"Our primary analysis looked at the risk of a motor vehicle collision while under the influence of cannabis and included all nine studies (relating to 49 411 participants). The pooled risk of a motor vehicle collision while driving under the influence of cannabis was almost twice the risk while driving unimpaired (odds ratio 1.92 (95% confidence interval 1.35 to 2.73); P=0.0003); we noted heterogeneity among the individual study effects (I2=81%).
"We also assessed culpability and non-culpability studies separately and explored differences between motor vehicle collisions resulting in deaths and non-fatal injuries. Meta-analyses on subgroups of studies explored the potential effect of specific features related to study design and potential biases: case-control studies versus culpability studies, fatal collisions versus non-fatal collisions, and high quality studies versus medium quality studies (fig 3?).
"High quality studies had a pooled odds ratio that was higher than that for medium quality studies, although both results showed a significant association at the 0.05 level. Furthermore, case-control studies (2.79 (1.23 to 6.33); P=0.01) estimated the effect of cannabis use on crash risk to be higher than that estimated by culpability studies (1.65 (1.11 to 2.46); P=0.07). Studies of fatal collisions (2.10 (1.31 to 3.36); P=0.002) had a pooled odds ratio that was statistically significant, but studies of non-fatal collisions (1.74 (0.88 to 3.46); P=0.11) did not show significant results.
"In all studies assessing cannabis use in conjunction with alcohol, the estimated odds ratio for cannabis and alcohol combined was higher than for cannabis use alone, suggesting the presence of a synergistic effect."


Asbridge, Mark, et al., "Acute Cannabis Consumption and Motor Vehicle Collision Risk: Systematic Review of Observational Studies and Meta-analysis," British Medical Journal, 2012;344:e536 doi: 10.1136/bmj.e536 (Published 9 February 2012).