Glasgow, United Kingdom: Laws prohibiting motorists from operating a vehicle with any detectable level of marijuana or marijuana metabolites in the driver's blood or urine improperly classify occasional marijuana smokers as impaired, concludes a report presented this week at the 17th International Conference on Alcohol, Drugs and Traffic Safety (ICADTS) in Glasgow.
To date, ten states have adopted such DUID (driving under the influence of drugs) laws, known as zero tolerance per se laws, and similar federal legislation is pending in Congress. Under these statutes, motorists may be criminally prosecuted if trace levels of cannabis or cannabis metabolites (inactive compounds that remain detectable in the urine for days or weeks after past use) are found in the driver's bodily fluids, even if the individual is neither under the influence nor impaired to drive.
"While [zero tolerance laws] facilitate law enforcement, [they] may unfairly classify many occasional drug users as impaired, even if they responsibly separate use and driving," concluded presenters from an international working panel of physicians, forensic toxicologists and traffic scientists, led by Drs. Franjo Grotenhermen of Germany's nova-Institut and Gunter Berghaus of the University of Cologne. "This is a problem particularly with driving under the influence of cannabis [because] THC, the psychoactive constituent of cannabis, and its metabolites may be detectable in blood and urine for days or even weeks after use. Per se laws specifying a science-based, finite legal limit for a suitable indicator of impairment by cannabis would minimize this problem and motivate drivers to separate cannabis use and driving."
Though no scientific consensus exists specifying per se impairment levels for marijuana, researchers said that culpability crash studies consistently report that drivers with THC levels in their blood below 5ng/ml are not associated with elevated risks of having a traffic accident.
"Commonly consumed doses of THC may cause maximum psychomotor impairment in some behavioral areas comparable to that equivalent to a BAC of above 0.08 percent," researchers determined. However, they also noted, "Relevant acute effects typically subside within 3-4 hours after smoking. ... According to culpability studies, THC levels in blood serum below 5 ng/ml were not associated with an elevated accident risk. Even a THC serum level of between 5 and 10 ng/ml may not be associated with an above normal accident risk. [Therefore,] unless they are under the acute influence of the drug, both frequent and infrequent users of cannabis do not seem to have a higher accident risk than non-users."
Consequently, researchers recommend replacing zero tolerance per se laws with scientific-based cut-off levels in order to delineate between drivers who operate a motor vehicle while impaired by cannabis versus those drivers who had previously consumed cannabis but were no longer under its influence. To ascertain this distinction, researchers recommended blood testing, which screens for the presence of THC, rather than urinalysis, which only detects the presence of marijuana metabolites.
"Despite its limitations, the THC level in blood serum is currently the most appropriate indicator of impairment by cannabis, while the presence of THC metabolites in blood or urine indicates past use but not present impairment. Current evidence suggests that a legal limit for THC in blood serum of 5-10ng/ml will effectively separate unimpaired drivers from those driving under the influence of cannabis," researchers concluded.
Previously, an Australian review of 3,398 fatally injured drivers published in the journal Accident & Analysis Prevention found that the culpability ratio for drivers with 5 ng/ml THC or less in their blood was lower than that of drug free drivers.
For a summary of European DUID laws, please visit: