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Alcohol
Cannabis
Stimulants
Prescribed Drugs
The
Inexperienced Driver
Publications
Cannabis is grossly impairing of driving skill with a fatal crash risk greater than a blood alcohol concentration of 0.15%. Unfortunately there is a widespread belief in the community that driving after using cannabis is safe.
The community is not aware of the dangers of cannabis smoking. Over the past 25 years, cannabis has become a serious drug with more obvious effects on behaviour and mental function than the relatively harmless weed that many think it to be. Careful breeding and selection of plant material has increased drug yield. "Commercial crops" are often grown under controlled hydroponic conditions. What we learned about cannabis in the past was misleading.
Outdated data
One of the problems has been that the literature is dated. Cannabinoids are highly fat soluble, and accumulate in fatty tissues. The most important of the many cannabinoids is Delta- 9-tetra-hydro-cannabinol (THC). It is only in the past decade that it has been possible to accurately quantify THC rather than its inactive metabolite.
THC levels in blood are highest as smoking ceases and fall to 5 - 10% of the peak level within an hour. THC is mostly metabolised to an inactive metabolite, 11-nor-9-carboxy-delta-tetrahydocannabinol (THC-COOH).
Much of the literature is based on experiments conducted before it was possible to measure active THC, so 20 years ago Moskowitz concluded that low doses of marijuana had almost no effects on car control other than slowness of decision making. Recent laboratory studies using more realistic simulations show that low doses of THC change driving similarly to a blood alcohol concentration of .05%. On-road studies show highly automated behaviours such as car handling and road position are the most obviously affected.
Current data
A recent analysis of 3398 fatally injured drivers across Victoria, New South Wales, and Western Australia between 1990 and 1999 found THC was associated with increased risk of causing a fatal collision. If the THC levels was > 5 µg/L the risk of a deceased driver being responsible for the collision in which (s)he died was 6.6 times higher than those drivers found to be drug free: this is similar to the risk of driving with a blood alcohol concentration greater than 0.15%.
The availability of screening tests and reliable assays for THC means that there will be a great deal more reliable research data in the next few years and practitioners should be in a better position to advise their patients about recreational drug use and the adverse outcomes of impaired driving performance. There is enough data now to start counseling about the hazards of cannabis in the workplace and in vehicles.
Recent studies have shown that low dose THC cigarettes showed changes in braking and increase in lane weaving similar to driving with a blood alcohol concentration of .05%.
Several on-road studies have been conducted in the past decade. Participants on a closed driving circuit showed poor car handling with the number of cones hit related to the dose of THC. [Detrimental effects of THC are more prominent in certain driving tasks than others. Highly automated behaviours, such as road tracking control, are more affected by THC than more complex driving tasks requiring conscious control.
Modern analytical techniques, that determine the level of THC rather than the inactive metabolite, have found that THC is associated with increased culpability for fatal collision. Recent consumption of marijuana causing THC levels ~0.005 mg/L increase the odds of being responsible for a fatal collision is 6.6 times higher than those drivers found to be drug free. This is similar to the risk of driving with a blood alcohol concentration of 0.15%.
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