Cannabis and impaired driving
Cannabinoids present in the cannabis plant and its derived drugs are known to cause impaired driving in users, with effects on a driver similar to those of alcohol. Meta-analysis of epidemiological studies shows that recent cannabis use elevates the risk of motor vehicle accident 1.5 to 2 times above control. Many jurisdictions have laws forbidding cannabis-impaired driving, and some have per se impairment levels determined by metabolites detected in body fluids.
Effects on driving
While several studies have shown increased risk associated with cannabis use by drivers, other studies have not found increased risk. Cannabis usage has been shown in some studies to have a negative effect on driving ability. The British Medical Journal indicated that "drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a vehicle collision as those who are not under the influence of drugs or alcohol".
In Cannabis and driving: a review of the literature and commentary, the United Kingdom's Department for Transport reviewed data on cannabis and driving, finding "Cannabis impairs driving behaviour. However, this impairment is mediated in that subjects under cannabis treatment appear to perceive that they are indeed impaired. Where they can compensate, they do, for example ... effects of driving behaviour are present up to an hour after smoking but do not continue for extended periods". The report summarizes current knowledge about the effects of cannabis on driving and accident risk based on a review of available literature published since 1994 and the effects of cannabis on laboratory based tasks. The study identified young males, amongst whom cannabis consumption is frequent and increasing, and in whom alcohol consumption is also common, as a risk group for traffic accidents. The cause, according to the report, is driving inexperience and factors associated with youth relating to risk taking, delinquency and motivation. These demographic and psychosocial variables may relate to both drug use and accident risk, thereby presenting an artificial relationship between use of drugs and accident involvement.
Kelly, Darke and Ross show similar results, with laboratory studies examining the effects of cannabis on skills utilised while driving showing impairments in tracking, attention, reaction time, short-term memory, hand-eye coordination, vigilance, time and distance perception, and decision making and concentration. An EMCDDA review concluded that "the acute effect of moderate or higher doses of cannabis impairs the skills related to safe driving and injury risk", specifically "attention, tracking and psychomotor skills". In their review of driving simulator studies, Kelly et al. conclude that there is evidence of dose-dependent impairments in cannabis-affected drivers' ability to control a vehicle in the areas of steering, headway control, speed variability, car following, reaction time and lane positioning. The researchers note that "even in those who learn to compensate for a drug's impairing effects, substantial impairment in performance can still be observed under conditions of general task performance (i.e. when no contingencies are present to maintain compensated performance)."
A 2012 meta-analysis found that acute cannabis use increased the risk of an automobile crash.
An extensive 2013 review of 66 studies regarding crash risk and drug use found that cannabis was associated with minor, but not statistically significant increased odds of injury or fatal accident. The estimated fatal crash odds for cannabis (1.26) were lower than: opiates (1.68), antianxiety medications (2.30), zopiclone (sleep medicine) (2.60), cocaine (2.96), and amphetamines (5.17). The estimated injury odds for cannabis (1.10) were lower than: antihistamines (1.12), penicillin (1.12), antianxiety meds (1.17), antidepressants (1.35), antiasthmatics (1.31), zopiclone (sleep medicine) (1.42), cocaine (1.66), and opiates (1.91). The study concluded: "By and large, the increase in the risk of accident involvement associated with the use of drugs must be regarded as modest...Compared to the huge increase in accident risk associated with alcohol, as well as the high accident rate among young drivers, the increases in risk associated with the use of drugs are surprisingly small."
A report from the University of Colorado, Montana State University, and the University of Oregon found that on average, states that have legalized medical cannabis had a decrease in traffic-related fatalities by 8–11%. The researchers hypothesized "it’s just safer to drive under the influence of marijuana than it is drunk....Drunk drivers take more risk, they tend to go faster. They don’t realize how impaired they are. People who are under the influence of marijuana drive slower, they don’t take as many risks". Another consideration, they added, was the fact that users of marijuana tend not to go out as much.
In the largest and most precisely controlled study of its kind carried out by the U.S. Department of Transportation’s National Highway Traffic Safety Administration to research the risks of cannabis and driving, it was found that other "studies that measure the presence of THC in the drivers' blood or oral fluid, rather than relying on self-report tend to have much lower (or no) elevated crash risk estimates. Likewise better controlled studies have found lower (or no) elevated crash risk estimates". The study found that "after adjusting for age, gender, race and alcohol use, drivers who tested positive for marijuana were no more likely to crash than those who had not used any drugs or alcohol prior to driving". The study however cautions that "these results do not indicate that drug use by drivers is risk-free."
On the other hand, a recent study of Journal of Transport & Health indicated that the numbers of fatal crashes involving marijuana after the recreational marijuana legalization or decriminalization have significantly increased in Colorado, Washington, and Massachusetts.
Detection of impairment
A major US insurance company states that "there are no widely accepted methods for detecting impairment from marijuana". The U.S. drug czar Gil Kerlikowske said in 2012, "I'll be dead from old age, before we know the impairment levels" for cannabis. A 2017 Canadian government report stated "science is unable to provide general guidance to drivers about how much cannabis can be consumed before it is unsafe to drive".
Some users seem to be able to perform risk compensation by driving slower or other behaviors, and some users appear to develop a physiological tolerance, making determining a standard impairment dose difficult. A 2010 study found "cannabis and alcohol acutely impair several driving‐related skills in a dose‐related fashion, but the effects of cannabis vary more between individuals because of tolerance, differences in smoking technique, and different absorptions of THC". A 2014 study found "no correlation between degree of impairment and THCA-A blood concentration". Because of these findings, some safety organizations have advocated that police use behavioral impairment tests instead of metabolite testing.
Despite these issues, some jurisdictions have taken to a urine or blood metabolite per se test standard.
False indications of driving impairment
Testing for metabolites of THC, versus the actual THC intoxicant, can result in DUID convictions of users who aren't actually impaired. According to National Institute on Drug Abuse, "the role played by marijuana in crashes is often unclear because it can be detected in body fluids for days or even weeks after intoxication".
Fat solubility of THC also makes detection in body fluids, which are mostly water, problematic, leading to false indication of non-impairment for users.
In Uruguay, the first nation to legalize cannabis, "any THC detectable in the body will deem a motorist impaired to drive". Reporting has indicated that blood tests would be utilized. Canadian draft regulations proposed in 2017 (Bill C-46) had summary conviction offences starting at 2 nanograms per milliliter.
At the federal level, the United States Department of Transportation regulates public transportation (e.g. pilots, bus drivers, and train operators) and commercial truckers. 49 CFR Part 199 and 49 CFR Part 40 set maximum urine concentration levels of THC-A as the threshold for impairment for regulated transportation occupations. As of January 1, 2018, the THC-A testing cutoffs were 50 ng/mL for an initial immunoassay test and 15 ng/mL for a confirmatory gas chromatography–mass spectrometry test.
In some U.S. jurisdictions that have legalized cannabis, such as Colorado (5 ng/ml), Nevada (2 ng/ml), Washington State (5 ng/ml), a maximum blood concentration level has been set by the state as a threshold for driving while intoxicated (DWI/DUI).
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