Alcohol is straightforward. Blood alcohol levels climb. Impairment follows. A roadside breath test gives a number that authorities can defend in court. Cannabis is a drug mur different story. 
THC, the high producing chemical, leaves your bloodstream fast. Within an hour of inhaling, the level can fall steeply even though the high remains. Then it tapers off, settles into fat tissue, and lingers. A user who smokes daily may even have measurable THC days after that last joint, long after impairment has stopped; an infrequent user can even be overtly 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. Some states, such as Colorado and Washington, have a per se limit of 5 nanograms of THC per milliliter of blood. Experts cringe at that number. 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 pick up metabolites, which show the body has had THC at some time, and those can linger for weeks. A positive result shows the person has used cannabis, not that they're impaired right now. How you get there counts. Smoking acts quickly and wears off. Edibles can take over an hour to get in and then peak and persist for hours. Someone could be tested too early, show a near-zero result, and then be truly impaired on the road. So what do officers look for? Mainly what they can see and hear: slurred speech, bloodshot eyes, unsteady balance, and field sobriety failures. 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 have no set limit at all and depend entirely on the prosecutor drawing an inference.