Since the legalization of marijuana, police have had trouble enforcing a consistent policy for impairment while driving. Washington and Colorado have the same legal limit of 5 nanograms per milimeter of blood, but in Oregon there is no such limit. If an officer in Oregon believes the driver to be impaired then they will be placed under arrest and made to take a blood or urine test which can be very excessive when the driver comes out clean. It took the US about 30 years to fully commit to a .08 limit for blood alcohol content and implement it in something as convenient as a breathalyzer. A limit of 5 nanograms is vague and can be reached by a completely sober person who smokes regularly due to THC staying in the blood long after the high has worn off, but creating a limit was a political compromise to see legalization through. Experts say that an effective method of roadside impairment testing for marijuana is atleast a decade away, and until it's ready there will likely be variation to see what works best.
"It's not against the law to drive a car after drinking alcohol, it's also not against the law to drive a car after smoking pot," Hingson said. It's only illegal to drive while impaired, he points out.
"I think it would be a long time before marijuana breathalyzers are scientifically vetted and supported by law enforcement and by the courts the way alcohol breathalyzers are," Knott said