The failings of per se limits to detect cannabis-induced driving impairment: Results from a simulated driving study

Many jurisdictions use per se limits to define cannabis-impaired driving. Previous studies, however, suggest that THC concentrations in biological matrices do not reliably reflect cannabis dose and are poorly correlated with magnitude of driving impairment. Here, we first review a range of concerns associated with per se limits for THC. We then use data from a recent clinical trial to test the validity of a range of extant blood and oral fluid THC per se limits in predicting driving impairment during a simulated driving task. Simulated driving performance was assessed in 14 infrequent cannabis users at two timepoints (30 min and 3.5 h) under three different conditions, namely controlled vaporization of 125 mg (i) THC-dominant (11% THC; per se limits of 1.4 and 7 ng/mL THC in plasma (meant to approximate 1 and 5 ng/mL whole blood) and 2 and 5 ng/mL THC in oral fluid reliably predicted impairment (defined as an increase in standard deviation of lateral position (SDLP) of >2 cm relative to placebo). For all participants, plasma and oral fluid THC concentrations were over the per se limits used 30 min after vaporizing THC-dominant or THC/CBD equivalent cannabis. However, 46% of participants failed to meet SDLP criteria for driving impairment. At 3.5 h post-vaporization, 57% of participants showed impairment, despite having low concentrations of THC in both blood (median = 1.0 ng/mL) and oral fluid (median = 1.0 ng/mL). We highlight two individual cases illustrating how (i) impairment can be minimal in the presence of a positive THC result, and (ii) impairment can be profound in the presence of a negative THC result. There appears to be a poor and inconsistent relationship between magnitude of impairment and THC concentrations in biological samples, meaning that per se limits cannot reliably discriminate between impaired from unimpaired drivers. There is a pressing need to develop improved methods of detecting cannabis intoxication and impairment.