Cannabis is one of the most widely used drugs of abuse. Approximately 6% of the general US population aged 12 years and older admitted to recent cannabis use . The widespread use of cannabis has also increased the prevalence of cannabis in the general driving population. Overall, 6.8% of drivers tested positive for D9-tetrahydrocannabinol (THC), the active ingredient of cannabis, in blood. The prevalence in young drivers aged 16–20 years was even higher, at 15.2% . Moreover, 13% of drivers involved in fatal accidents were positive for THC . High prevalence rates of THC among drivers may pose a serious problem, as experimental and epidemiological studies have demonstrated that THC increased driving impairment and crash risk in a dose-related manner [4–6].