Over the past few years, quite a few states have legalized marijuana for medical and recreational use. With this legalization, police officers needed a way to test drivers who appeared to be too stoned to drive.
This, in turn, has made the marijuana blood test the go-to way to check for impairment during a traffic stop.
Roadside tests and demonstrated impairment still play a role and assessment by a Drug Recognition Expert is now common. But most police departments attempt to verify these results with a blood test. If the allegedly impaired driver has more than the state’s per se limit (often 1 to 5 ng/mL of THC), a DUI conviction usually becomes much more likely.