While no-one disputes that driving under the influence of the drug alcohol makes your driving worse, a new survey reports that people driving under the influence of cannabis say their driving is unaffected or even improved by cannabis. Unfortunately, this is simply not true. Although the dose of cannabis per smoke is variable, a single joint can produce functional impairment similar to a BAC of 0.05% (the proposed new limit) in all the areas normally associated with driving skills, including reaction time and speed of decision making. A 2003 review in Drug and Alcohol Dependence cites a study suggesting that the risk of accident is between 3 and 7 times higher than normal. This association was recently confirmed by a Christchurch-based study in New Zealand.
It is true, however, that cannabis use does not appear to be correlated with the incidence of fatal accidents in the same way as alcohol. This is because cannabis preserves higher cognitive reasoning while reducing automatic functions, the exact opposite effect of alcohol which depresses your reasoning after the first drink. Thus, those impaired by cannabis tend to know that they are wasted and compensate by driving slower and using less complex manoeuvering. In contrast, the drunk does not recognise the level of his/her impairment and tends to drive faster and more recklessly. This is why the person stoned on cannabis thinks their driving skills are better – they are driving more slowly and cautiously.
The real problem occurs when the stoned person decides to have alcohol. Even a single bottle of beer will sufficiently depress higher reasoning to remove the inhibition and cause the driver to act recklessly. After a couple of joints, even a person well within the legal limit becomes an extreme hazard on the road. Unfortunately, this is by far the most common scenario and it leads to a gross underestimation of the amount of harm caused by cannabis. Most of the police data seems to blame alcohol to the exclusion of any other drug. This is likely to be a serious problem in New Zealand as data from fatal accidents shows that:
Preliminary results showed 40 percent of drivers killed between 2004 and 2006 had used alcohol, cannabis or both before their death. (emphasis mine)
This is a scary statistic and it thoroughly reinforces the rationale behind random drug testing at checkpoint. If someone passes the breath test but seems inordinately intoxicated for the level of breath alcohol, they should immediately be subject to a mandatory blood test for cannabis (the urine test shows past smoking as well as current, so it is not suitable). Well-defined legal penalties should be enforced if a mixture of alcohol and cannabis is found. This is the only real way to stop this kind of behaviour.
Or you could drop the BAC limit from 0.08% to 0.05% and pat yourself on the back – and achieve nothing at all.