"Several meta-analyses of multiple studies found that the risk of being involved in a crash significantly increased after marijuana use13 -- in a few cases, the risk doubled or more than doubled.14-16 However, a large case-control study conducted by the National Highway Traffic Safety Administration found no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.17"
Prevalence of Driving While Under the Influence of Alcohol and Other Drugs in the US
Impairment Thresholds for Blood THC Level Compared With Blood Alcohol Content: "To combat drug-driving, most countries either operate a zero tolerance policy or take into account degree of impairment, sometimes in a two-tier system. Legal limits may be set low, at the limit of detection, or higher to take effects into consideration. For example, while the project set a detection limit of 1 ng/ml in whole blood for THC in the roadside surveys, it was found that 2 ng/ml THC in whole blood (3.8 ng/ml THC in serum) seems to cause impairment equivalent to 0.5 g/l BAC. Such equivalents could not be calculated for other drugs. It is not realistic to develop cut-off limits for all substances."
Prevalence of Drug Use Among Drivers: "Analyses of the oral fluid samples obtained from daytime drivers indicated an overall drug use prevalence of 11 percent, and for nighttime drivers, 14.4 percent (Table 19). This includes illegal, prescription, and over-the-counter drugs combined. This overall difference between day and night is statistically significant (p < .01).