Exploring the effects of marijuana use on driving ability

WATCH ABOVE: Dr. Samir Gupta explains the effects marijuana use can have on a person’s ability to drive.

TORONTO — A new law has been passed which imposes hefty fines for driving under the influence of marijuana in this province. I thought this was a good time to look at what evidence we actually have for the effects of marijuana on driving and accidents.

We are talking here about the effects of tetrahydrocannabinol (THC), which is the main active component in marijuana, on driving. There are a few different types of studies which have tried to examine this. The first type of study simply looks at the acute physical and mental effects of the drug. On the physical side, studies have shown that marijuana decreases motor coordination and reaction time, and on the mental side, it increases risk-taking behaviour and impairs judgment.

Do these actually impair driving ability? To answer that, researchers have taken subjects with varying blood THC levels and subjected them to either a driving simulator or actual driving on a closed course. They look at several variables, such as “body sway” (a measure of equilibrium), reaction time to specific road situations, and “brake latency” (the time it takes to apply the brakes).

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Many of these studies do show effects on driving skills, but the effects have generally been surprisingly small, and we can’t definitively conclude from these studies whether this would translate into accidents in the real world.

However, there are also interesting epidemiologic (including cohort and case-control) studies that have examined the issue. Mark Asbridge and colleagues published a systematic review of the literature in the British Medical Journal in 2011. They looked at nine different studies, and showed that driving under the influence of marijuana was associated with about double the odds of having a motor vehicle collision, compared with unimpaired driving. This relationship was seen in both fatal collisions and in collisions where the driver was at fault. Accordingly, there seems to be a valid signal for the impact of marijuana on accident rates.

It is worth mentioning that the effects of THC on driving are quite different than the effects of alcohol.

While both THC and alcohol impair driving skills in a dose-related fashion (in other words, the higher the blood level, the more impaired the driver is), determining the appropriate maximum cut-off and enforcing this cut-off is much trickier with marijuana than with alcohol, because the effect of marijuana on individuals is harder to predict than that of alcohol. For example, different smoking techniques lead to different blood levels of THC. There is considerable variation in how much THC different people absorb from smoking the same amount of marijuana, and some people develop a tolerance to THC after prolonged use.

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In terms of actual effects, THC seems to predominantly affect the automatic functions we complete while driving, whereas alcohol affects the complex tasks required for driving. It’s also interesting that marijuana smokers seem to be more aware of their impairment, and studies show that they tend to compensate by driving more conservatively.

One final thought to keep in mind is that what is quite clear is that the combination of marijuana with alcohol is much more dangerous in terms of both impairment and accident risk, than either one of these alone.

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