Hurth, Germany: US laws prohibiting motorists from operating a vehicle with any detectable level of cannabis or cannabis metabolites in the driver’s blood or urine improperly classify occasional marijuana smokers as impaired, concludes a report issued this month by an international panel of experts.
“Many recent per se laws for DUID [driving under the influence of drugs] prescribe a zero tolerance for specific drugs, classifying drivers as being under the influence of a drug if any amount of a listed drug or its metabolites can be detected in blood or other body fluids. … This strict approach facilitates law enforcement, but is not based on science and does not only target impaired drivers,” authors state. “Per se laws specifying non-zero limits may offer a fairer and possibly more effective alternative … than zero tolerance laws, provided these limits are, as for alcohol, derived rationally from scientific evidence.”
To date, ten states have enacted so-called “zero tolerance” drugged driving laws, making it a criminal offense for an individual to operate a motor vehicle with any detectable level of a Schedule I substance present in his or her bodily fluids. In six of these states, the law also prohibits motorists from operating a motor vehicle if they have trace levels of non-psychoactive marijuana metabolites in their system. Three states – Nevada, Pennsylvania, and Virginia – have enacted per se drugged driving standards, prohibiting individuals from operating a motor vehicle if they have levels of Schedule I drugs present in their body above a specific threshold. All other states employ an “effect based” standard for DUID, which penalizes motorists only if their observed impairment may be linked to the recent ingestion of a controlled substance.
Authors estimate that a per se threshold for THC in the driver’s blood of approximately 5 ng/ml (equal to 10 ng/ml as measured in blood serum) may be reasonable for determining relative psychomotor impairment in non-habitual users. “The most meaningful recent culpability studies indicate that drivers with THC concentrations in whole blood of less than 5 ng/ml have a crash risk no higher than that of drug-free users,” authors write. “The crash risk apparently begins to exceed that of sober drivers as THC concentrations in whole blood reach 5-10 ng/ml.”
THC blood levels typically fall below 5 ng/ml in recreational cannabis users within 60 to 90 minutes after inhalation.
Authors add that a driver who tests positive for THC in the blood at levels of 5 ng/ml may suffer from psychomotor impairment comparable to those drivers who have blood alcohol levels of .08%. However, previous studies of on-road accidents indicate that cannabis’ impact on actual driving performance appears to be more limited than alcohol because subjects under its influence are generally aware of their impairment and compensate accordingly, such as by slowing down and by focusing their attention when they know a response will be required. This behavior is largely the opposite of that exhibited by drivers under the influence of alcohol, who tend to drive in a more risky manner proportional to their intoxication.
Publication of the panel’s report comes less than a month after Congress approved legislation authorizing the Department of Transportation and the National Institutes of Health to “submit to Congress a report on the problem of drug-impaired driving,” including “an assessment of methodologies and technologies for measuring driver impairment resulting from use of the most common illicit drugs.”
For more information, please contact Paul Armentano, NORML Senior Policy Analyst, at (202) 483-5500. Full text of the expert panel’s report, “Developing Science-Based Per Se Limits for Driving Under the Influence of Cannabis,” is available upon request. A comprehensive breakdown of state drugged driving laws appears in NORML’s report, “You Are Going Directly to Jail: DUID Legislation: What It Means, Who’s Behind It, and Strategies to Prevent It,” available online at:
http://www.norml.org/index.cfm?Group_ID=6492