Marijuana-impaired driving tests may be flawed

Massachusetts officials have been reviewing options on enforcing impaired driving laws after marijuana was legalized in the state. A report from a Michigan state commission studying the effects of marijuana on drivers may be instructive for Massachusetts’ enforcement and an OUI defense once these plans are finalized.

Michigan’s impaired driving safety commission recommended that there should be no legal test limit for the active ingredient in cannabis, tetrahydrocannabinol, or THC, in a motorist’s blood. It recommended that police continue to use roadside sobriety tests to determine whether a motorist is driving while impaired.

Michigan law currently allows criminal prosecution if a driver has THC in their blood. It approved recreational use of marijuana last year and medical marijuana in 2008.

Marijuana metabolites can stay in a person’s bloodstream for up to one month. There is a very weak link between THC levels in the blood and whether a driver is impaired, according to a Michigan State University professor of pharmacology and toxicology. He said that blood levels rise greatly after exposure to cannabis, but THC quickly drop by half within six to 10 minutes after it is in the bloodstream.

The commission also reported that everyone’s body metabolizes marijuana differently. Accordingly, a fixed testing limit would not give an accurate or reliable assessment of a driver’s impairment.

Without a test, police and prosecutors would have to rely on the roadside sobriety tests used to spot whether drivers are too drunk to drive. These include the suspected driver walking a straight-line heel to toe, following the driver’s eye movement with a light, and standing on one leg while counting to 30. The police could also testify about their observations about the motorist’s driving.

The absence of accurate testing complicates Massachusetts’ enforcement of its OUI laws for marijuana. An attorney may also help challenge arrests based upon invalid tests.