Marijuana and Impaired Driving

Driving Performance and Cannabis Users’ Perception of Safety: A Randomized Clinical Trial

JAMA Psychiatry 2022 March

Conclusions and relevance: Smoking cannabis ad libitum by regular users resulted in simulated driving decrements. However, when experienced users control their own intake, driving impairment cannot be inferred based on THC content of the cigarette, behavioral tolerance, or THC blood concentrations. Participants’ increasing willingness to drive at 1 hour 30 minutes may indicate a false sense of driving safety. Worse driving performance is evident for several hours postsmoking in many users but appears to resolve by 4 hours 30 minutes in most individuals. Further research is needed on the impact of individual biologic differences, cannabis use history, and administration methods on driving performance.

Marijuana Effects on your Driving

Attentiveness, vigilance, perception of time and speed, and use of acquired knowledge are all affected by marijuana.

Feel Different, Drive Different
Feel Different, Drive Different

Legalization of Marijuana Linked to Increased Traffic Crashes, Fatalities
Journal of Studies on Alcohol and Drugs 2022, July 19

States that legalized recreational marijuana saw a subsequent increase in traffic crashes and fatalities, according to new research in the Journal of Studies on Alcohol and Drugs.

Farmer and colleagues’ analysis of five states that allow the recreational use of marijuana revealed a 5.8% increase in the rate of traffic crash injuries and a 4.1% increase in fatal crash rates after legalization and the onset of retail sales. The researchers found no increase in a comparison group of states that did not legalize the drug.

Overall, the initial jump in the injury rate occurred after legalization but before retail sales began. Traffic crash injuries rose 6.5% after legalization but decreased slightly (-0.7%) after retail sales commenced. However, fatal crashes increased both after legalization (+2.3%) and after retail sales were authorized (+1.8%)…

Changes in Traffic Crash Rates After Legalization of Marijuana: Results by Crash Severity
National Center for Biotechnology Information 2022, July 8

Legalization of the recreational use of marijuana was associated with a 6.5% increase in injury crash rates and a 2.3% increase in fatal crash rates, but the subsequent onset of retail marijuana sales did not elicit additional substantial changes. Thus, the combined effect of legalization and retail sales was a 5.8% increase in injury crash rates and a 4.1% increase in fatal crash rates. Across states, the effects on injury crash rates ranged from a 7% decrease to an 18% increase. The effects on fatal crash rates ranged from a 10% decrease to a 4% increase…

The Legalization of Marijuana in Colorado: The Impact Rocky Mountain High Intensity Drug Trafficking Area 2021 September

Since recreational marijuana was legalized in 2013, traffic deaths where drivers tested
positive for marijuana increased 138% while all Colorado traffic deaths increased 29%.
• Since recreational marijuana was legalized, traffic deaths involving drivers who tested
positive for marijuana more than doubled from 55 in 2013 to 131 people killed in 2020.
• Since recreational marijuana was legalized, the percentage of all Colorado traffic deaths
involving drivers who tested positive for marijuana increased from 11% in 2013 to 20%
in 2020…

Simulated driving performance among daily and occasional cannabis users
The National Center for Biotechnology Information 2021, September 6

We observed a decrement in driving performance assessed by SDLP after acute cannabis smoking that was statistically significant only in the occasional users in comparison to the nonusers. Direct contrasts between the occasional users and daily users in SDLP were not statistically significant. Daily users drove slower after cannabis use as compared to the occasional use group and non-users. The study results do not conclusively establish that occasional users exhibit more driving impairment than daily users when both smoke cannabis ad libitum…

Users of Marijuana and Alcohol Take More Risks Behind the Wheel
AAA Survey 2021, April 13

With 4/20 rapidly approaching, AAA is issuing a warning about the dangers of driving impaired. New AAA research suggests that users of both alcohol and marijuana (not necessarily at the same time) are often some of the most dangerous drivers on the road. According to the research, drivers who consumed marijuana and alcohol within a 30 day period were more likely to engage in risky behavior like speeding, texting, intentionally running red lights, and aggressive driving.

“Although some drivers think marijuana makes them a better driver, research shows it can inhibit concentration, slow reaction times, and cloud judgment. That judgement is even more compromised by a marijuana user who also drinks alcohol. It’s important that drivers know the risk that comes with these two drugs and never drive impaired.”…

Risky Driving Behaviors of Drivers Who Use Alcohol and Cannabis
Journal of the Transportation Research Board 2021, January 29

Many drivers in the United States use alcohol or cannabis, including some who co-use both substances. Using data from a nationally representative survey, self-reported engagement in various risky driving behaviors is examined among drivers who co-use alcohol and cannabis, those who use alcohol but not cannabis, those who use cannabis but not alcohol, and those who use neither.

Compared with those who neither drink nor use cannabis, those who use only cannabis were more likely to drive under the influence of prescription drugs, engage in aggressive driving, and ride with an intoxicated driver….

“Although some drivers think marijuana makes them a better driver, research shows it can inhibit concentration, slow reaction times, and cloud judgment. That judgement is even more compromised by a marijuana user who also drinks alcohol. It’s important that drivers know the risk that comes with these two drugs and never drive impaired.”…

Colorado drugged driving data
DUID Victim Voices 2021, January 11

Lukas Myers knows this.  The photo shows him being extracted from a car when he was 12 years old after a crash caused by a marijuana-impaired driver.  Most bones in both of Lukas’s legs were broken as well as both wrists.  

For some, stories like this are convincing.  Others need data.  So here is a summary of relevant data from 11 sources.

Percent of drivers who admit driving after marijuana use:

-18.6% of past-30 day adult marijuana users
-54.4%% of past-30 day high school student marijuana users (11.2% of students/20.6% of past-30 day users)…

Changes in Traffic Crash Rates After Legalization of Marijuana: Results by Crash Severity
JAMA Internal Medicine 2020, June 22

Marijuana use impairs driving,1 but researchers have not yet conclusively determined if a state’s legalizing recreational marijuana is associated with traffic fatality rates. Two early studies reported no significant change in roadway deaths following legalization in Colorado and Washington,2,3 whereas a study including Oregon reported a temporary increase.4 A more recent study, including 2017 data, found a statistically significant increase in fatal crashes only after commercial stores opened, suggesting that the effect of legalization may take more time to observe.5

Following the recent release of 2018 roadway fatality reports by the US Department of Transportation, we analyzed data from more states over a longer period of commercial sales to get a better understanding of the relationship between legalization of recreational marijuana and traffic fatalities…

Recreational Pot Laws May Boost Traffic Deaths Associated Press 2020, June 22

Previous research has had mixed results and the new studies, published Monday in JAMA Internal Medicine, can’t prove that the traffic death increases they found were caused by marijuana use.

One study found an excess 75 traffic deaths per year after retail sales began in Colorado in January 2014, compared with states without similar laws. But it found no similar change in Washington state.

The other study looked at those states plus two others that allow recreational pot sales, Oregon and Alaska. If every state legalized recreational marijuana sales, an extra 6,800 people would die each year in traffic accidents, the researchers calculated. They found an increase of 2 deaths per billion miles traveled compared with 20 states without those laws. That change was slightly higher than in the other study…

Recreational cannabis use impairs driving performance in the absence of acute intoxication
Science Direct 2020, March 1

Cannabis users had poorer driving simulator performance relative to controls.

Users were not intoxicated during testing suggesting residual effects of use.

Impairment was primarily localized to early onset (before age 16) cannabis users.

Earlier age of onset of cannabis use was associated with more severe impairment.

Covariate analyses indicate that impulsivity influences performance differences…

Fatal Crashes Involving Drivers Who Test Positive for Marijuana Increase After State Legalizes Drug
AAA Newsroom 2020, January 30

A concerning number of Washington state drivers involved in fatal crashes are testing positive for recent use of marijuana, according to new research from the AAA Foundation for Traffic Safety.

The share of drivers who, after a fatal crash, tested positive for active THC – the drug’s main psychoactive ingredient – has doubled since the state legalized marijuana in December 2012. AAA believes the increase raises important traffic safety concerns for drivers across the country, because recreational marijuana use is now legal in 11 states and Washington, D.C.

The latest AAA Foundation research found that between 2008 and 2012 – the five-year period before the drug was legal – an estimated 8.8% of Washington drivers involved in fatal crashes were positive for THC. That rate rose to 18% between 2013 and 2017…

Recreational Cannabis Impairs Driving Even When Driver Not High: Harvard Research
Insurance Journal 2020, January 16

Recreational cannabis use affects driving ability even when drivers are not intoxicated by marijuana, according to a study from a Harvard Medical School hospital.

Published in the Drug and Alcohol Dependence journal, the study “Recreational Cannabis Use Impairs Driving Performance in the Absence of Acute Intoxication,” finds that in addition to chronic, heavy, recreational cannabis use being associated with poorer driving performance in non-intoxicated individuals compared to non-users, the researchers linked earlier onset of marijuana use (under age 16) to worse performance…

Marijuana and Drugged Driving
National Institute on Drug Abuse 2019, December 31

The effects of specific drugs on driving skills differ depending on how they act in the brain. For example, marijuana can slow reaction time, impair judgment of time and distance, and decrease coordination. Drivers who have used cocaine or methamphetamine can be aggressive and reckless when driving. Certain kinds of prescription medicines, including benzodiazepines and opioids, can cause drowsiness, dizziness, and impair cognitive functioning (thinking and judgment). All of these effects can lead to vehicle crashes.

Research studies have shown negative effects of marijuana on drivers, including an increase in lane weaving, poor reaction time, and altered attention to the road. Use of alcohol with marijuana makes drivers more impaired, causing even more lane weaving. Some studies report that opioids can cause drowsiness and impair thinking and judgment…

Any dose of alcohol combined with cannabis significantly increases levels of THC in blood Science Daily 2015, May 15

Cannabis plus alcohol is one of the most frequently detected drug combinations in car accidents, yet the interaction of these two compounds is still poorly understood. A study appearing online in Clinical Chemistry, the journal of AACC, shows for the first time that the simultaneous use of alcohol and cannabis produces significantly higher blood concentrations of cannabis’s main psychoactive constituent, Δ9-tetrahydrocannabinol (THC), as well as THC’s primary active metabolite, 11-hydroxy-THC (11-OH-THC), than cannabis use alone…

The Effect of Cannabis Compared with Alcohol on Driving 2010, February 18

The prevalence of both alcohol and cannabis use and the high morbidity associated with motor vehicle crashes has lead to a plethora of research on the link between the two.Cannabis and alcohol acutely impair several driving-related skills in a dose-related fashion, but the effects of cannabis vary more between individuals than they do with alcohol because of tolerance, differences in smoking technique, and different absorptions of Δ9-tetrahydrocannabinol (THC), the active ingredient in marijuana. Detrimental effects of cannabis use vary in a dose-related fashion, and are more pronounced with highly automatic driving functions than with more complex tasks that require conscious control, whereas alcohol produces an opposite pattern of impairment. Because of both this and an increased awareness that they are impaired, marijuana smokers tend to compensate effectively while driving by utilizing a variety of behavioral strategies…