How does cannabis affect driving ability?

Cannabis impairs driving ability, though the magnitude and duration of impairment differ from alcohol. THC impairs: reaction time, divided attention, lane tracking, and speed regulation. Meta-analyses show cannabis approximately doubles the risk of a motor vehicle accident. Impairment is most pronounced in the first 1–3 hours after use and correlates with blood THC levels, though the relationship is less predictable than for alcohol. Unlike alcohol, blood THC levels are a poor proxy for impairment — THC is detectable in blood for days to weeks after use in regular users, long after impairment has resolved. CBD does not appear to impair driving at therapeutic doses. Combining cannabis with alcohol produces additive impairment greater than either alone. Most jurisdictions have per se THC limits for driving (typically 2–5 ng/mL blood THC), though these thresholds are scientifically contested.