Of the 9772 drivers studied, 681 were positive for cannabis (7.0%) and 2096 for alcohol (21.4%), including 285 for both (2.9%). The other three families of drugs were, proportionally, more often associated with cannabis than with alcohol (). Men, more often involved in crashes than women, were also more often positive for both cannabis and alcohol, as were the youngest drivers, and users of mopeds and motorcycles. Positive detection was more commonly associated with nighttime crashes.
Characteristics for all drivers, and for those tested positive for cannabis and for alcohol. Values are numbers (percentages) of drivers
Agreement between the two responsibility evaluation methods was seen to be satisfactory (Κ test 0.71). shows the estimated odds ratios of responsibility, according to the Robertson and Drummer method, associated with drugs and alcohol. Cannabis was significantly related to responsibility of the driver. Using expert evaluation did not modify this non-adjusted odds ratio (2.87, 95% confidence interval 2.07 to 3.97).
Drivers' responsibility associated with drugs and alcohol. Values are numbers (percentages) of drivers unless otherwise indicated
Amphetamines, cocaine, and alcohol were also significantly related to responsibility, although opiates were not. We highlighted a significant dose effect for cannabis (and for alcohol), adjusted or not for alcohol (respectively for cannabis), shown in (test for trend for non-null concentrations or Δ9tetrahydrocannabinol P < 0.01; for non-null blood alcohol concentration P < 0.001). These dose effects were taken into account in the rest of the analysis.
Odds ratios of driver responsibility associated with blood concentration of Δ9 tetrahydrocannabinol and alcohol
Case drivers and control drivers also differ according to sex (P < 0.05), age (P < 0.001), vehicle type (P < 0.001), and time of crash (P < 0.001). We performed multivariate analysis to adjust the relation between responsibility and blood concentration of Δ9tetrahydrocannabinol for these cofactors, added to blood alcohol concentration, amphetamines, and cocaine. In addition to blood concentrations of Δ9tetrahydrocannabinol (P < 0.001) and alcohol (P < 0.001), shown in , the final model included driver's age (P < 0.001), type of vehicle (P < 0.001), and time of crash (P < 0.001). It excluded amphetamines (P = 0.20), cocaine (P = 0.07), and driver's sex (P = 0.08). shows adjusted odds ratios associated with these cofactors, included or not in the final model. No interaction reached significance, particularly that between blood concentrations of Δ9tetrahydrocannabinol and alcohol (P = 0.99): we estimated the adjusted joint effect corresponding to blood concentrations of both Δ9tetrahydrocannabinol and alcohol present at any dose to be 14.0 (8.00 to 24.7), very close to the value obtained from the product of the adjusted individual effects (1.78×8.51 = 15.1).
Table 4 Adjusted odds ratios of driver responsibility associated with the cofactors of blood concentrations of Δ9tetrahydrocannabinol and alcohol, either included in the final model (age, vehicle type, crash time) or not (amphetamines, cocaine, sex). (more ...)
Comparing our controls with drivers not at fault involved in a slight injury crash allowed us to identify their distinguishing characteristics: driver's sex and age; type of vehicle; and place, time, and type of crash. The prevalence of cannabis in our controls was 2.8%, compared with 2.9% when standardised for these variables; both these prevalences were 2.7% for alcohol.
Similarly, comparing our cases with the other drivers responsible for a fatal crash enabled us to estimate the prevalence of cannabis and alcohol for the other drivers: for cannabis, the standardised prevalence was 8.5% compared with 8.8% for the cases, and for alcohol, it was 29.3% compared with 29.8%.
The adjusted fraction of fatal crashes attributable to cannabis (present at any dose) was 2.5% (4.3% when adjusted only for alcohol). The adjusted fraction for alcohol was 29% and 31% when adjusted only for cannabis. When considering only blood alcohol concentration over 0.5 g/l, it was 25% and 27% respectively ().
Adjusted fractions of attributable risks of fatal crashes associated with blood concentrations of Δ9 tetrahydrocannabinol and alcohol. Values are percentages (95% confidence intervals)