A new study published in Forensic Science International reveals that while alcohol and benzodiazepines have a clear and significant impact on driver impairment as their blood concentrations rise, the effects of THC are much less consistent and harder to predict, calling into question per se THC driving limits.
The research, which examined over 15,000 cases of impaired driving, highlights the complexity of assessing impairment from different substances, with alcohol and benzodiazepines showing a strong correlation between concentration and impairment, unlike THC and amphetamines.
The study aimed to determine how blood concentrations of ethanol, benzodiazepines, amphetamines, and tetrahydrocannabinol (THC) correlate with the severity of driver impairment, as measured by a Clinical Test of Impairment (CTI). Researchers utilized data from a national database, analyzing 15,514 individual mono-drug cases collected over nine years. The study population was predominantly male, with a median age of 34 years. In addition to these cases, 3,684 drug-free drivers with similar age and gender distribution were included as a reference group.
The results revealed a strong correlation between increased blood concentrations of ethanol and benzodiazepines and the severity of driver impairment. The percentage of drivers classified as impaired increased significantly with higher concentrations of these substances—from 60% to 97% for ethanol and from 38% to 76% for benzodiazepines.
Conversely, the correlation between blood concentrations and impairment was notably lower for amphetamines and THC. The study found that impairment increased only modestly with higher concentrations of these drugs, from 43% to 58% for amphetamines and from 41% to 55% for THC. The correlation between drug concentration and the degree of impairment was highest for ethanol (Spearman’s rho=0.548, p<0.001) and relatively high for benzodiazepines (Spearman’s rho=0.377, p<0.001), but low for amphetamines (Spearman’s rho=0.078, p<0.001) and THC (Spearman’s rho=0.100, p<0.001).
The percentage of impaired drivers increased with increasing blood drug concentration for all four drug classes, most pronounced for ethanol and benzodiazepines and much less for amphetamines and THC. The median blood drug concentration increased with increasing magnitude of impairment for ethanol and benzodiazepines, while this was much less pronounced for amphetamines and THC. The ranges of drug concentrations, however, were wide for all four drug classes in all impairment categories as assessed by individual clinical examination.
The results call into question the validity of per se THC driving limits, which find an individual guilty of a DUID if they have a specific amount of THC in their blood, regardless of actual impairment. Currently, there are at least six states with per se THC driving limits: Colorado, Montana, Nevada, Ohio, Illinois, and Washington.
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