Imagine if you had to tell a family that their child was never coming home again...because a driver had a few too many drinks and they were too lazy to get a taxi? How would you feel if it was your child? Your brother, your parent, your best friend? Now imagine that you're the one who had a few drinks and thought...Home isn't too far. I'll make it without getting busted. While on the back streets worrying if the booze bus will catch you, you hit someone. How do you live with that for the rest of your life?
The recent amendment to Victorian driving laws, set to take effect March 1, marks a significant shift in how medicinal cannabis users who test positive for THC while driving will be treated under the law. While proponents celebrate this as a victory for medical cannabis patients, serious concerns about road safety emerge from extensive research on cannabis impairment and driving ability.
Under the new legislation, magistrates will have discretionary power when dealing with drivers who test positive for THC (tetrahydrocannabinol) if they hold a valid prescription for medicinal cannabis and appear unimpaired. This changes the previous mandatory six-month licence suspension and fine system. However, driving with THC present in one’s system remains an offence.
The complexity of THC impairment presents significant challenges for road safety. Unlike alcohol, where blood alcohol concentration correlates strongly with impairment levels, THC’s effects are far more variable and complex. Research from the Colorado Department of Public Health and Environment shows that driving impairment can persist for at least six hours after smoking cannabis, even with doses as low as 18mg of THC. However, typical cannabis consumption often involves much higher doses – a standard joint containing 400mg of flower with 15% THC concentration can deliver around 60mg of THC, potentially extending impairment duration significantly.
Recent research published in Scientific Reports (2021) raises particular concern about cannabis users’ visual function and self-perception of impairment. The study found significant adverse effects on all visual parameters analysed, including visual acuity, contrast sensitivity, stereoacuity, and night-vision disturbances. Crucially, the research revealed that cannabis users may be unaware of their visual impairment, creating a dangerous situation where drivers might feel capable of driving while actually experiencing significant visual degradation.
Data from jurisdictions with longer histories of cannabis law reform provide sobering insights. Colorado’s Department of Public Safety reported that 66.3% of DUI case filings in 2018 tested positive for cannabinoids, with nearly half having THC levels at or above 5ng/mL, their legal limit. Studies from Washington State showed that cannabis-involved fatal crashes doubled after legalisation, increasing from 9% to 19%.
The Canadian perspective, detailed in research by the Traffic Injury Research Foundation, highlights how THC impairs crucial driving skills including:
- Speed control
- Lane positioning
- Reaction time
- Divided attention
- Route planning
- Decision-making
- Risk assessment
The argument that regular medicinal users develop tolerance to THC’s impairing effects is problematic. While some studies suggest partial tolerance development, research indicates that when tolerance occurs, users typically increase their dosage to achieve desired effects, potentially maintaining or even increasing impairment levels.
Furthermore, a 2024 study from Massachusetts General Hospital revealed that THC significantly disrupts the prefrontal cortex’s normal connections and activity – the brain region crucial for decision-making and self-control. This disruption was measurable and correlated with intoxication severity, potentially affecting drivers’ ability to respond to changing road conditions.
The new Victorian legislation creates a challenging situation for law enforcement. How will officers determine whether a THC-positive driver is truly unimpaired? The research consistently shows that subjective self-assessment of impairment is unreliable, with users often underestimating their level of impairment.
While the law change aims to protect legitimate medicinal cannabis users from unfair penalties, it may inadvertently create a dangerous precedent. Australia’s successful road safety record has been built on clear, evidence-based policies regarding impaired driving. Introducing subjective assessment of impairment for THC-positive drivers could compromise these standards and potentially lead to increased road safety risks.
The evidence suggests that any THC presence while driving poses potential risks to public safety, regardless of its source or legal status. As this legislative change takes effect, careful monitoring of its impact on road safety will be crucial, as will continued research into reliable methods for assessing cannabis impairment in drivers.
Dalgarno Institute
Sources
- Major change to cannabis laws for millions of Australians
- Effects of cannabis on visual function and self-perceived visual quality
- Cannabis (THC) messes with your Executive Functions – Can disrupt and impede good decision making and add to dysregulation
- Dalgarno Institute Website – ‘Medicinal’ Cannabis & Driving – Is it an Issue? (DRR)
