Cannabis and Driving

by: Paul Armentano

Fourteen US states have amended their longstanding, effect-based DUI drug laws to per se or zero tolerant per se statutes in regard to cannabis. Other states are considering enacting similar legislation. Under these amended traffic safety laws, it is a criminal violation for one to operate a motor vehicle with trace levels of cannabinoids or their metabolites in his or her blood or urine. Opponents of per se cannabinoid limits argue that neither the presence of cannabinoids nor their metabolites are appropriate or consistent predictors of behavioral or psychomotor impairment. They further argue that the imposition of such per se limits may result in the criminal conviction of individuals who may have previously consumed cannabis at some unspecified point in time, but were no longer under its influence. As more states enact statutory changes allowing for the legal use of cannabis under certain circumstances, there is a growing need to re-examine the appropriateness of these proposed per se standards for cannabinoids and their metabolites because the imposition of such limits may, in some instances, inadvertently criminalize behavior that poses no threat to traffic safety, such as the state-sanctioned private consumption of cannabis by adults.

Since 1996, 18 states and the District of Columbia have enacted legislation regulating the physician-authorized use of cannabis by patients diagnosed with specific qualifying diagnoses (NORMLa, n.d.). In November 2012, voters in two states – Colorado and Washington – decided in favor of ballot initiatives legalizing the private consumption of cannabis by those over the age of 21. These two latter state laws took effect in December 2012. Separate statewide legislative proposals to allow for the limited therapeutic use of cannabis and/or the substance’s social consumption by adults are pending in the various state legislatures and are increasingly gaining support among the public (Silver, 2011).

The ongoing political debate regarding the legal status of cannabis for adults, along with the recent relaxation of cannabis laws in certain jurisdictions in the United States, has coincided with renewed concerns among politicians, law enforcement personnel, and some members of the public regarding the substance’s potential impact on driving performance and accident risk. These concerns have provoked some state legislatures to amend their traffic safety laws in regard to cannabis.

Presently, the criminal laws in all 50 states prohibit the operation of a motor vehicle by a person who is proven to be under the influence of cannabis. These types of traffic safety laws are referred to as "effect-based DUI laws" because they mandate prosecutors establish that a motorist recently ingested cannabis and that doing so prohibited him or her from safely operating a motor vehicle. (In other words, the state must prove that a subject’s psychomotor impairment was a direct effect of the substance consumed.)

Recently, however, some states have begun to enact additional per se or zero tolerant per se statutes to their criminal traffic safety codes specific to cannabis. These per se laws create a new traffic safety violation based solely on whether or not specific quantities of cannabinoids or their inert metabolites are present in a subject’s blood or urine above a specific, state-imposed threshold. By definition, a zero tolerance per se limit for cannabinoids means that the presence of any amount of cannabinoids in the body above zero is a traffic safety violation. Under such statutes, prosecutors do not need to establish in court that the presence of these compounds caused a subject’s psychomotor impairment (or even that a subject was, in fact, impaired). As a matter of law, the only issue before the court is whether or not a defendant engaged in the act of driving with a detectable level of cannabinoids or cannabinoid metabolites in his or her bodily fluids. Proof that the defendant was behaviorally impaired is not required under the law for a prosecutor to gain a criminal conviction.

Patrick J. Silva Professional law Corporation


Policy debates regarding marijuana-law reform invariably raise the issue of marijuana and driving. This is a valid concern. In fact, NORML's own "Principles of Responsible Cannabis Use" invoke a "no driving" clause, stating: "Although cannabis is said by most experts to be safer than alcohol and many prescription drugs with motorists, responsible cannabis consumers never operate motor vehicles in an impaired condition."

Nevertheless, concerns regarding doped driving should not be an impediment to marijuana-law reform. Alcohol is legal in America, yet every state maintains tough laws punishing those who choose to drive impaired by it. There is no reason why similar principles should not regulate cannabis consumption.

Moreover, emerging scientific research indicates that cannabis actually has far less impact on the psychomotor skills needed for driving than alcohol does, and is seldom a causal factor in automobile accidents. The following documents provide a comprehensive overview of the scientific evidence regarding marijuana's impact on psychomotor skills and driving.


Acute Cannabis Intoxication May Be Associated With Changes In Psychomotor Performance

• Changes in performance are typically dose-related

• Changes in performance are most acute in naïve users

• Changes in performance are typically short-lived

– 70 percent of subjects manifest ‘significant’ psychomotor impairment 20-
40 minutes following cannabis inhalation; this percentage falls to 30 percent after 60 minutes (Berghaus et al., 1998 as cited by Gieringer)

– Peak acute effects following cannabis inhalation are typically obtained within 10 to 30 minutes (NHTSA. 2004. Drugs and Human Performance

– “Experimental research on the effects of cannabis … indicat[e] that any effects ... dissipate quickly after one hour.” (NHTSA. 2003. State of
Knowledge of Drug-Impaired Driving: FINAL REPORT)

– “[T]he cannabis effect (on driving performance) tends to disappear after
the first 60

Acute Cannabis Intoxication Is Associated With Changes In Psychomotor Performance

• Experienced users tend to become tolerant to many of cannabis’ performance-impairing effects

– “[F]requent marijuana users may show fewer behavioral signs of disruption during
intoxication than infrequent users, even when difficult memory tasks are used to
assess cognitive performance.” (Hart et al., 2010. Neurophysiological and cognitive
effects of marijuana in frequent users )

– “The present study confirms that heavy cannabis users develop tolerance to some of the impairing behavioral effects of cannabis.” (Theunissen et al., 2011. Neurophysiological functioning of occasional and heavy cannabis users during THC intoxication)

– “[T]he present study generally confirms that heavy cannabis users develop tolerance to the impairing effects of THC on neurocognitive task performance (Ramaekers et al., 2010. Tolerance and cross-tolerance to neurocognitive effects of THC and alcohol in heavy cannabis users)

– “Experienced smokers who drive on a set course show almost no functional impairment under the influence of marijuana.” (Sewell et al., op. cit.)