Two main questions arise in the law surrounding driving after having ingested cannabis: (1) whether cannabis actually impairs driving ability, and (2) whether the common practice of testing for THC (the main psychoactive substance in cannabis) is a reliable means to measure impairment. On the first question, studies are mixed. Several recent, extensive studies–including one conducted by the National Highway Traffic Safety Administration and one conducted by the American Automobile Association (AAA)–show that drivers with detectable THC in their blood are no more likely to cause car crashes than drivers with no amount of THC in their blood.Eric M. Langton, Regulating Cannabis-Using Drivers: Why Per Se Laws Are Scientifically Invalid, 34 W. Mich. U.T.M. Cooley L. Rev. 367, 387 (2018)Andrea Roth, The Uneasy Case for Marijuana as Chemical Impairment Under a Science-Based Jurisprudence of Dangerousness, 103 CAL. L. REV. 101, 105 (2015). Others show that cannabis can impair certain abilities important to safe driving (such as reaction time, divided attention, and cognitive functions)–but no studies have been able to show that this increases the actual risk of crashing, or that drivers with THC in their blood cause a disproportionate number of crashes. On the second question, the studies that have been conducted so far have consistently found that THC blood levels and degree of impairment are not closely related. No known relationship between blood levels of THC (the main psychoactive substance in cannabis) and increased relative crash risk, or THC blood levels and level of driving impairment, has been shown by single-crash or classic-control studies. Thus, even though it is possible that cannabis impairs driving ability to some extent, there are currently no reliable means to test or measure whether a driver was actually impaired.
Nonetheless, policymakers in the United States have generally dealt with cannabis-and-driving criminalization by importing the alcohol DUI regime into the cannabis context. This has led to complications down the road when cannabis-driving cases land in criminal court because cannabis detection science differs vastly from alcohol detection science. For example, blood alcohol content (BAC) has similar rates of absorption, distribution, and elimination across all humans, and there is also a fairly good correlation between BAC and level of impairment (in other words, impairment increases when BAC increases, and impairment decreases when BAC decreases). This has allowed law enforcement to use tools like breathalyzers and blood tests in criminal court because alcohol concentration is a relatively reliable indicator of how recently and how much alcohol was consumed. In contrast, THC levels can vary widely depending on the means of ingestion, THC is metabolized at an exponentially declining rate (as opposed to the steady metabolization rate for alcohol), and there is very poor correlation of THC blood levels with impairment. As stated in a report to Congress produced by the U.S. Department of Transportation's National Highway Traffic Safety Administration, "[I]n contrast to the situation with alcohol, someone can show little or no impairment at a THC level at which someone else may show a greater degree of impairment." The report also noted that, in some studies, THC was detectable as late as 30 days after ingestion–even though the acute psychoactive effects of cannabis last only for a few hours.
Effects on driving
Studies that are more carefully controlled, and that actually measure marijuana (THC) use by drivers rather than relying on self-report, generally show reduced crash risk estimates or no risk associated with marijuana use.
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." A 2012 British Medical Journal meta-analysis 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" and found that acute cannabis use increased the risk of an automobile crash.
In Cannabis and driving: a review of the literature and commentary, the United Kingdom's Department for Transport summarized available research on cannabis and driving. It concluded that there is no evidence that impairment resulting from cannabis use causes accidents, and that it is problematic to estimate the percentage of accident involvements associated with cannabis use alone given the lack of valid baseline data and that the presence of cannabis is often confounded by alcohol.Cannabis and driving: a review of the literature and commentary (No.12). nationalarchives.gov.uk. Retrieved on 2016-12-19. 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)."
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.
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 The former director of the U.S. Office of National Drug Control Policy 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.
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".
False positives According to a 2017 Report to Congress produced by the U.S. National Highway Traffic Safety Administration, many of the THC immunoassay screening tests used by law enforcement can give a false positive of THC metabolites. This is because these screenings tests "lack high specificity [and] are subject to cross-reactivity." After this initial screening, it is typical for a more accurate, specific test to be conducted.
In some areas, law enforcement uses on-site kits to collect oral fluid. (It is possible that law enforcement may be required to obtain a search warrant prior to taking this sample.) But it has not yet been shown whether these tests are accurate or reliable, and it is possible a false positive could result from environmental exposure.
It is possible that prior users of chronic cannabis users can test positive for THC even months or years after they have stopped using cannabis; there is evidence that cannabis metabolites can become stored in fat tissue and then be released due to stress or weight loss.
False negatives Fat solubility of THC also makes detection in body fluids, which are mostly water, problematic, leading to false indication of non-impairment for users.
Legal standards