What cannabis can impair
Cannabis can slow reaction time, interfere with coordination and lane tracking, distort perception, and make divided-attention tasks harder. Combining cannabis with alcohol or another impairing substance can increase impairment.
A person cannot reliably judge their own driving ability after using an impairing drug. The safe choice is to arrange transportation that does not depend on the person who used cannabis.
Why testing is complicated
THC and its metabolites can remain detectable after the impairing effects have passed. Unlike blood alcohol concentration, a THC concentration does not map consistently to an individual driver's degree of impairment.
Crash studies also face confounding by age, alcohol use, driving exposure, and patterns of testing. Those limits affect precise risk estimates, but they do not erase evidence that acute cannabis exposure can impair driving skills.
Law and safety point the same way
Driving while impaired is illegal across the United States, including where adult cannabis use is legal. Local rules and penalties vary. Do not drive or operate machinery after using an impairing cannabis product.