legal limit of 80mg in Britain. The study also found that more than 20 per cent of alcohol-related accidents were caused by drivers with previous convictions. These are the people the Government needs to target with education and punishment.

If the Government must legislate, then reducing the national legal limit would be more workable and more sensible. According to Alcohol Concern, a national agency concerned with alcohol misuse, the relative risk of a drink-driving accident increases significantly when a driver's BAC rises above 50mg per 100ml. Alcohol Concern is therefore campaigning to lower the UK limit to 50 mg.

Far more effective than legislation aimed at criminalising relatively small amounts of alcohol in the bloodstream would be to increase spending on advertisements which urge people to limit how much they drink before driving. Shock-tactic advertising campaigns in Australia have been shown to contribute to overall driver awareness, and campaigns in the UK have been awarded internationally for their powerful messages.

As part of a broader health initiative aimed at teen binge drinking, drug taking and unsafe sex there is an intuitive logic to advising young people not to get behind the wheel drunk. But in practice, zero tolerance is unlikely to get to the heart of the problem of drunk driving: legislation should target offenders, not age groups.