Driving While Under the Influence of Marijuana

California is expected to legalize marijuana for recreational use on November 8. Polls show Proposition 64, which will allow those 21 and over to use and possess marijuana for personal use, will pass with a double-digit margin. The new law will prohibit the use of marijuana while driving, but does not address the issue of how to determine if someone is driving while under the influence of marijuana.

Medical marijuana approved in 1996

California has allowed the use of medical marijuana since 1996 and decriminalized possession of one ounce or less of marijuana in 2011. The new law will undoubtedly increase the use of marijuana in the largest state in the nation, but does not address the specifics of how the police and the courts will determine if someone is driving impaired while using marijuana. The California legislature will likely need to clarify the definitions.
However, the properties of marijuana make that a difficult task. The key compound in cannabis that produces the mind-altered state that many users find attractive is THC. Unlike alcohol, which dissolves in water and leaves the body quickly, THC is fat-soluble and can stay in the body long after the effects have ceased, particularly among chronic users.

DUI-cannabis

The state of Washington, which legalized marijuana in 2012, has established a THC blood level to determine the safe driving level while under the influence of marijuana. If a driver has more than 5 nanograms of THC per milliliter in their blood, they will receive a DUI-cannabis. However, not all agree that it is a reasonable standard. There are significant individual differences in how long the body retains levels of THC in the blood.
If we are going to criminalize DUI marijuana, we need to take information from scientific studies and use it to decide if that risk is sufficiently high to be so morally blameworthy that we call it a crime. But we don't, so picking 5 nanograms per milliliter is arbitrary. — Andrea Roth, UC Law Professor
Colorado, another state where recreational marijuana use has been legalized, has a law similar to Washington state and uses the same 5 nanograms per milliliter as the level at which drivers are presumed to be impaired.

Oregon takes different approach

Oregon has taken a different approach and rejected using the 5 nanogram level of THC in a driver’s blood to determine if a driver is impaired. Instead, Oregon relies on the observations of the police officer to determine if the driver’s ability to drive is impaired. Oregon cited conflicting evidence about what level of THC indicates an impairment, noting that THC behaves differently depending upon the frequency of use and the individual.

Unclear what California will do

It is unclear if California will adopt the 5 nanograms per milliliter standard or opt for different criteria to determine driver impairment. There is general agreement that driving while under the influence of marijuana leads to some level of impairment and is a legal question that will need clarification.
There is clear evidence that cannabis, like alcohol, impairs the psychomotor skills required for safe driving. The study stated that while cannabis-impaired drivers tend to drive more slowly and cautiously than drunk drivers, evidence shows they are also more likely to cause accidents than drug and alcohol-free drivers. — B.C. Medical Journal
With five additional states set to legalize the use of recreational marijuana this fall, the issue of determining how to keep drivers who are too impaired to drive off the highways is one that will attract increasing attention across the nation.

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