April 20, 2020 Feature

The Legalization of Marijuana and Its Impact on Traffic Safety and Impaired Driving

Honorable Neil E. Axel

Around the country there is a growing trend to either decriminalize or legalize the possession and use of recreational and medical marijuana. This trend is evidenced by the legalization of marijuana in 11 states and the District of Columbia, by the decriminalization of marijuana possession in 23 states, and in the establishment of medical marijuana programs in 34 states and the District of Columbia. Many have argued that the push toward legalization has been based, at least in part, on the perception that marijuana is a harmless drug, criminal possession cases are not worthy of prosecution, the war on drugs has led to unnecessary incarceration, and the regulation of the marijuana industry leads to increased tax revenues. Regardless of the reason for this trend, there are implications to this change in social policy that impact both the safety of our nation’s highways and the investigation and adjudication of impaired driving offenses.

Legalization has implications for the investigation and adjudication of impaired driving offenses.

Legalization has implications for the investigation and adjudication of impaired driving offenses.

SDI Productions/E+ via Getty Images

According to one estimate, there are presently 22 million individuals in this country who use marijuana. Marijuana, N.I.D.A. Research Report Series (Sept. 2019). Although the precise number can be debated, what is clear is that marijuana use is on the rise, public acceptance of its use is increasing, and penalties for possession are declining. According to a web article from the financial service The Motley Fool, projected sales of recreational and medical marijuana in the United States in 2018 were estimated to be between $8.6 billion and $10 billion. S. Williams, U.S. Marijuana Sales May Triple to $30 Billion by 2023, New Report Finds, Motley Fool (June 2, 2019).

When Canada legalized marijuana in 2018, Mike Farnworth, the British Columbia Minister of Public Safety, was quoted as saying that “legalization of cannabis is the largest public policy shift this country has experienced in the past five decades. It is an octopus with many tentacles and there are many unknowns. I don’t think that when the federal government decided to legalize marijuana, it thought through all of the implications.” D. Bilefsky, Legalizing Recreational Marijuana, Canada Begins a National Experiment, N.Y. Times (Oct. 17, 2018). Around the same time, the Canadian Medical Association published an editorial calling the legalization plan an “uncontrolled experiment in which the profits of cannabis producers and tax revenues are squarely pitched against the health of Canadians.” D. Kelsall, Watching Canada’s Experiment with Legal Cannabis, 190 Can. Med. Ass’n J. E1218 (Oct. 15, 2018). However one might characterize this shift in public policy, the legalization initiatives throughout the United States are having a profound impact upon our communities.

Last year, former New York Times reporter and author Alex Berenson published a book entitled Tell Your Children: The Truth About Marijuana, Mental Health & Violence (Free Press 2019). In that book, Berenson writes that using cannabis or any drug is ultimately a personal choice. What to do about legalization is a political decision. But whether marijuana is dangerous to the brain and can ultimately cause violence is a scientific question, with a hard yes or no answer. Id. at xxxvi. Berenson reviews the extensive research undertaken thus far and concludes that marijuana is dangerous and that there is a link between its use and psychosis. By way of example, Berenson cites a 2017 National Academies of Sciences, Engineering and Medicine report, which stated:

The association between cannabis use and development of a psychotic disorder is supported by data synthesized in several good-quality systematic reviews. The magnitude of this association is moderate to large and appears to be dose-dependent. . . . The primary literature reviewed by the committee confirms the conclusions of the systematic reviews, including the association between cannabis use and psychotic outcome and the dose-dependency of the effects, further bolstering the overall strength of evidence for our conclusions.

Id. at xx (citing Nat’l Acads. of Sci., Eng’g & Medicine, The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research at 294–95 (Washington, DC, 2017)).

Because of separation of powers, judges have no control over what the legislature does, and, of course, no control over the scientific research being done on the impact of marijuana use. Judges do play a significant role, however, in responding to driving behaviors and conduct that brings individuals before the courts of limited and general jurisdiction for impaired driving. This article will look at some of the ways that marijuana has impacted traffic safety and the adjudication of marijuana-impaired driving cases.

Impact on Traffic Safety

Although the impact on traffic safety may be difficult to quantify, one measure is the data collected by jurisdictions around the country regarding the presence of alcohol and drugs in drivers involved in motor vehicle crashes. Historically, however, the collection of such data in drug-impaired driving cases has been sporadic or nonexistent, depending upon the jurisdiction. Even though more jurisdictions are testing drivers killed in automobile crashes for the presence of drugs, with the absence of good, hard historical data, it is difficult to make the necessary comparisons to reliably ascertain the precise impact that marijuana legalization has had on the number of drug-impaired fatalities. We know from studies in a number of states, including Colorado and Washington, that marijuana-related fatalities have increased significantly. See generally B. Hansen et al., Early Evidence on Recreational Marijuana Legalization and Traffic Fatalities (Feb. 2018). What perhaps we don’t know is whether those increased fatalities are directly related to the legalization of marijuana for recreational or medical purposes.

When we look at the impact of marijuana on traffic safety, first we need to look at what is involved in the act of driving. It is generally accepted that driving is “a complex activity requiring alertness, divided yet wide-ranging attention, concentration, eye-hand-foot coordination, and the ability to process visual, auditory, and kinesthetic information quickly.” P. Larkin, Medical or Recreational Marijuana and Drugged Driving, 52 Am. Cr. L. Rev. 453, 454 (2015). Each of these cognitive and psychomotor functions is impacted by the psychoactive ingredients of marijuana. In a 2017 report to Congress, the National Highway Traffic Safety Administration (NHTSA) noted that studies have shown that marijuana can impair critical abilities necessary for safe driving including slowed reaction time, impaired road tracking, decreased divided attention, impaired cognitive performance and sensory-perception functions, and impaired executive functions. R. Compton, Marijuana-Impaired Driving—A Report to Congress, DOT HS 812 440 (July 2017).

Marijuana impairment was also judicially recognized by the Supreme Judicial Court of Massachusetts in its decision in Commonwealth v. Gerhardt, 477 Mass. 775, 81 N.E.3d 751 (2017), where the court noted that

the primary psychoactive substance in marijuana, tetrahydrocannibol (THC), is known to have an impact on several functions of the brain that are relevant to driving ability, including the capacity to divide one’s attention and focus on several things at the same time, balance, and the speed of processing information. While not all researchers agree, a significant amount of research has shown that consumption of marijuana can impair the ability to drive.

While the use of marijuana has increased over the past 20 years, studies have also shown that the typical impaired driver is more likely to have not only alcohol in his or her system, but also marijuana, drugs, or a combination of drugs. For the past 45 years, the NHTSA has conducted roadside surveys to determine the percentage of motorists operating a vehicle with alcohol in their systems. More recently, the surveys have also tested for the presence of drugs. Typically, these surveys involved thousands of subjects who voluntarily submitted to alcohol and drug testing at more than 300 locations around the country. In each instance, the motorist stops at a checkpoint and is asked to voluntarily submit to testing with the promise not to be arrested or prosecuted as a result of the testing.

These surveys have shown dramatic decreases in the incidence of alcohol-impaired drivers who were operating with a blood alcohol level of 0.08 or higher. For example, the percentage of drivers in the sample who tested positive for alcohol at 0.08 or higher decreased from 7.5 percent in 1973 to 1.5 percent in 2015. A. Berning et al., Results of the 2013–2014 National Roadside Survey of Alcohol and Drug Use By Drivers, NHTSA Traffic Safety Facts Research Note, DOT HS 812 118 (Feb. 2015). At the same time, when the survey compared the percentage of drivers who had drugs in their system, the numbers show a significant increase in individuals driving with potentially impairing drugs in their system. From 2007 to 2015, while the presence of any alcohol declined from 12.4 percent to 8.3 percent, the presence of marijuana rose from 8.6 percent to 12.6 percent. In the latest survey in 2015, the presence of any drug other than alcohol was found in 22.5 percent of the subjects. Id. Although the presence of drugs in one’s system does not necessarily demonstrate a level of impairment, these statistics point to a significant increase in the presence of drugs on our highways. When coupled with other studies and statistics, we are seeing an alarming number of drug-impaired drivers on our roadways.

At the same time, the public’s perception of the dangers associated with drugs and driving is declining. In Colorado, in a survey of more than 11,000 marijuana users, 40 percent of recreational users and 34 percent of medical marijuana users believed that marijuana had no impact of their ability to drive. CDOT Survey Reveals New Insight on Marijuana and Driving, Colo. Dep’t of Transp. (Apr. 17, 2018).

Per Se and Zero Tolerance Impaired Driving Statutes

As with alcohol-impaired driving, states have approached drug-impaired driving cases with both per se or “zero tolerance” statutes as well as statutes that require proof that one is operating a motor vehicle while one’s ability to do so is impaired or impacted by a drug, combination of drugs, or combination of drugs and alcohol. In the case of per se or zero tolerance laws, the state sets a limit as to what level of drugs in one’s body is permissible when operating a motor vehicle. One example is Pennsylvania’s law that prohibits one from driving with any amount of controlled substance or metabolite in one’s blood, or Washington’s law that prohibits one from driving with a THC concentration of at least 5 ng/ml in one’s blood. In such cases, all that is necessary to convict one of operating under the influence is evidence that the defendant was operating a motor vehicle and had a level of drugs in his or her body above the statutory minimum. In the case of marijuana, 17 states have per se or zero tolerance statutes.

Notwithstanding the adoption of per se drug-impaired driving statutes, within the scientific community, there is little correlation between the measurable level of drugs in one’s blood and a degree of impairment. R.L. DuPont et al., The Need for Drugged Driving Per Se Laws: A Commentary, 13 Traffic Injury Prevention 31 (2012). A variety of factors contribute to the extent that one’s ability to drive is impacted by drugs in one’s system including varying rates of absorption, distribution and elimination from the body, one’s sensitivity to effects of drugs, the potency of the drug, and one’s prior use of drugs.

In states that have both medical marijuana and per se statutes, a conflict may arise when courts are called upon to reconcile the legislative intent in enacting a law that prohibits one from operating a motor vehicle with any amount of marijuana in one’s system and a law that permits certain individuals to lawfully use marijuana for medical purposes. In some circumstances, one may lawfully use medical marijuana and then operate a motor vehicle days later and be charged and convicted for operating under the influence of marijuana even though that operator is not cognitively or physiologically impaired by marijuana. A number of states have addressed this issue. In People v. Koon, 832 N.W.2d 724 (Mich. 2013), for example, the court reconciled the two statutes by holding that in the case of a medical marijuana user, the state could not simply rely upon the zero-tolerance impaired driving statute and would need to prove impairment in order to convict. In Dobson v. McClennen, 361 P.3d 374 (Ariz. 2015), the Arizona court took a slightly different approach and placed the burden on the medical marijuana cardholder to show that marijuana or its metabolite was in a concentration insufficient to cause impairment.

Drug Recognition Expert Testimony

In the course of the investigation, prosecution, and trial of impaired driving cases, the legalization of marijuana has also led to a number of issues that have increased the complexity of such cases. These can include Fourth Amendment issues, the admissibility of scientific and opinion evidence, and new technologies and investigative tools used to detect and prove impairment.

In jurisdictions without a per se statute, or where a defendant refuses a blood test, opinion evidence from a specially trained police officer, often called a “drug recognition expert” or “drug recognition evaluator” (DRE), may be an important part of the trial evidence presented to the court. DRE are specially trained police officers who are certified as proficient in administering a 12-step protocol under the Drug Evaluation and Classification Program. This protocol is a standardized set of steps employed to assist the specially trained police officers to make observations about one arrested for drug-impaired driving. The specially trained officers then apply their specialized training and experience to conclude whether their observations fit established indicia of impairment by particular classes of drugs. The DRE program has been in use for almost 40 years, and currently is employed in all 50 states, the District of Columbia, Canada, and several other countries around the world. Presently, the appellate courts of 20 states have judicially accepted DRE testimony as scientifically reliable and admissible under either Frye or Daubert standards, or admissible as nonscientific evidence based upon specialized knowledge. In holding DRE testimony to be admissible, the Wisconsin Supreme Court in State v. Chitwood, 369 Wis. 2d 132, 160, 879 N.W.2d 786, 799 (2016), noted that “every court to have considered the issue has concluded that testimony based upon the DRE protocol is admissible into evidence.” Maine and North Carolina also allow DRE testimony in court proceedings by statute.

DRE testimony, when combined with all of the facts, circumstances, observations, driving behavior, toxicology results, and admissions, helps the trier of fact to determine whether one was operating a motor vehicle while impaired by drugs. No single piece of evidence is determinative, and the judge or jury must weigh all of the evidence in its deliberations.

Blood Testing and the Fourth Amendment

Although the criminal justice system has become accustomed to easy noninvasive breath testing to prove alcohol-impaired driving cases, at this time there is no comparable breath test or noninvasive testing mechanism that can test for the presence and level of drugs in the body sufficiently reliably to be admissible in court as substantive evidence. As a result, law enforcement has had to rely upon the taking of blood from impaired driving suspects in order to test for drugs. This increased reliance on blood testing due to the increase in drug-impaired driving as well as breath test refusals has led to the increased use of blood testing results in court proceedings.

In the context of criminal prosecutions, the issue is under what circumstances may the state lawfully obtain a sample of an arrestee’s blood. As Justice Brennan wrote, in the majority opinion in Schmerber v. California, 384 U.S. 757, 767 (1966), the taking of blood “plainly constitute[s] searches of ‘persons,’ and depend[s] antecedently upon seizures of ‘persons,’ within the meaning” of the Fourth Amendment. Therefore, “[t]he starting point for analyzing the validity of a warrantless search is the underlying precept that ‘searches conducted outside the judicial process, without prior approval by a judge or magistrate, are per se unreasonable under the Fourth amendment—subject only to a few specifically established and well-delineated exceptions.’” Arizona v. Gant, 556 U.S. 332, 338 (2009). Some of those exceptions have traditionally included search incident to an arrest, exigent circumstances, and consent.

For more than 40 years, law enforcement officers, relying upon Schmerber, were able to obtain breath, blood, and urine samples for testing from DUI suspects without obtaining a search warrant. In Schmerber, the Court ruled that a police officer properly compelled a DUI subject to provide a warrantless blood sample because the officer “might reasonably have believed that he was confronted with an emergency, in which the delay necessary to obtain a warrant, under the circumstances, threatened ‘the destruction of evidence.’” 384 U.S. at 770. In doing so, the Court recognized the presence of exigent circumstances because “the percentage of alcohol in the blood begins to diminish shortly after drinking stops, as the body functions to eliminate it from the system.” Id.

By 2016, however, as a result of the Supreme Court’s decisions in Missouri v. McNeely, 569 U.S. 141 (2013), and Birchfield v. North Dakota, 136 S. Ct. 2160 (2016), the judicial landscape had shifted significantly. First, in McNeely, the Court held that the natural dissipation of alcohol in one’s blood did not create a per se exception to the Fourth Amendment’s warrant requirement based on exigent circumstances. Instead, the Court left it to the trial courts to determine on a case-by-case basis when exigent circumstances exist.

Next, in Birchfield, although the case involved the circumstances under which one may be prosecuted for the separate crime of refusing to submit to a warrantless breath or blood test, the Court addressed the “search incident to arrest” exception to the warrant requirement. In this instance, after conducting a balancing test weighing the state’s interest in obtaining the evidence versus one’s right to privacy, the Court held that the Fourth Amendment does not permit a warrantless blood test incident to arrest for impaired driving.

Most recently, in 2019, the Supreme Court considered once again under what circumstances the Fourth Amendment may permit police to obtain blood from an impaired driver without a warrant. In Mitchell v. Wisconsin, 139 S. Ct. 2525 (2019), police obtained a blood sample from an unconscious driver without a warrant pursuant to Wisconsin’s implied consent law. The Supreme Court granted certiorari on the question of “[w]hether a statute authorizing a blood draw from an unconscious motorist provides an exception to the Fourth Amendment warrant requirement.”

In this case, Mitchell was arrested for driving while intoxicated following a probable cause determination by the arresting officer, including a preliminary breath test that was three times the state’s legal limit. He was taken to the police station for a “more reliable” breath test using “evidence-grade” equipment. By the time Mitchell got to the police station, he was too lethargic for a breath test and was taken to the hospital. By the time he got to the hospital, though, he was unconscious. Under Wisconsin’s implied consent law, when one is incapable of withdrawing one’s implied consent to BAC testing, then one is presumed not to have done so. Accordingly, the Wisconsin Supreme Court held that Mitchell had consented to the taking of his blood and upheld the admissibility of the warrantless blood test result. State v. Mitchell, 383 Wis. 2d 192 (2018).

Although certiorari was granted on an issue related to implied consent, the court decided the case based upon Schmerber and the exigent circumstances exception to the warrant requirement. The court held that when a driver is unconscious and cannot be given a breath test, the exigent circumstances doctrine permits a warrantless blood test to prevent the imminent destruction of evidence caused by the rapidly dissipating blood-alcohol evidence. In so holding, the court noted that Mitchell’s medical condition created the same type of urgency that the automobile accident created in Schmerber, and redefined exigency in impaired driving cases to exist

when (1) BAC evidence is dissipating and (2) some other factor creates pressing health, safety, or law enforcement needs that would take priority over a warrant application. Both conditions are met when a drunk-driving suspect is unconscious, so Schmerber controls: With such suspects, too, a warrantless blood draw is lawful.

Mitchell, 139 S. Ct. at 2537. Left unresolved at this point is the applicability of implied consent laws in light of traditional Fourth Amendment jurisprudence on the scope of the consent exception to the Fourth Amendment warrant requirement. Generally, implied consent laws around the country provide that a motorist implicitly agrees to submit to a blood or breath test by obtaining a driver’s license and/or driving on the state’s roadways. If a motorist, for example, elects to withdraw his or her consent and not agree to a blood test, although there may be administrative penalties for his or her decision, may the police still lawfully obtain a blood sample without a warrant? At this point, we know only the view of three of the Court’s nine justices. Writing for Justices Ginsburg and Kagan, Justice Sotomayor dissented from the plurality decision, and noted that Wisconsin’s implied consent law “cannot itself create the actual and informed consent that the Fourth Amendment requires.” Id. at 2546 (Sotomayor, J., dissenting).

Medical Marijuana

The state-sanctioned use of marijuana for medical purposes adds a new dimension to the legalization of marijuana and its impact on impaired-driving cases. At first blush, perhaps it would not have a significant impact other than the increased numbers of individuals using a substance that impairs one’s cognitive and physical abilities to safely operate a motor vehicle. After all, drug-impaired driving laws have historically covered all forms of drugs—prescribed, illicit, over-the-counter, and other impairing substances—in its statutory scheme. But rough estimates suggest that there are more than 3.5 million medical marijuana patients nationally, and yet the scientific literature at this point may not support its effectiveness for widespread use.

In 2017, the National Academies of Sciences, Engineering, and Medicine issued a comprehensive report entitled The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. The report concludes that based on the body of current research, the use of cannabis and cannabinoids provides only varying degrees of effectiveness for certain health conditions. The report highlights the need for significantly more research to determine what medical conditions and symptoms may be improved by cannabis and what forms or derivatives of cannabis may be therapeutic. In its introduction, the report recognizes where we are today:

This is a pivotal time in the world of cannabis policy and research. Shifting public sentiment, conflicting and impeded scientific research, and legislative battles have fueled the debate about what, if any, harms or benefits can be attributed to the use of cannabis or its derivatives.

Among the conclusions set forth in the report, based upon current research, there is

  • conclusive or substantive evidence that cannabis may be effective for chemotherapy-induced nausea, chronic pain, and multiple sclerosis–related spasticity;
  • limited evidence that cannabis is effective for increasing appetite, post-traumatic stress disorder, or anxiety; and
  • insufficient or no evidence of effectiveness in treating epilepsy, amyotrophic lateral sclerosis, or schizophrenia.

At the same time, the report concluded that there is

  • substantial evidence that cannabis use prior to driving increases crash risk, and
  • moderate evidence that cannabis use may lead to alcohol, drug, or tobacco dependence.

Most recently, in October 2019, a new study was reported in The Lancet: Psychiatry (Deepak Cyril D’Souza, Canabis in Psychiatric Disorders: The Cart Before the Horse?, 6 Lancet: Psychiatry 968 (2019)) on the effectiveness of medical marijuana in the treatment of mental health disorders. The study was based on a meta-analysis of 83 studies and 3,000 subjects. The study concluded that the use of cannabinoids for depression, anxiety, attention-deficit hyperactivity disorder, Tourette syndrome, post-traumatic stress disorder, or psychosis cannot be justified based upon the current evidence.

Proponents of medical marijuana, however, argue that it is a safe and effective treatment for the symptoms of numerous medical conditions, citing numerous studies and the use of marijuana as medicine throughout world history. Opponents, on the other hand, argue that medical marijuana is merely an excuse to use marijuana recreationally and that it is too dangerous to use, is not effective, and is unnecessary in light of other legal drugs that are effective treatments.

Regardless of who is correct, the use of medical marijuana, and one’s right to do so, has raised a number of questions around the country in the adjudication and sentencing of impaired drivers. For example,

  • May one who is authorized by state law to obtain and use medical marijuana be convicted under a state’s zero tolerance or per se drug-impaired driving statute without evidence of impairment?
  • May a court prohibit a medical marijuana patient from possessing or using medical marijuana as a condition of probation?
  • Does a probationer violate his or her probation by using medical marijuana?
  • May a DUI treatment court restrict or prohibit a participant from using medical marijuana while in the program?

Courts around the country are wrestling with these and other questions as the medical marijuana industry continues to expand. Nonetheless, regardless of whether a drug is a lawfully or unlawfully used controlled substance, prescription drug, or over-the-counter medication, the right to use the substance does not vitiate the responsibility to operate a motor vehicle unimpaired.


The push to legalize or decriminalize the use of marijuana around the country may well represent a number of “political” decisions, as characterized by Alex Berenson, but its impact is on the one hand significant, and on the other hand unknown. More research is needed into a host of issues: the health risks of marijuana use, the health benefits of various forms of medical marijuana, the impact of marijuana on crash risks, and the development of tools to determine the presence of drugs in motorists who may be endangering others by operating motor vehicles while impaired.

We see already the increased complexity in impaired-driving cases, and trial delays occasioned by unavailability of witnesses and significant backlogs in state and local drug testing laboratories. Perhaps we are now experiencing some of the unknown consequences of legislative enactments that occur faster than the development of science.


Honorable Neil E. Axel

District Court of Maryland

Honorable Neil E. Axel has served as a judge of the District Court of Maryland since 1997. Presently he serves as a senior judge and is the ABA National Judicial Fellow for traffic safety issues. In addition, he is a member of the faculty of the National Judicial College.