Opinion: Zero tolerance for cannabis and driving makes zero sense

published on November 25, 2016 by Stephanie Lake and M-J Milloy in Vancouver Sun

Maybe lost amidst the sound and fury from last Tuesday’s U.S. presidential election results were the outcomes from ballot initiatives in eight states approving plans for legal medical or recreational cannabis. Like citizens and policymakers in Canada, those in California, Maine, Nevada and Massachusetts now must grapple with how to set up a legal framework to govern the production, distribution and use of cannabis by adults.

As in Canada, perhaps no aspect of cannabis legalization “” other than how to curb youth cannabis use “” sparks as much public concern as cannabis and driving. Recently, authorities in Washington and Colorado reported sharp jumps in positive tests for cannabis among victims of motor vehicle accidents following their states’ enactment of legal cannabis, stoking fears that it sparked a surge in cannabis-impaired driving. B.C.’s Solicitor-General Mike Morris recently advocated for a zero-tolerance policy: Driving with any THC (the psychoactive ingredient in cannabis) in your bloodstream would be an offence.

While zero tolerance might make for a clear legal standard and straightforward enforcement, a quick look at the science behind cannabis and driving reveals it to be unworkable.

1. Unlike alcohol, the amount of cannabis in a driver’s bloodstream does not indicate whether they are impaired.

Blood alcohol level (BAC) has been repeatedly shown to tightly correlate with level of driving impairment. This has led to the establishment of BAC cut-off points (e.g., 0.08 mg of alcohol per mL of blood) that are universally accepted as valid indicators of impairment. When it comes to cannabis, however, determining a standard of impairment has been met with many challenges. Currently, there is no consensus on how much THC causes impairment. In part, this is because THC is fat-soluble (rather than water-soluble like alcohol), so THC can be detected in the body weeks after use. For example, individuals who regularly use cannabis can store residual levels of THC that exceed the legal limit in some U.S. states.

2. Scientific experts do not support a zero-tolerance approach to driving and cannabis.

Similar to designing an effective medical diagnostic test, the standard of impairment for cannabis will need to strike a delicate balance between minimizing false positives and false negatives while maximizing true positives and true negatives. Just as it is not possible to design a perfect diagnostic test, it will not be possible to determine a perfect standard of impairment for THC; but setting the standard at 0 ng/mL would grossly inflate the proportion of false positives. In 2005, a European expert panel recommended a legal limit of THC in blood serum between 7-10 ng/mL (which translates to 3.5-5 ng/mL in whole blood), in order to reduce the probability of false positives. However, given the issues covered under item 1, there is still no universally accepted standard.

3. Zero tolerance is impractical after legalization.

Although Morris lauded the Australian zero-tolerance approach to cannabis and driving, importing their standard is problematic. An obvious difference between Australia and Canada is that recreational cannabis will be legal here, whereas it remains prohibited under Australian law. This means that when a driver tests positive for any level of THC in Australia, whether impaired or not, the presence of THC indicates a criminal offence; this will not be the case in Canada, where the presence of THC may simply mean that the driver used cannabis at some point in the previous few days. Another practical shortcoming is that there is currently no objective curb-side test to measure THC (e.g., breathalyzer).

4. Increased detection of cannabis among drivers in motor vehicle accidents after legalization does not necessarily mean more people are driving (and crashing) impaired.

Earlier this week, road safety officials reported that positive tests for cannabis among victims of fatal motor vehicle accidents doubled in the state of Washington and tripled in the state of Colorado following legalization. The statistics need to be interpreted in light of other social and policy changes induced by legalization, including increased surveillance of cannabis among fatally injured drivers and increased prevalence of cannabis use “” resulting in the rate of unimpaired drivers who crashed with residual levels of THC “” post-legalization. To get a better picture of the impact of cannabis legalization on motor vehicle fatalities, we need to also examine overall trends in motor vehicle fatalities over time, the proportion of fatally injured drivers whose blood THC level exceeded that allowed by state law, and the overall trends in alcohol-related motor vehicle fatalities (since alcohol and cannabis will interact to produce an additive effect).

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