Alcohol is straightforward. Your blood alcohol rises. Impairment follows. You blow a number in a roadside breath test that authorities can support in court. Cannabis doesn\'t work like that. 
THC, the high producing chemical, clears out of the blood quickly. Within an hour of inhaling, the level can fall steeply even though the high remains. After that it slows, moves into fat tissue, and stays there. Someone who smokes every day can carry measurable THC for days after the last joint, well after impairment is gone; an occasional user can be clearly impaired at a low reading. That figure doesn't line up cleanly with how someone drives, thinks, and responds. That gap causes some real pain. A few states, including Colorado and Washington, set a per se limit of 5 nanograms of THC per milliliter of blood. Many experts are uneasy about that threshold. Research continues to show tenuous relationships between blood THC and different driving skills; a habitual user can sail through every test at a level that would knock out a novice. Urine tests drug impairment training for school nurses complicate things further. They pick up metabolites, which show the body has had THC at some time, and those can linger for weeks. A positive result proves past cannabis use, but not current impairment. The method of consumption matters. Smoking hits fast and fades. Edibles can take over an hour to get in and then peak and persist for hours. A driver tested too soon might read near zero and then become impaired behind the wheel. What do officers look for then? Mostly observation: slurred speech, bloodshot eyes, poor balance, and failed field sobriety exercises. Trained drug recognition experts follow a systematic twelve-step evaluation. It's useful, yet subjective and open to challenge in court. Some states have no fixed limit and leave it to the state prosecutor to make an inference.
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