[6] 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.
[6] Nonetheless, policymakers in the United States have generally dealt with cannabis-and-driving criminalization by importing the alcohol DUI regime into the cannabis context.
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.
[10] 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,[11] 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.
The cause, according to the report, is driving inexperience and factors associated with youth relating to risk taking, delinquency and motivation.
[14] 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.
"[15] 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).
[20] 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.
[21] 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".
[28] 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".
[30] Fat solubility of THC also makes detection in body fluids, which are mostly water, problematic, leading to false indication of non-impairment for users.
[28] In May 2016, the American Automobile Association (AAA) Foundation for Traffic Safety conducted an extensive study concluding that, "a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported.
"[4] Another study has stated that “[z]ero-limit DUID laws for cannabis based on analysis of carboxy-THC in blood or urine lack scientific support and cannot be defended.
[32][33] Denmark Ireland Luxembourg Netherlands (if other drugs are present) United Kingdom Norway Norway Driving under the influence of drugs is a kind of road safety traffic offense information which can be exchanged by member states of the European Union in the scope of directive 2011/82/EU of 25 October 2011.
[35] In Uruguay, the first nation to legalize cannabis, "any THC detectable in the body will deem a motorist impaired to drive".
[22][38] Some legal scholarship has argued that per se cannabis-DUID laws violate the Due Process Clause under the Fourteenth Amendment of the Constitution because they "subject drivers to criminal prosecutions without any real culpability.
[44] Arizona Revised Statute § 28-1381(A)(3)[45] makes it illegal to drive or be in actual physical control of a vehicle while there is cannabis or its metabolite in the body.
The statute was challenged as violating the equal protection clauses of the federal and Arizona constitutions in State v. Hammonds,[46] but the Court of Appeals of Arizona upheld the statute on the grounds that the "presence of an inactive and nonimpairing metabolite of an illicit drug in a driver's urine does not necessarily mean that there is no active component of that drug present in the driver's blood."
[57] In People v. Gassman, 622 N.E.2d 845, appeal denied 642 N.E.2d 1291, defendant argued that 11-501(a)(6) violated due process and the equal protection clause, but the Appellate Court of Illinois, Second District, rejected his arguments.
Iowa Code section 321J.2(1)(c)[60] criminalizes the operation of a motor vehicle while "any amount of a controlled substance is present" in the driver's blood or urine.
If, as a result of consuming a controlled substance, the person's ability or capacity to [drive] is impaired, the statute has been violated.
"[69] These standard jury instructions were cited by the Court of Appeals of Minnesota in an unpublished case, State v. Suber, 2008 WL 942622.
But the court still held that this evidence was not sufficient because the state's drug recognition expert testified that cannabis could bed detectable in urinalysis for seven to ten days after ingestion, the state did not have any direct proof that the defendant ingested cannabis in the hours leading up to him driving, and there was evidence to support defendant's story of innocence: that his eyes were red and watery due to lack of sleep, and that his performance on field sobriety tests was affected by the fact that he had Asperger's syndrome.
Oklahoma Statute 47 § 11-902 makes it unlawful to drive or be in actual physical control of a vehicle who has any amount of cannabis or one of its metabolites or analogs in their "blood, saliva, urine or any other bodily fluid at the time of a test .
[77] Tennessee does not have a per se law for drug impaired driving;[78] rather, Tennessee courts take a totality of the circumstances approach, examining factors such as erratic driving, performance on field sobriety tests, presence of drugs or paraphernalia, and observed behavior such as slurred speech and unsteadiness.
"[85] Wisconsin Statute § 346.63(1)(am)[86] makes it a crime to drive with "a detectable amount of a restricted controlled substance in his or her blood."
[88] In State v. Smet, the defendant argued that § 346.63(1)(am) violated due process and the equal protection clause, but the Court of Appeals of Wisconsin rejected his arguments.