Road traffic accidents (RTAs) account for about 25% of all deaths by injury worldwide, which compares to 6% for wars (Peden et al., 2004). Traffic deaths have been declining somewhat in western countries, but RTAs continue to be a major epidemic of modern times. Consequently, all contributory factors must be evaluated thoroughly. The take-home message from this review by Penning and co-authors is that drugs of abuse alone or in combination with alcohol are an important and increasing cause of impaired driving. The research that they review shows clear representation of these drugs amongst driving related deaths and injuries. Some of the facts they present are startling - did you know that over 2 million Europeans use cannabis daily and drive (Coullé et al., 2008).

Alcohol, of course, is the archetypal impairing factor in driving, and the methodology adopted to assess the effects of other drugs on driving generally follows that established for alcohol. Penning et al. identify three main types of evidence: (1) actual driving performance; (2) laboratory studies of performance on tasks assumed to assess driving-related skills and processes; and (3) epidemiology. While actual driving studies have a direct connection to the target performance that is attractive, they have several problems. Carrying out studies of impairing drugs on the public roads is not permitted in many countries, and such studies are expensive, both in resource costs, and in the time a driver volunteer must spend accumulating data on a single or small group of measures. Laboratory studies avoid some of the legal problems, and allow more detailed measures of performance impairments to be made. The final problem is common to both real driving and laboratory measures, that there is no direct way of quantifying the impact of the observed performance decrement on accident risk. A 20% increase in the standard deviation of lateral position (weaving) is obviously a bad thing, but how much increase in the risk of a collision does it mean? Epidemiological studies are needed here. They can assess the risk due to alcohol, but it is then much harder to allocate this to specific components of driving performance.

Thus the three methods complement one another, and contribute to different aspects of our understanding of the impact of drugs on driving. And all three methods clearly show the dangers of alcohol: blood alcohol concentrations above about 0.04-0.05%, are associated with rapidly increasing accident risk, and impairment to a broad range of performance measures. Lower concentrations can produce detectable impairments for some measures, but the picture is not so clear-cut, and 0.05% seems to be the evidence-based legal limit that is the most widely accepted in Europe.

Of the illicit drugs, only cannabis has convincing results from all three types of evidence, and this drug is clearly a hazard, especially in combination with alcohol. Penning et al. cite the work of Ramaekers and colleagues (2009) showing that alcohol at a ‘legal’ level (0.04% BAC) combined with cannabis (THC) had an additive effect increasing lateral deviation above that reported for 0.1% BACs. Such a driver might thus be seriously impaired, but still pass a breathalyser.

Penning et al's review concerned only drugs of abuse, but we should not forget that some widely-used prescription drugs can impair driving and increase accident risk (e.g. Skegg et al., 1979). Some of these, such as the benzodiazepines and opiates, are of course also drugs of abuse.

Evidence for other drugs is less conclusive, but impairments are commonly found. In some cases, for example stimulant drugs, aspects of performance may be improved. This is reason why some lorry drivers (or truckers) use stimulants to reduce fatigue and avoid falling asleep at the wheel - which is itself a major cause of deaths and accidents for this group (Alford, 2009). However this should not reassure us too much, as there is evidence both from experimental research, police reports and user interviews that these drugs can lead to overconfidence and risk-taking behaviour. Such a 'risky shift' can, of course, also occur with alcohol.