Marijuana-positive drivers possess a mildly elevated risk of being in accidents compared to drug and alcohol free drivers—but this risk is not positively correlated with higher blood/THC levels, according to results published online in a journal, Accident Analysis and Prevention.
Researchers assessed the risk of accident associated with drivers who tested positive for the presence of drugs or alcohol in their blood compared to drug and alcohol free drivers in 1,046 fatal auto accidents.
Alcohol-positive drivers were most likely to be culpable in fatal accidents (OR=13.7). Drivers who tested positive for drugs other than alcohol, such as opiates, sedatives, and cannabis had a ratio of 3.5.
Drivers who tested positive for marijuana only, as indicated by presence of THC in their blood, were only weakly associated with accident culpability (OR=1.3).
Drivers who tested positive for the presence of THC in blood at levels of 5ng/ml or higher possessed no elevated rate of accident culpability (OR=1), but, oddly, those with trace levels of THC (less than 2 ng/ml) did possess greater rates of culpability (OR=3.1).
According to the US National Highway Traffic Safety Administration: “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH (metabolite) concentrations.”
Another study commissioned by the United States Department of Transportation concludes similarly: “One of the program’s objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC … in single samples. The answer is very clear: it is not. Plasma of drivers showing substantial impairment in these studies contained both high and low THC concentrations, and, drivers with high plasma concentrations showed substantial, but also no impairment, or even some improvement.”
What are the states doing?
Washington and Montana define drivers with 5ng/ml or more of THC in their blood as per se impaired. Colorado law presumes impairment if more than 5ng/ml of THC in their blood. Three states – Nevada (2ng/ml), Ohio (2ng/ml), and Pennsylvania (1ng/ml) – impose lower per se limits for THC in blood.
Eleven other states impose zero tolerance per se laws for the presence of cannabis. Lastly, some states require evidence of recent drug ingestion as well as evidence that a driver was under the influence of the substances that he or she had consumed for a DUI drug conviction to stick.
In a 2013 Humboldt Journal of Social Relations article, NORML Deputy Director Paul Armentano argued: “The sole presence of THC and/or its metabolites in blood, particularly at low levels, is an inconsistent and largely inappropriate indicator of psychomotor impairment in cannabis consuming subjects. … As additional states consider amending their cannabis consumption laws, lawmakers would be advised to consider alternative legislative approaches to address concerns over DUI cannabis behavior that do not rely solely on the presence of THC or its metabolites in blood or urine as determinants of guilt in a court of law. Otherwise, the imposition of traffic safety laws may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish those who have engaged in legally protected behavior and who have not posed any actionable traffic safety threat.”
According to a 2013 article published in Accident Analysis and Prevention, drivers found positive for amphetamines (OR=6.19), opiates (OR=1.91) and benzodiazepines (OR=1.17) represent the highest odds of traffic accident injury, while drivers testing positive for penicillin (OR=1.12), antihistamines (OR= 1.12), cannabis (OR=1.10), and analgesics (OR=1.02) possess the lowest odds ratios.
As Donnie Da Kine put it: “A 100% unimpaired 65 year-old male is a greater risk while driving than stoned 30 year-old male.”