In all states including permissive use states such as California, it is illegal to drive a motor vehicle while under the influence of drugs or alcohol including legal-use marijuana. This should not be surprising, since all states make it illegal to operate a motor vehicle while under the influence of any drug, recommended by a physician or not, if that drug actually causes the impairment of the drivers ability to safely operate a vehicle.
A major problem for law enforcement in proving the crime is that traces of marijuana can be detected in the blood stream and urine for as much as thirty days after its original use. This means that a person can end up submitting to a blood or urine test at the request of law enforcement and end up testing positive for marijuana even though the driver had not used marijuana for days or even weeks.
California law for example, like most other states, makes it illegal to operate a motor vehicle if the driver was under the influence “at the time” of the subject driving. The good news for the defendant is that most states allow the jury to consider whether the marijuana trace found in the blood was from another day perhaps even from another week. The prosecution will often find it very difficult to overcome this challenge when the prosecutors case is based mostly or solely on a positive blood or urine test.