One of the first large-scale studies of the presence of drugs in the bloodstream of drivers involved in non-fatal crashes found the level of drug use "is disturbingly high".
The study was carried out in the Australian state of Victoria, and tested 436 drivers admitted to hospital after a crash, for a range of legal and illegal drugs.
Researchers found that just under half (46 per cent) tested positive for cannabis, and over 7 per cent had the active agent of cannabis - tetrahydrocannabinol (THC) - in their bloodstream, indicating they had inhaled cannabis smoke in the previous hour. One in 10 drivers had two or more drugs in their system.
While traces of cannabinoids can remain in the bloodstream for up to a week, THC levels peak around 20 minutes after the last inhalation, and remain measurable for around three hours.
Researchers in Victoria think that time delays while the patient is taken to hospital and then treated would significantly reduce the concentrations of THC.
As a result, the authors of the study say the true level of THC is underestimated. Cannabinoids have been found to slow reaction times, and reduce a driver's ability to judge distances and speeds.
THC was almost exclusively found in drivers aged between 15 and 44-years-old.
The next most commonly found drugs were benzodiazepines, a class of psychoactive drugs often prescribed to treat conditions including anxiety, stress and panic attacks. These drugs were found in over 60 of the drivers tested, almost a third of whom were women aged 65 and over.
Opiates (11 per cent) and amphetamines (4 per cent) were the next most common forms of drugs, followed by methadone which was found in 12 drivers. Cocaine was found only in 1.4 per cent of drivers, suggesting a lower incidence of this drug use than in the US or some European countries. Some 70 per cent of the drivers were men who also showed a slightly higher prevalence of illegal drugs compared to women, in whom prescription drugs were more common.
The most commonly mixed drugs were benzodiazepines and opiates, with 17 drivers testing positive for both. Men aged between 15 and 44 accounted for half of all crashes.
Victoria recently passed legislation allowing for the compulsory testing of drivers for drugs and alcohol.
Blood samples are taken from all drivers involved in a crash.
Among the proposals contained in a draft of the new Road Safety Strategy 2007-2011 is an increase in drug driving enforcement.
Because there is currently no viable roadside testing device, the enforcement is likely to be based on a roadside examination of a driver's co-ordination. If the driver fails, a blood sample is taken and analysed for drugs.
Unlike alcohol, for which all drivers can now be randomly tested, motorists are sent for a drugs test only after being stopped for driving erratically but found to be under the alcohol limit.
The extent of drug driving is Ireland is almost impossible to gauge, as the Garda and court services statistics only list convictions for "impaired driving", and do not distinguish between impairment caused by alcohol or drugs.
The results of the Australian study were in the July edition of Emergency Medicine Australasia.
