Alcohol is easy to measure. Your blood vision impairment technology alcohol rises. You get impaired. A roadside breath test gives a number that authorities can defend in court. Cannabis does not work that way. 
THC, the high producing chemical, leaves your bloodstream fast. An hour after inhaling, levels can drop sharply while you still feel high. 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 infrequent user can even be overtly impaired at a low reading. That number just doesn't translate smoothly into how the person 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. Studies keep finding weak links between blood THC and various driving skills; a habitual user might breeze through all the tests at a level that would drop a novice. Urine tests complicate things further. They detect metabolites, meaning the body processed THC at some point, and that window can last weeks. A positive test means the person has used cannabis, but it doesn't mean they're currently impaired. The method of consumption matters. First, smoking gets there faster and wears off. Edibles can take over an hour to get in and then peak and persist for hours. It's possible you could get tested too soon and find out near zero, then actually be impaired on the street. So what do officers look for? 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 helpful but also subjective and susceptible to being overturned in court. A few states not only don't have a set limit, they rely solely on the state prosecutor making an inference.