Imagine if you had to tell a family that their child was never coming home again...because a driver had a few too many drinks and they were too lazy to get a taxi? How would you feel if it was your child? Your brother, your parent, your best friend? Now imagine that you're the one who had a few drinks and thought...Home isn't too far. I'll make it without getting busted. While on the back streets worrying if the booze bus will catch you, you hit someone. How do you live with that for the rest of your life?October 24, 2025
As jurisdictions worldwide continue to legalise cannabis for both “medicinal” and recreational use, a disturbing pattern has emerged on our roads. Recent data from Ohio revealing that over 40% of drivers killed in car crashes tested positive for THC represents just the latest confirmation of what extensive research has been warning us about for years: cannabis-impaired driving has become a critical public safety issue that demands immediate attention.
The Scope of the Problem
The Ohio findings are far from an isolated incident. A 2022 study from the National Highway Traffic Safety Administration found that more than 25% of all those killed or seriously injured in road accidents who tested positive for any drug tested positive for marijuana, a higher rate than that found for alcohol at 23%. Even more alarming, a September 2024 study by researchers at London School of Economics found that the legalisation of recreational marijuana increased traffic fatalities by 75% in Alaska, 18% in California, 16% in Oregon, and 15% in Colorado.
The implications are staggering. Analysis from Harvard and New York Medical College researchers shows that if marijuana were legalised nationwide, the U.S. would suffer an additional 6,800 fatal crashes per year. These numbers represent real lives lost, families devastated, and communities forever changed by preventable tragedies.
Direct Impairment of Critical Driving Skills
Cannabis consumption significantly impairs multiple cognitive and motor functions essential for safe driving. Research has consistently demonstrated that marijuana use affects motor control and reaction time, with studies showing marijuana increases driver reaction time and the number of incorrect responses to emergencies. Users demonstrate difficulty maintaining consistent lane position and experience problems with speed variability. The ability to process multiple inputs simultaneously becomes compromised, affecting divided attention and attention maintenance capabilities that drivers rely upon constantly.
Executive function deteriorates under the influence of THC, affecting route planning and judgment in ways that can have fatal consequences. Perhaps most concerning, recent research reveals that cannabis has significant adverse effects on visual acuity, contrast sensitivity, stereoacuity, and causes night-vision disturbances. Laboratory studies examining the impairment effects of marijuana use on psychomotor and cognitive functions have shown that cannabis consumption can impair driving task-related abilities such as motor control, executive function, visual processing, short-term memory, and working memory in a dose-dependent fashion.
Reviews of studies on the effects of marijuana on driving skills have demonstrated that marijuana specifically impairs certain skills necessary for safe driving. These include controlling speed variability, maintaining proper lane positioning, sustaining adequate reaction time, managing divided attention, maintaining attention over time, planning routes effectively, making sound decisions, and properly assessing risks. In some driving simulator studies, marijuana use increased driver reaction time and the number of incorrect responses to emergencies. Drivers crashed more frequently into sudden obstacles when on high doses of THC, though this effect was not observed at lower doses.
Demonstrable Increase in Crash Risk
The crash risk data leaves no doubt. Multiple studies have shown that marijuana use increases the risk of fatal crash involvement, with drivers facing injury risk between 1.8 and 2.8 times higher than non-users. Research on drivers in fatal crashes has shown that THC-positive drivers were more than twice as likely to crash as drivers without THC in their system. The odds of drivers being found responsible for a crash increase substantially with rising marijuana concentrations in the blood.
At very high THC levels, the odds ratio for crashes can reach 10.0, representing an extraordinarily elevated risk. Studies investigating cannabis use as a risk factor for motor vehicle crash fatalities have found that while the degree of impairment varies by tetrahydrocannabinol level, the association between cannabis use and significantly increased risk of fatal crash involvement remains consistent across research. Meta-analyses confirm that acute THC administration impairs aspects of driving performance in measurable and significant ways.
The results are devastating
Dangerous THC Concentration Levels
The Ohio data revealed extremely high THC levels among deceased drivers, averaging 30.7 ng/ml. Impairment thresholds typically fall around 2-5 ng/ml, meaning these drivers had concentrations more than six times higher than levels associated with significant impairment. These elevated concentrations indicate recent use before driving and demonstrate a complete disregard for impairment risks. The data suggest an absence of effective deterrence or awareness and point to a potential normalisation of drugged driving behaviour that should alarm anyone concerned with public safety.
Colorado’s Department of Public Health and Environment examined driving impairment and motor vehicle crash risk relative to marijuana use, and evidence indicating how long it takes for impairment to resolve after marijuana use. Their findings confirmed that the risk of a motor vehicle crash increases among drivers with recent marijuana use. Using alcohol and marijuana together increases impairment and the risk of a motor vehicle crash more than using either substance alone. For less-than-weekly marijuana users, using marijuana containing 10 milligrams or more of THC is likely to impair the ability to safely drive, bike, or perform other safety-sensitive activities.
The Post-Legalisation Effect
Evidence from jurisdictions with experience in cannabis legalisation tells a cautionary tale that policymakers ignore at their peril. In Washington State, researchers assessed cannabis involvement and THC levels among fatally injured drivers before and after legalising non-medical cannabis use. Using data from all motor vehicle crash decedent drivers based on observed and imputed values, the prevalence of cannabis involvement in fatalities was 9% prior to legalisation and 19% after. In adjusted analyses, the proportion of decedent drivers with high THC levels (greater than 10 ng/ml) increased nearly five-fold after legalisation.
Although cannabis testing rates increased during the study period, findings remained generally similar when restricted to those with completed cannabis testing. This study was one of the first to impute cannabis involvement in motor vehicle crash fatalities among decedents without testing and to measure and impute THC levels rather than simply the presence or absence of THC. The results add to literature suggesting that legalising cannabis may increase motor vehicle crash fatalities and highlight the need to better characterise and mitigate those risks.
Colorado’s experience has been similarly troubling. The state’s Division of Criminal Justice analysed driving under the influence case filings and found that among cases with cannabinoid screens, 66% tested positive for cannabinoids, with 57% of all screened cases testing positive for Delta 9-THC specifically. The median value of Delta 9-THC among individuals screened was 5.2 ng/mL and the mean was 8.2 ng/mL, both of which exceeded the permissible inference level. About half of the case filings with Delta 9-THC confirmation tests had levels at or above the permissible inference level of 5 ng/mL.
The Myth of Compensatory Behavior
While some studies have suggested that marijuana users may adopt compensatory behaviours like driving slower or maintaining greater following distances, this finding creates a misleading impression of safety. Simulator studies investigating behavioural changes when driving under the influence of marijuana have concluded that marijuana use by drivers may cause decreased speeds, fewer attempts to overtake, and increased following distance to the vehicle in front. These findings stand in sharp contrast to studies investigating the effects of alcohol use.
However, these behaviours do not eliminate the fundamental impairment caused by cannabis use. They merely reflect user awareness of diminished capacity. Recent research shows that users’ perception of when impairment has resolved often occurs before actual driving performance improves. One study measuring driver performance in a simulator showed subjects perceived the impairing effects of THC to be eliminated before a measurable improvement in driving performance was seen. The false sense of security created by perceived compensation can itself be dangerous, as drivers may believe they are driving safely when objective measures show continued impairment.
Furthermore, compensatory behaviours do not prevent the fundamental cognitive and motor impairments that increase crash risk. Studies have shown that despite potentially driving more slowly, marijuana-impaired drivers still hit more pedestrians, exceed speed limits more often, make fewer stops at red lights, and make more centreline crossings than sober drivers. The notion that slower driving compensates for impairment ignores the reality that safe driving requires far more than speed control.
The Myth of Tolerance
Cannabis advocates often claim that regular users develop tolerance to impairing effects, making them safer drivers than occasional users. Evidence strongly contradicts this. While some tolerance to THC’s effects may develop, it does not eliminate impairment or reduce crash risk. In reality, as tolerance builds, users tend to increase their dose to reach the same psychoactive effects—a pattern well documented in both research and clinical settings. This makes the supposed safety benefit of tolerance an illusion, as users simply consume more to overcome reduced sensitivity. Studies comparing occasional and daily cannabis users found that both groups showed significant driving impairment. Although daily users drove slower, this compensatory behaviour failed to remove measurable performance deficits.
Chronic Impairment and False Comparisons
Regular users may also experience chronic impairment due to THC’s fat-soluble nature, which allows it to linger in brain tissue. The body continues to release and metabolise this residual THC, sometimes converting it into 11-hydroxy-THC—an even more intoxicating compound. This ongoing effect undermines claims that frequent users are safer. Similar arguments were once made about alcohol tolerance, with drinkers claiming experience made them safer drivers. Society rightly rejected that logic when setting drunk driving laws. The same principle applies to cannabis: impairment remains impairment, regardless of tolerance, and driving under its influence poses an unacceptable risk (Source: WRD News)
