Measuring levels of THC as evidence of a driver’s impairment is not supported by scientific evidence

Australian researchers have reported that tests which detect the concentration of THC in blood or oral fluids are not reliable indicators of driving impairment, nor are they able to accurately assess when, or even how much cannabis has been taken by an individual.  

Researchers in a recent clinical trial have assessed the relationship between THC levels and driving performance in 14 volunteers who inhaled vaporized cannabis samples of varying potencies of THC and CBD before taking to a driving simulator. Three differing potency levels were given; high THC/low CBD, equal ratios THC and CBD, and nominal THC/placebo. Thirty minutes into the study all volunteers had both blood and oral fluid samples are taken, and then again 3.5 hours later. 

The findings were inconclusive.  For all participants, plasma and oral fluid THC concentrations were over the per se limits 30 minutes after vaporizing THC-dominant or THC/CBD equivalent cannabis, but 54% of participants displayed no driving impairment.  However, 57% of participants displayed driving impairment 3 ½ hours later, even though their recorded THC levels in their blood were below per se limits.  In two cases, one volunteer had a positive THC result but displayed minimal impaired driving, whilst another volunteer had a negative THC result, but driving impairment was profound.

If the authorities had hoped that these tests could provide traffic enforcement officers with a reliable measure of cannabis consumption and associated driving impairment, this research and other similar studies have clearly debunked this idea.  Unlike BAC (blood alcohol concentrations) for alcohol, the findings show an inconsistency between the magnitude of impairment and THC concentrations.  The study concludes that the presence of THC in an individual is an unreliable predictor of driving impairment.  The authors of the study conclude by stating that ‘there is a pressing need to develop improved methods of detecting cannabis intoxication and impairment.’ 

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