Yet more research is appearing suggesting that drivers with THC in their system are not necessarily a danger on the roads.
Determining how impaired a driver is based solely on the levels of substances in their blood can be quite complex.
A recent study from Norway has provided insights into the intricate relationship between blood concentrations of common intoxicating substances and actual driving impairment.
These findings carry significant implications for drug-driving laws and enforcement.
The Varying Impact of Different Substances
The researchers examined data from over 15,000 drivers in Norway who were stopped on suspicion of impaired driving between 2012 and 2020.
Each driver underwent a clinical test of impairment conducted by a physician, evaluating factors such as coordination, cognitive function, and balance.
Blood samples were also collected to assess levels of alcohol, benzodiazepines, amphetamines, and THC (the primary psychoactive component in cannabis).
As you might expect, alcohol demonstrated the strongest correlation between blood levels and impairment. As blood alcohol concentration increased, the percentage of drivers classified as impaired rose significantly—from 60% at the lowest levels to 97% at the highest.
Benzodiazepines, a category of sedative medications, exhibited a similar but less pronounced trend.
The percentage of impaired drivers increased from 38% at low blood levels to 76% at the highest concentrations.
In contrast, the relationship for amphetamines and THC was much weaker. Even at the highest blood concentrations, only 58% of amphetamine-positive drivers and 55% of THC-positive drivers were assessed as impaired.
The correlation between blood levels and degree of impairment was strong for alcohol, moderate for benzodiazepines, but notably weak for amphetamines and THC.
These findings underscore the varying effects different substances have on driving ability.
While alcohol’s effects are relatively predictable based on blood concentration, the same cannot be said for stimulants like amphetamines or cannabis.
This poses challenges for setting legal limits and assessing impairment in drivers under the influence of these drugs.
The Complexity of Individual Responses
One of the most interesting findings of this study was the significant variation in impairment among individuals who had similar blood drug concentrations.
Across all types of substances, there were instances of drivers with high blood levels who displayed no signs of impairment, alongside drivers with low levels who were clearly impaired.
This highlights the complexity of individual responses to intoxicating substances.
Factors such as tolerance, genetics, and the context in which the substance is used can all play a role in determining how impaired a person becomes after consuming a certain amount of a drug.
Regular users might develop a tolerance that enables them to function relatively normally even with elevated blood levels. In contrast, occasional users could experience significant impairment from a smaller dose.
The researchers pointed out that for amphetamines specifically, the connection between blood concentration and impairment was not strong.
Some drivers with very high levels of amphetamines showed no clear signs of impairment, while others with lower levels were assessed as significantly impaired.
This could be due to the stimulant effects of amphetamines potentially masking some impairment signs, as well as the differing responses of regular versus occasional users.
For cannabis, the weak correlation likely relates to how THC affects the body over time. Peak impairment often occurs soon after smoking, when blood levels are highest.
But significant impairment can persist even as blood THC levels drop rapidly. This makes it very difficult to infer impairment based on a single blood measurement taken potentially hours after use.
Implications for Drug-Driving Laws and Enforcement
These findings have important implications for how we approach drugged driving from a legal and enforcement perspective.
Many jurisdictions have implemented per se limits for drugs other than alcohol, where exceeding a certain blood concentration is automatically considered an offense regardless of actual impairment.
But this research suggests such limits may be problematic, especially for drugs like cannabis and amphetamines.
For alcohol, the strong correlation between blood levels and impairment provides reasonable justification for per se limits.
But for other substances, there’s a much higher risk of both false positives (sober drivers exceeding the limit) and false negatives (impaired drivers below the limit).
This could lead to unfair penalization of unimpaired drivers in some cases, while allowing dangerously impaired drivers to avoid consequences in others.
The study’s authors suggest that for drugs other than alcohol, assessment of actual impairment through methods like the clinical test of impairment may be more appropriate than relying solely on blood levels.
However, they note that even clinical tests have limitations.
In this study, 34% of drivers with no drugs detected still failed the impairment test, highlighting the potential for false positives.
Ultimately, a more nuanced approach combining biological testing, clinical assessment, and consideration of individual factors may be needed to fairly and accurately identify drug-impaired drivers.
Technologies like cognitive tests and driving simulators could potentially play a role in more comprehensive impairment assessment.
Balancing Safety and Justice
Keeping impaired drivers off the road is crucial for public safety. But we must also strive for fairness and accuracy in how we identify and penalize those drivers.
This research highlights the need for ongoing refinement of our approaches to drugged driving.
More research is clearly needed to better understand how various substances impact driving ability across different individuals and contexts.
We could develop more sophisticated ways of assessing impairment that go beyond simple biological tests.
And our laws and enforcement practices must evolve to reflect the complex realities of how drugs affect drivers, instead of being driven by political ideologies.
