Driving While Stoned: Marijuana and Impaired Driving

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Jeffrey Johnson is a legal writer with a focus on personal injury. He has worked on personal injury and sovereign immunity litigation in addition to experience in family, estate, and criminal law. He earned a J.D. from the University of Baltimore and has worked in legal offices and non-profits in Maryland, Texas, and North Carolina. He has also earned an MFA in screenwriting from Chapman Univer...

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UPDATED: Jul 16, 2021

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MarijuanaMore than half of the American states have reformed their drug laws to allow people in those states to use marijuana legally under specified circumstances. Most of those states permit the medical use of marijuana, although seven states and the District of Columbia now allow the use of marijuana for recreational purposes.

No state allows drivers to drive a vehicle while under the influence of alcohol or any other drug, whether or not the driver’s consumption was legal. Some law enforcement agencies and activist groups that oppose impaired driving are worried that legalization of marijuana will cause more people to drive while they are impaired. That poses a problem, they say, because it is difficult to prove whether a driver is under the influence of marijuana.

Marijuana and Impaired Driving

According to the National Highway Traffic Safety Administration (NHTSA), 8.3% of weekend nighttime drivers have “some alcohol” in their system, while 12.6% test positive for THC, the active chemical in marijuana. Those statistics are meaningless, however, since it is legal to drive with “some alcohol” in the driver’s blood (unless the driver’s blood alcohol concentration exceeds 0.08%), just as it is legal for some drivers in some states to drive with THC in the driver’s blood unless the driver is actually impaired.

Significant marijuana use slows reaction times and impairs judgment, although marijuana users tend to compensate by driving more cautiously if they recently smoked a joint. For that reason, the Canadian Senate Special Committee on Illegal Drugs concluded that smoking marijuana, “particularly in low doses, has little effect on the skills involved in automobile driving.”

Some studies have found that substantial marijuana use causes the kinds of impairment that increase the risk of a traffic accident. Other studies conclude that heavy users have the least impairment, presumably because they develop a tolerance to the drug or have learned how to cope with its effects. Studies based on traffic accidents, rather than clinical measures of reaction times, have been less likely to find a correlation between marijuana consumption and an elevated crash risk.

Significantly, comparing crash studies involving alcohol use to crash studies involving marijuana use shows that alcohol use elevates the risk of an accident while marijuana use does not pose a similar risk. That doesn’t mean that driving while stoned is safe, but it does suggest that the problem of stoned driving is less worrisome than the problem of drunk driving.

Marijuana and DUI Arrests

Evidence of THC consumption remains in the blood long after the driver’s “high” has vanished. Drug tests for THC return a positive result well after the last use of marijuana. Since THC is stored in fat cells, chronic users can have a positive test result for several weeks after they stop smoking. Even a one-time user might have a positive test result for up to seven days after consuming marijuana. Studies show that impairment caused by THC begins to decline two hours after ingestion and that most users are no longer impaired after three or four hours have passed.

Unlike chemical tests for alcohol, a breath test cannot determine whether THC is present in a driver’s system. Blood and urine tests for THC show only that marijuana was consumed in the past. No chemical test establishes whether a smoker is under the influence of marijuana. There is no consensus that a particular concentration of THC in the blood can be correlated with impairment, in the way that a .08% blood alcohol content is generally regarded as proof of unfitness to drive.

How, then, can police officers and courts decide whether a driver was impaired by the consumption of marijuana? The question is not easy to answer. Before state legislatures made it a crime to drive with a .08% blood alcohol content, prosecutors proved DUI cases the old-fashioned way. They presented proof that (1) the driver had been drinking and (2) the driver was driving erratically. They then asked a jury to draw a connection between those two facts.

The fact that prosecutors didn’t always win DUI cases with old-fashioned proof of impaired driving (particularly when the driver was driving normally) was an impetus for the 0.08 laws, leading to the arrest of drivers who might pose an elevated risk of unsafe driving even if they are driving well. Prosecutors have no similar fallback law when it comes to prosecuting cases of stoned driving, and that means they might lose cases when the driver recently smoked marijuana but was not driving erratically. Whether that is good or bad is a matter of perspective.

Field Sobriety Tests

Most traffic officers are trained to use three field sobriety tests to decide whether to arrest a driver for DUI (or, in some cases, to administer a preliminary breath test). According to NHTSA, failing the three tests makes it probable that a driver has a blood alcohol concentration of 0.08% or higher. Courts generally accept NHTSA’s claim, despite the dubious science that underlies it and the subjective nature of the tests.

Field sobriety tests require an officer to assess a driver’s balance and ability to follow directions while walking in a straight line (the “walk-and-turn” test) and while standing on one leg (the “one leg stand” test) — tasks that are never performed while driving. The third test (horizontal gaze nystagmus) asks an officer to decide whether and when the driver’s eyes become jittery as they move from side to side. Those tests were developed to detect a driver’s illegal blood alcohol content and NHTSA has never validated them as proof that a driver is impaired by the consumption of marijuana.

Massachusetts Considers Stoned Driving

Coupled with the odor of marijuana, two of those test results (the one leg stand and the walk-and-turn) were the primary evidence against Thomas Gerhardt. An officer stopped Gerhardt’s car because the taillights were not illuminated. Gerhardt admitted that he smoked marijuana several hours before driving. The recreational consumption of marijuana is legal in Massachusetts.

The arresting officer claims that Gerhardt could not maintain his balance on one leg for 30 seconds and that he forgot to turn around and repeat the test after walking in a straight line. Gerhardt was charged with driving while impaired despite the absence of evidence of bad driving. The trial judge asked the Massachusetts Supreme Judicial Court to decide whether expert evidence of a driver’s impairment is required to convict a defendant of driving under the influence of marijuana in the absence of erratic driving.

Gerhardt’s lawyer told the state’s highest court that the field sobriety tests have been validated for blood alcohol content but not for driving impairment — a fact that NHTSA’s training manual acknowledges. With no other evidence of impairment, can Gerhardt be prosecuted?

The prosecution argues that studies have validated the field sobriety tests as evidence of impaired driving, but other studies show no correlation between HGN and walk-and-turn performance in test subjects who recently consumed of marijuana. And even NHTSA admits that officers who administer field sobriety tests in controlled laboratory conditions make improper decisions to arrest about 15% to 20% of the time. That might be adequate to establish probable cause for a DUI arrest, but a test with a 20% error rate under controlled conditions should not suffice as proof beyond a reasonable doubt of impaired driving.

The Massachusetts Supreme Judicial Court will be challenged to establish a standard of proof for a charge of driving under the influence of marijuana when there is no evidence of bad driving. As states increasingly legalize use of the drug, the court’s decision may influence the reasoning of appellate courts across the nation.

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