A new federal report harshly calls into question the accuracy and reliability of marijuana impairment testing methods that are commonly used by police departments across the country.
“A number of states have set a THC limit in their laws indicating that if a suspect’s THC concentration is above that level (typically 5 ng/ml of blood), then the suspect is to be considered impaired,” it says, referring to tetrahydrocannabinol, a component of marijuana that is associated with its psychoactive effects. “This per se limit appears to have been based on something other than scientific evidence. Some recent studies demonstrate that such per se limits are not evidence-based.”
In addition to blood tests, the report that the National Highway Traffic Safety Administration (NHTSA) submitted to Congress last month also slams saliva, sweat, hair and urine testing, none of which have been shown to correlate with actual impairment behind the wheel.
“Thus, there are currently no evidence-based methods to detect marijuana-impaired driving,” the report concludes.
Nonetheless, while the federal agency says “research has demonstrated the potential of marijuana to impair driving related skills,” it also cites studies showing that cannabis appears to be less of a problem than is alcohol.
“Subjects dosed on marijuana showed reduced mean speeds, increased time driving below the speed limit and increased following distance during a car following task,” NHSTA says. “Alcohol, in contrast was associated with higher mean speeds (over the speed limit), greater variability in speed, and spent a greater percent of time driving above the speed limit. Marijuana had no effect on variability of speed.”
And while “both alcohol and marijuana alone increased lane position variability and when combined the effects were additive, “only alcohol increased lane departures.”
That is not to say that driving under the influence of marijuana, or any drug, is a good idea. Cannabis can “impair critical abilities necessary for safe driving,” the report argues. Studies have indicated that some of its effects include slowed reaction time, problems with road tracking and divided attention.
That said, NHSTA acknowledges that “many studies have not shown impairment on these psychomotor tasks, cognitive and executive functions as have shown statistically significant impairments,” saying, “it is not clear why this is the case.”
Congress mandated that the U.S. Department of Transportation, of which NHSTA is a component agency, issue the report as part of a broader highway funding bill signed into law by then-President Obama last year.
The report lays out the difficulties in testing for marijuana impairment as compared to alcohol intoxication detection methods that are in common use thusly:
“While ethyl alcohol is readily soluble in water, and hence blood, THC is fat soluble. This means that once ingested, THC is stored in fatty tissues in the body and can be released back into the blood sometimes long after ingestion. Some studies have detected THC in the blood at 30 days post ingestion. Thus, while THC can be detected in the blood long after ingestion, the acute psychoactive effects of marijuana ingestion last for mere hours, not days or weeks.”
Elsewhere in the document, the federal agency argues that marijuana’s status as a Schedule I controlled substance is likely blocking research that could potentially lead to developing better ways to detect impaired driving.
“Unlike alcohol, marijuana is classified as a Schedule I substance under the Controlled Substances Act. A much smaller number of studies have looked at the impairing effects of marijuana use on driving-related skills [compared to alcohol],” it says. “The extra precautions associated with conducting research on a Schedule I drug may contribute to this relative lack of research. For example, these include the need for a government license to obtain, store and use marijuana, the security requirements for storage, and documentation requirements and disposal requirements.”
In conclusion, the 44-page report recommends that efforts to train police to detect impairment, such as drug recognition expert programs, be expanded.
It also suggests that efforts be made to develop “a psychomotor, behavioral or cognitive test that would indicate the degree of driving impairment and elevated risk of crash involvement due to marijuana use.”
But, it says, no such test that is “practical and feasible” exists at this time.
However, the recommendation comes as the National Institute on Drug Abuse (NIDA), another federal agency, is seeking proposals to develop a phone app that police could potentially use to detect marijuana impairment.
NIDA’s solicitation, issued last month, similarly slammed current testing methods as “unreliable” and “not precise.”