The US Department of Transportation (DOT) reported in its Fact Sheet on "Cannabis / Marijuana," published on the DOT website (accessed Dec. 30, 2005):
"It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects...
Marijuana has been shown to impair performance on driving simulator tasks and on open and closed driving courses for up to approximately 3 hours. Decreased car handling performance, increased reaction times, impaired time and distance estimation, inability to maintain headway, lateral travel, subjective sleepiness, motor incoordination, and impaired sustained vigilance have all been reported.
Some drivers may actually be able to improve performance for brief periods by overcompensating for self-perceived impairment...
Low doses of THC moderately impair cognitive and psychomotor tasks associated with driving, while severe driving impairment is observed with high doses, chronic use and in combination with low doses of alcohol. The more difficult and unpredictable the task, the more likely marijuana will impair performance."
The Michigan Supreme Court ruled 5-4 on June 21, 2006 in the combined cases of Michigan v. Derror (Case No. 129269) and Michigan v. Kurts (Case No. 129364):
"In these consolidated appeals, we are called upon to determine whether 11-carboxy-THC, a 'metabolite' or byproduct of metabolism created when the body breaks down THC (tetrahydrocannabinol), the psychoactive ingredient of marijuana, is a schedule I controlled substance under MCL 333.7212 of the Public Health Code. We hold that it is.
Thus, a person operating a motor vehicle with 11-carboxy-THC in his or her system may be prosecuted under MCL 257.625(8), which prohibits the operation of a motor vehicle with any amount of a schedule I controlled substance in the body...
Additionally... [we] hold that, in a prosecution under MCL 257-625(8), a prosecutor is not required to prove beyond a reasonable doubt that the defendant knew that he or she might be intoxicated. Rather, the prosecutor need only prove that the defendant had any amount of a schedule I controlled substance in his or her body."
Does Marijuana Use Significantly Impair Driving Ability Even When Not under the Immediate Effects of the Drug?
Stephen J. Heishman, PhD, Research Psychologist at the National Institute on Drug Abuse (NIDA), presented the following at the 1995 National Conference on Marijuana Use:
"Driving and marijuana do not mix; that's the bottom line.
The data from these laboratory studies show that marijuana impairs balance and coordination -- functional components important to driving -- in a dose-related way. These effects may be related to reported marijuana-induced impairment of automobile driving."
The Canadian Public Health Administration (CPHA) stated in its Nov. 21, 2005 internet "The Pot and Driving Campaign":
"Drugs that can help reduce the symptoms of a disease can also affect a person's ability to drive safely. That is why some prescription drugs come with warnings not to drive for a certain amount of time after taking them.
Cannabis impairs driving skills most severely during what is known as the acute phase, which typically lasts for up to 60 minutes after smoking.
That is followed by post-acute (the phase after the acute one) and residual phases. The residual phase is 150 minutes or more after smoking [marijuana], during which impairment subsides rapidly.
The degree of impairment during the residual phase depends on the amount of THC consumed. After smoking a so-called typical dose (about 20 mg) of THC, the residual phase lasts 2-3 hours."
Oakley Ray, PhD, Emeritus Professor of Psychology and Pharmacology at Vanderbilt University, and Charles Ksir, PhD, Professor of Psychology at the University of Wyoming, wrote in their 2004 textbook Drugs, Society, and Human Behavior:
"In everyday use while intoxicated, the marijuana user is unable to easily recall information he or she learned just seconds or minutes before...
[A]lthough reaction time is not greatly affected, if affected at all, there is a great impairment in the ability to engage in tracking behavior, such as keeping a pointer on a spot on a rotating turntable...
Behaviorally, the intoxication produced by marijuana does present some danger, especially if the user is driving."
Accident Analysis & Prevention, a peer-reviewed journal, reported in its July 2004 article titled "Psychoactive Substance Use and the Risk of Motor Vehicle Accidents," by K.L.L. Movig, et al.:
"The objective of this study was to estimate the association between psychoactive drug use and motor vehicle accidents requiring hospitalization.
The risk for road trauma was increased for single use of benzodiazepines and alcohol... High relative risks were estimated for drivers using combinations of drugs and those using a combination of drugs and alcohol. Increased risks, although not statistically significant, were assessed for drivers using amphetamines, cocaine, or opiates.
No increased risk for road trauma was found for drivers exposed to cannabis."
The International Association for Cannabis as Medicine (IACM) reported in its presentation "Developing Science-Based Per Se Limits for Driving Under the Influence of Cannabis: Finding and Recommendations by an Expert Panel," presented Sep. 2005 at the 3rd Conference on Cannabinoids in Medicine:
"Even frequent users of cannabis do not seem to have a higher accident risk than non-users, as long as they are not under the acute influence of the drug, i.e. there appear to be no extended effects of cannabis use on traffic safety beyond the period of acute impairment...
Most studies found no effect of cannabis on psychomotor function after 4 hours... a waiting period of about three hours after smoking a medium to strong social dose (15-20 mg) will be sufficient to reduce a driver's impairment."
The National Association for the Reform of Marijuana Laws (NORML) wrote in an Aug. 26, 2005 article titled "You Are Going Directly to Jail," by Paul Armentano, NORML Senior Policy Advisor:
"Unlike with alcohol, the accidental risk caused by cannabis -- particularly among those who are not acutely intoxicated -- appears limited because subjects under its influence are generally aware of their impairment and compensate to some extent, such as by slowing down and by focusing their attention when they know a response will be required."