When it comes to drunken-driving laws, the standards are clear: A blood alcohol content reading of .08 percent or above means a person is too impaired to drive, case closed. That is what years of research say, and that is what the law says.

It is tempting to do the same with marijuana, to establish a limit that can be measured through a simple test, so that law enforcement has another tool for curbing driving under the influence.

Unfortunately, it is not that easy, not yet anyway. Unlike blood alcohol tests, the methods for gauging the amount of marijuana in one’s system do not necessarily measure impairment, and far too little is understood about how marijuana affects one’s ability to drive.

Marijuana OUI blood tests will be part of legislation proposed this session by the Maine Department of Public Safety. Maine would follow Colorado, Washington and Montana, all of which have set an intoxication limit of 5 parts per billion of THC, the main psychoactive ingredient in marijuana. In addition, Nevada has a limit of 2 parts per billion, while 11 other states have “zero tolerance” laws for the presence of THC.

There are some state-to-state differences on how the law is used in practice, but typically if drivers are found through a blood test to have exceeded the limit, they are charged with operating under the influence, just the same as if they had blown more than .08 on the breath test for alcohol.

The charges come with the same consequences of prison time, prolonged loss of license, rights restrictions and debilitating court fees and fines as an alcohol OUI, but with none of the certainty that the driver was under the influence.

That’s because while alcohol metabolizes quickly before leaving the system, THC from marijuana remains well after its intoxicating effects have subsided. A blood test could show THC that was consumed a day — or two, or three — before the test but has absolutely no psychoactive effects at the time of the test.

What’s more, the level of THC in the blood has little correlation to a person’s ability to drive. A heavy smoker — a medical marijuana patient, for example — could have steadily high levels of residual THC with little or no impairment, while an inexperienced user could feel particularly impaired after smoking but only register low levels of THC.

According to the National Highway Transportation Safety Board, “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. … It is inadvisable to try and predict effects based on blood THC concentrations alone.”

Finally, little is known about marijuana effects on drivers, a key to determining how strict enforcement should be. Some preliminary research suggests marijuana use doubles the chance of an accident, about equal to the effect of texting while driving, and far less than drunken driving. But the research thus far is slim.

So while the blood tests would lead to more OUI charges in Maine, they’d hardly make the roads any safer.

Until the tests improve — and more is known about marijuana and driving — field sobriety exams, and more training for police officers to recognize impaired driving, are the most effective ways to curb drugged driving.

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