Measuring ‘drunk’ is pretty easy; the more alcohol someone drinks, the more alcohol shows up in that person’s blood and the more impaired that person becomes, falling somewhere on a scale of tipsy to wasted. Measuring ‘high,’ on the other hand, is far hazier—much to the dismay of some states’ law enforcement.
Blood tests that try to quantify marijuana use are in fact useless at assessing how impaired a driver is, according to a study by the AAA Foundation for Traffic Safety. In other words, the study found that people with low blood amounts of THC—or delta-9-tetrahydrocannabinol, the main psychoactive component of pot—may still act as if they’re really stoned. On the other hand, some people may have THC measurements off the charts yet still act normally.
They just need to devise a test that measures how hungry and connected to the universe you feel.
The problem is that pot stays in your system for a long time even after you are no longer “high”. You could still test positive for some time after you last smoked.