A Feasibility Test of the In Vivo Driving Impairment Research Method: Examining Cannabinoid Concentrations as Predictors of Risky Drinking

We tested the feasibility of a new approach to examining drugged driving – the In Vivo Driving Impairment Research Method (IVDIRM). Heavy cannabis users with a history of driving after using were recruited. The volunteers agreed to have their cars instrumented with data loggers and drive normally for six to 10 days. Participants also agree to provide oral fluid samples, which later were assayed for cannabinoid concentrations, whenever they drove. We anticipated that participants, of their own volition, would produce multiple occurrences of drugged driving at different measured drug levels. Cannabinoid concentrations then would be used to predict driving behavior measured by the instrumentation, using a within-subjects design to accommodate individual differences in personality, driving style, risk-taking, etc. We tested the hypothesis that measuring and modeling cannabidiol (CBD) would improve prediction of driving impairment beyond simply looking at delta-9-tetrahydrocannabinol (THC). A sample of 30 participants provided 358 oral fluid samples that were linked to driving data, indicating indeed that the IVDIRM method was viable. Mixed-model analysis found that subjects’ CBD levels were important forpredicting risky driving; participants with high THC concentrations who also tested positive for CBD demonstrated a higher rate of elevated g-force events while driving than those who tested negative for CBD. When CBD was zero, the predicted proportion of elevated X-axis G-force events (from acceleration and braking) ranged from approximately 0.05 to 0.08 across the distribution of THC scores. When CBD was positive, the predicted proportion ranged from approximately zero to above 0.14. For elevated Y-axis G-force events (from turning and swerving), when CBD was zero the proportion was close to 0 and flat across THC scores. When CBD was positive, predicted elevated Y-axis events ranged from 0 to approximately 0.025.