Marijuana has overtaken alcohol as the most commonly detected intoxicant found in US drivers, according to Science Daily, citing a new article published in the Journal of Drug Policy Analysis.

According to the report, approximately 13% of drivers pulled over by police test positive for marijuana, compared to 8% with a measurable amount of alcohol (measurable, not necessarily over the limit). That said, cannabis remains detectable for much longer than alcohol - which makes it difficult to gauge the number of actively stoned drivers vs. drunk drivers.

Driving drunk vs. stoned

The average drunk driver (BAC > 0.01%) is around 6.5 times more likely to crash than someone driving sober, however those with a BAC of .09% or more are 11 times more likely to be involved in a fatal crash as a sober driver. Drivers with a BAC of 0.125% are 30 times more likely to be involved in a fatal crash, while those driving plastered with a BAC of 0.22% or higher are 380 times more likely to be involved in a fatal crash.

Age matters too. A 16-year-old male with a BAC of .09% is five times as likely as the average driver with the same BAC.

Overall, drivers with a BAC above 0.08% are responsible for over 80% of all deaths involving detectable amounts of alcohol according to the report, citing a 2014 NHTSA study.

The effects of stoned driving, meanwhile, is much more difficult to gauge - however "three relevant facts are clear" according to the report; driving under the influence of cannabis adds to crash risk, especially in combination with alcohol and other drugs; the risk of driving under the influence of cannabis alone, even at high levels, is much lower than the risk of driving under the influence of high levels of alcohol; and the pharmacokinetics of cannabis make it difficult to to empirically demonstrate impairment.

Cannabis use acutely degrades driving ability, particularly on automated driving responses (Asbridge, Hayden, and Cartwright 2012; Grotenhermen et al. 2005). Cannabis use impairs both attention and psychomotor performance (Ramaekers et al. 2004). Additionally, consumption can cause drowsiness and lethargy, slowed down reaction times, and alter time perception, which can lead a driver to swerve or to follow other cars too closely (Ramaekers et al. 2004). Neither the quantity of cannabis (nor its primary active agent THC) consumed, or the blood level of THC, strongly predicts the degree of impairment. -Journal of Drug Policy Analysis

Studies of the effects of stoned driving have varied as well. A 2012 study found that drivers who consumed cannabis at least three hours before driving were around twice as likely to be involved in a fatal crash vs. drivers who don't consume pot. A 2013 study, however, found no significant increase in risk of fatal motor vehicle accidents, however it did find a significant increase in the risk of a crash resulting in property damage.

Driving stoned may be safer than driving drunk due to the way cannabis affects cognitive functions vs. alcohol.

Even at levels nearly twice the 5 ng/ml legal limit in some states, the measured performance degradation with respect to perceptual and motor tasks is approximately equivalent to that at the legal BAC threshold of 0.08 (Grotenhermen et al. 2005). This discrepancy can be partially explained by the relatively limited impact of cannabis on higher cognitive functions associated with driving, such as divided attention tasks. This means that complex tasks requiring conscious control, such as interpreting and anticipating traffic, are less affected by cannabis (Grotenhermen et al. 2005). Further, drivers subjectively under the influence of cannabis are generally aware that they are impaired and adjust their driving accordingly by taking fewer risks and acting less aggressively–indeed, there is evidence they may overestimate their impairment, which is the opposite reaction of those under the influence of alcohol (Sexton et al. 2000; 2009). This heightened awareness of impairment may account for the ability of cannabis impaired drivers to correctly respond to a driving situation if given a warning; however, “where events are unexpected, such compensation is not always possible” -JDPA

Driving drunk and stoned

Bad idea. The report notes that driving while stoned and drunk produces a greater level of impairment than simply combining the risk factors of each method of intoxication alone.

"Experimental studies that evaluated the impact of cannabis and alcohol on driving skills determined that standard deviation of lateral position, time driven out of lane, reaction time, and standard deviation of headway were all more-than-additively impaired by the combination of the two drugs (Ramaekers, Robbe, and O’Hanlon 2000). The substantial impairment and high vehicle crash risk from simultaneous alcohol and cannabis use suggests a synergistically deleterious effect on driving ability (Asbridge 2014)."

Problems with detection

One of the more frustrating issues with establishing safe levels of marijuana consumption is the difficulty in accurately detecting cannabis levels in the system.

There is no breath test for cannabis, although research is underway. Blood tests cannot be conducted by law enforcement officers roadside, and the very rapid but not perfectly predictable decrease in THC concentration means that a blood test conducted one or two hours after the initial stop is likely to be inconclusive. The long half-lives of cannabinoid metabolites mean that positive urinalysis results demonstrate some use of cannabis in the several days (or, for frequent heavy users, weeks) before the test, but not that the person tested had used recently enough to be still impaired. A breath test or a cheek swab might be designed to give a positive result for about as long as actual impairment lasts, but there are to date no such tests whose results have been accepted as valid in court.

That said, the report's authors suggest that should a reliable test level of marijuana intoxication emerge - driving under the influence of pot alone "should be treated as a traffic infraction rather than a crime, unless aggravated by recklessness, aggressiveness, or high speed," while drivers who combine marijuana and alcohol "are large enough to justify criminalization."

Read the report below: