Background on: Marijuana and impaired driving


More states are passing legislation permitting medical and/or recreational marijuana use, which raises concerns about users driving under the influence of marijuana. This piece will discuss:

  • Marijuana consumption and characteristics of marijuana impairment;
  • Marijuana legalization’s impact on auto accidents;
  • Difficulties related to measuring user impairment; and
  • Insurance impacts

Historical perspective

Marijuana is a type of hemp plant of the species Cannabis sativa L., part of the genus Cannabis L. Unlike industrial hemp, however, marijuana contains appreciable amounts of delta-9-tetrahydrocannabinol (THC), a psychoactive cannabinoid– it’s the active chemical that induces user intoxication. The plant also contains several other, non-psychoactive cannabinoids such as “cannabidiol” (CBD).  

There is evidence that cannabis has been consumed for thousands of years, often for medicinal purposes. The plant was used as a patent medicine in the U.S. since at least 1850, when the United States Pharmacopoeia described the plant for the first time. Cannabis was first regulated under federal law under the Marihuana Tax Act of 1937.

Marijuana was subsequently subjected to countrywide prohibition under the Controlled Substances Act of 1970 (CSA), which established a schedule for substances regulated under federal law. Marijuana is currently a Schedule I drug under the CSA, which defines Schedule I drugs as substances that have “no currently accepted medical use in the United States, a lack of accepted safety for use under medical supervision, and a high potential for abuse." Other substances under Schedule I include heroin, LSD, and peyote.

Despite the treatment of marijuana under federal law, in 1996 California became the first state in the U.S. to pass legislation permitting a medical marijuana program. By April 2021, 36 states and the District of Columbia have passed legislation permitting so-called “comprehensive” medical marijuana programs, which typically allow qualifying patients to access marijuana and marijuana-related products.

Since 2012, 18 states and the District of Columbia have passed legislation permitting anyone over the age of 21 to possess and use marijuana, subject to certain limitations. Most of those states also have or are developing regulations for a commercial market to support recreational marijuana sales.

Marijuana impairment

The THC in marijuana plants causes intoxication in a user. (THC levels in other hemp plants are typically so low that they cannot induce intoxication.)

Effects of marijuana consumption can vary. Marijuana can affect users differently, depending on a variety of factors, including user tolerance. Common experiences while intoxicated include feelings of euphoria and relaxation; some may also experience heightened sensory perceptions and altered perceptions of time.

Marijuana cannot cause overdose, but can potentially cause temporary psychosis. There are no documented instances of an adult dying from an overdose of marijuana alone. However, in rare instances a user may experience a psychotic reaction to the drug or high levels of anxiety – in some cases, these side effects could lead a user to seek medical treatment. Such negative effects are often experienced after consuming edible marijuana products, which are often more potent and take longer to induce intoxication.

Method of consumption alters impairment profile. Several factors influence intoxication onset, intensity, and duration, including method of consumption, type of marijuana product consumed, product potency, and user characteristics.

Marijuana and related products can be consumed in several ways, including inhalation (either by smoking or vaporizing) of dried plant matter or concentrates (such as hashish or kief), oral ingestion (edibles, capsules, infusible oils), sublingual ingestion (lozenges), or topical application (lotions, salves, oils).

Smoking often causes almost immediate intoxication, with impairment typically lasting 2 to 4 hours. Intoxication onset is more delayed for other methods, sometimes up to two hours for edibles – and impairment may last much longer.

Product potency is dependent on THC levels. Potency varies considerably across marijuana products and can influence the degree of impairment. Smokable marijuana plant matter can range anywhere from 8 percent to 30 percent THC, whereas high-quality hash oil could reach up to 80 percent THC. There is evidence that marijuana products have become more potent over time.

User characteristics will also influence impairment. For example, chronic users may experience less acute impairment than non-chronic users.

Marijuana and impaired driving

Marijuana intoxication can cause impaired driving, thereby increasing the risks of accidents. Marijuana legalization is associated with an increase in impaired driving.

Marijuana impairment degrades cognition and motor skills.  Marijuana alters a user’s perception. As such, most studies agree that marijuana use results in impaired coordination, memory, associative learning, attention, cognitive flexibility, and reaction time. Driving ability is thereby degraded to some degree – but by how much remains a matter of study and is subject to several factors, including the level of impairment and user characteristics.

For example, there is some evidence that user impairment may also result in limited “compensatory defensive” driving, in which a user drives more carefully to compensate for a degradation in motor functioning – but this may only mitigate degradation for some skills and may not apply to non-chronic users.

Marijuana impairment increases the risk of accidents. Nonetheless, the evidence suggests that acute impairment increases the risk of traffic accidents – though the magnitude of the increased risk is still a matter of study and can vary widely, depending on the study.

One literature review found evidence that 20 to 30 percent of crashes involving marijuana occurred because of the marijuana use. (This compares to roughly 85 percent of crashes involving alcohol that occurred because of alcohol use.) The review estimated that the crash risk increased 22 percent while under the influence of marijuana, controlling for concurrent alcohol use.

Another review found that someone driving under the influence of marijuana is 1.65 times more likely to be culpable in a fatal accident.

The greater the impairment, the worse the driving abilities. As noted above, level of impairment can influence the degree to which driving ability degrades. Indeed, there is strong evidence that the more impaired the driver, the worse their driving abilities.

Mixing marijuana and alcohol produces additive effects. There is evidence that mixing marijuana and alcohol increases impairment greater than the net effects of each individual substance. There also may exist the possibility for alcohol to increase THC levels. Potential compensatory defensive driving is nullified when a user mixes alcohol and marijuana.

The number of crash rates could increase after legalization. Researchers at the Insurance Institute for Highway Safety (IIHS) and the Highway Loss Data Institute (HLDI) since 2014 have been examining how legalization has affected crash rates and insurance claims, and evidence is emerging that crash rates go up when states legalize recreational use and retail sales of marijuana. The most recent of these studies, released in June 2021 by the IIHS, shows that injury and fatal crash rates in California, Colorado, Nevada, Oregon, and Washington jumped in the months following relaxation of marijuana laws in each state. The five states experienced a 6 percent increase in injury crash rates and a 4 percent increase in fatal crash rates, compared with other Western states where recreational marijuana use was illegal during the study period. However, only the increase in injury crash rates was statistically significant. These findings are consistent with a 2018 IIHS study of police-reported crashes, most of which did not involve injuries or fatalities. This study found that legalization of retail sales in Colorado, Oregon and Washington was associated with a 5 percent higher crash rate compared with the neighboring control states.

Fatal crashes involving drivers who tested positive for THC have increased. Some studies indicate that more people with “detectible” levels of THC in their bloodstreams were involved in fatal accidents after legalization. However, as discussed below, the mere presence of THC does not necessarily indicate marijuana impairment. Furthermore, regarding fatal crash rates overall, at least one study found no significant annual changes in crash fatality rates for Colorado and Washington when compared to 8 control states.

A 2020 study by the AAA Foundation for Traffic Safety shows that the percentage of drivers in Washington involved in fatal crashes who tested positive for marijuana increased 100 percent after the state made the drug legal for recreational use. The study considered the presence of detectable THC in the blood of fatal-crash-involved drivers. In general, the presence of detectable THC in blood suggests, but does not conclusively prove, that a person has recently used cannabis.

Collision claim frequency appears to have increased. Insurance records show an increase in claims under collision coverage, which pays for damage to an at-fault, insured driver’s own vehicle, according to HLDI’s latest analysis. The legalization of retail sales in Colorado, Nevada, Oregon, and Washington was associated with a 4 percent increase in collision claim frequency compared with the other Western states from 2012 to 2019. The 4 percent decline is down slightly from the 6 percent increase HLDI identified in a previous study, which covered 2012 to 2018.   

Higher risk demographics also have higher rates of marijuana-impaired driving. Younger drivers are at greater risk of traffic accidents than older drivers. Younger male drivers are at high risk of traffic accidents. Early evidence suggests that younger male drivers are most likely to drive under the influence of marijuana.

Use of recreational marijuana impairs driving even when the driver is not high. A study published in the journal Drug and Alcohol Dependence suggests that chronic, heavy use of recreational marijuana impairs driving skills even when the driver is not high. The researchers used a driving simulator to evaluate the potential impact of cannabis use on driving performance. The study concluded that driving impairment was significantly worse among the study participants who began using marijuana regularly before age 16. The study, by researchers at Harvard Medical School’s McLean Hospital, found that cannabis users hit more pedestrians, exceeded the speed limit more often, and drove through more red lights compared with non-users. At the time of the study, the marijuana users had not used for at least 12 hours and were not intoxicated.

Determining intoxication: “THC persistence”

A key issue raised in many studies examining the effects of marijuana-impaired driving and its risks is “THC persistence.” Unlike alcohol, THC levels in a user’s body may not be an accurate indication of impairment.

Compared with marijuana, determining alcohol intoxication is relatively straightforward. The human body processes alcohol at a rate that allows blood alcohol concentration (BAC) to closely correlate with intoxication, making it an effective and accurate benchmark for measuring impairment.

THC presence does not necessarily indicate impairment. The human body processes THC differently than alcohol. As the AAA noted in a major 2016 study, THC can remain in a user’s blood or urine for weeks after they consume marijuana, depending on various factors. Furthermore, THC levels spike immediately after consumption, but decline to low levels very quickly – long before impairment ends. It is therefore not currently possible to accurately determine when a user consumed marijuana based on the THC levels in their body.

Additionally, the length and intensity of intoxication depends not only on the strength of the marijuana product, but also on how the drug is consumed. Inhaling marijuana typically causes onset of intoxication within five minutes, with symptoms of intoxication lasting a couple of hours. On the other hand, ingesting marijuana (e.g. “special brownies”) can delay onset of intoxication between one to four hours, and intoxication can last much longer than that.

These and other reasons led the AAA to conclude that “simply detecting any THC does not therefore indicate impairment.”

A U.S. National Highway Traffic Safety Administration (NHTSA) report came to similar conclusions, noting that most studies have found that levels of THC do not closely correlate to the degree of impairment – and that often peak impairment occurs when THC levels have already begun to decline.

In sum, THC detection in a user post-accident does not necessarily mean that marijuana impairment contributed to a traffic accident.

There is no agreed-upon impairment limit. As noted above, greater impairment leads to worse driving skills. There is some evidence that higher blood THC concentrations are associated with a driver’s culpability in an accident. However, there is no agreed-upon impairment limit above which an individual is indisputably impaired.

State responses to marijuana-impaired driving

Some states enforce per se limits on THC concentrations. Nonetheless, several states currently enforce per se limits to determine marijuana impaired driving, typically 5 ng/ml of THC, though the limit in some states is as low as 1 ng/ml. Operating a vehicle with blood THC concentrations above the per se limit is illegal. Colorado enforces a “reasonable inference” standard, in which any THC concentration above 5 ng/ml can be inferred to indicate impairment. (Other states enforce a zero-tolerance policy for THC – any level of THC is prohibited.)

However, per se limits have been criticized for their potential to incriminate drivers who are not impaired, since THC can persist for long periods of time in a user. Unfortunately, the opposite may also be true: impaired drivers may not always be prosecuted, since high levels of THC quickly leave the bloodstream before impairment subsides. One study found that only 10 percent of its participants would have been prosecuted for impaired driving, even though many self-reported recent marijuana use.

Furthermore, the time between a roadside traffic stop and subsequent blood testing could take hours, making potential impairment difficult to measure since THC levels might have declined long before testing.

The AAA has therefore concluded that “a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported.”

Other states use “behavioral evaluations” to help determine impairment. Several states prohibit a driver from being under the influence of THC. In these states, determining whether a driver was marijuana-impaired depends on a variety of evidence, including behavioral evaluations of the driver by a law enforcement officer.

There is currently no scientifically-sound roadside impairment test. There is currently no “breathalyzer”-equivalent for marijuana impairment, in part due to the various difficulties of scientifically measuring impairment outlined above. Some have argued that saliva testing may help in determining THC-levels during a roadside stop, but others have argued that the mere presence of THC still cannot consistently and scientifically determine impairment.

Impact on insurance

Personal auto: the standard personal auto policy does not address driving under the influence of any drug, including alcohol and marijuana. However, auto insurance rates may be impacted by the spread of marijuana legalization, particularly if such legalization is associated with an appreciable increase in impaired driving and related accidents. An individual’s auto insurance rates may rise if they are convicted of driving under the influence of marijuana. Risky driving behavior may also influence rates.

Commercial auto: the standard commercial auto policy also does not address driving under the influence of drugs. However, the U.S. Federal Motor Carrier Safety Administration (FMCSA) governs the drug and alcohol testing rules and regulations for employees driving vehicles that require a commercial driver’s license (CDL).

The FMCSA requires employers to test a prospective employee for drugs, including marijuana, before permitting the individual to operate a commercial motor vehicle (CMV). The FMSCA may also require post-accident drug testing in the event of certain vehicle accidents, including those that result in a human fatality.

Random testing throughout the year is also required for CDL operators. Any CMV operator who is under the reasonable suspicion of being under the influence of drugs can be tested immediately. An operator who fails a drug test is prohibited from operating a CMV. The FMSCA prescribes a “return-to-duty” process for such an operator.

Of note, the U.S. Department of Transportation (DOT) has stated that a drug test cannot be verified as negative based on the fact that an employee has been certified to use medical marijuana: “It remains unacceptable for any safety‐sensitive employee subject to drug testing under the Department of Transportation’s drug testing regulations to use marijuana.”



Additional resources

American Automobile Association, “Overview of Major Issues Regarding the Impacts of Alcohol and Marijuana on Driving

Governors Highway Safety Association, “Drug Impaired Driving

National Association of Insurance Commissioners, “Cannabis and Insurance

National Conference of State Legislatures, “Drugged Driving – Marijuana-Impaired Driving

U.S. Department of Transportation, National Highway Traffic Safety Administration, “Marijuana-Impaired Driving



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