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Marijuana and driving

Police have long utilized methods for field testing suspected DUI/DWI drivers for alcohol. Recent legalization, however, has raised new legal issues in California for determining whether a motorist is driving while impaired by marijuana. Law enforcement is seeking new methods to conduct field sobriety tests for suspected drivers.

Under California law, a driver gives implied consent for the police to obtain a blood, breath or urine test when they suspect that an individual is driving under the influence of marijuana. However, these tests are not precise.

A significant flaw is that these do not show whether the marijuana was consumed within a few hours before the suspect was stopped and merely show that marijuana was smoked at some time. This could be anytime within an hour or several weeks.

Blood and alcohol tests can detect the active marijuana ingredient, Tetrahydrocannabinol, in the blood stream for several weeks after it was consumed. This makes it difficult for proving that a driver consumed THC when testing for driving under the influence. The blood alcohol content for suspected drunk drivers, however, dissipates within a short time and more accurately determines whether it exceeds the legal limit.

California law enforcement generally rely on a field sobriety test for arresting a suspected driver for operating under the influence, according to These include the horizontal gaze nystagmus test, the walk and turn test and the one-leg stand test.

Police may also use a saliva swab test which can detect THC, crystal meth, methadone, cocaine and several prescription drugs. However, the accuracy of these tests has been disputed. Saliva may contain traces of a drug up to three days after it was consumed.

A driver suspected of driving while impaired under marijuana may have grounds to challenge their arrest. An attorney can help assure that they can preserve their ability to challenge evidence and defend their rights after an arrest.

Source: Los Angeles Times, HS Insiders, "Commentary: 'Weeding' out the suspects," March 2, 2017

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