Alcohol is simple. Your blood alcohol rises. Impairment follows. A roadside breath test gives a number that authorities can defend in court. Cannabis does not work that way. 
THC, the compound behind the high, exits your system quickly. An hour after inhaling, levels can drop sharply while you still feel high. Then it slows down and goes into fat tissue and remains there. A daily smoker may still show measurable THC days after the last joint, long after impairment has ended; an infrequent user can be obviously impaired even with a low reading. That number just doesn't translate smoothly into how the person drives, thinks, and responds. That mismatch causes genuine headaches. 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 keeps showing only a loose connection between blood THC and different driving abilities; a habitual user might breeze through all the tests at a level that would drop a novice. Urine tests make it worse. They detect metabolites, meaning the gaize body processed THC at some point, and that window can last weeks. A positive result shows the person has used cannabis, not that they're impaired right now. How the THC enters the body counts, too. Smoking acts quickly and wears off. Edibles may need more than an hour to take effect, then peak and linger for hours. It's possible you could get tested too soon and find out near zero, then actually be impaired on the street. What do police rely on instead? Mostly observation: slurred speech, bloodshot eyes, poor balance, and failed field sobriety exercises. Drug recognition experts with specialist training run a twelve-step systematic assessment. It's useful, yet subjective and open to challenge in court. A few states have no set limit at all and depend entirely on the prosecutor drawing an inference.