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The most comprehensive marijuana driving research from all over the world says.... marijuana drivers drive slower to compensate for being high.

Michael Komorn

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Marijuana makes drivers drive slower.

That's about it.

Read on to see the consensus from research spanning 50 years from USA, UK, Canada, Australia, and everywhere else. Just under 300 studies along with independent and government reports were collected, yet the research keeps pointing to the same thing. People driving after using cannabis drive a little bit slower. States with Legalized and Medical Marijuana have fewer fatal car accidents. My guess is that the fewer fatal accidents are due to substituting alcohol and other medications for cannabis.

Don't believe me, take the official government word directly from NIDA:


However, a large case-control study conducted by the National Highway Traffic Safety Administration found no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.

NHTSA 2017 Marijuana-Impaired Driving A Report to Congress


Description and Assessment of the Role of Marijuana as a Causal Factor in Traffic Crashes and the Extent of the Problem of Marijuana-Impaired Driving

The scope and magnitude of the marijuana-impaired driving problem in this country cannot be clearly specified at this time.


The National Highway Traffic Safety Administration did its own tests in 2015 and found that THC showed no increased crash risk.


However, analyses incorporating adjustments for age, gender, ethnicity, and alcohol concentration level did not show a significant increase in levels of crash risk associated with the presence of drugs. This finding indicates that these other variables (age, gender, ethnicity and alcohol use) were highly correlated with drug use and account for much of the increased risk associated with the use of illegal drugs and with THC.



Crash Fatality Rates After Recreational Marijuana Legalization in Washington and Colorado.


We found no significant association between recreational marijuana legalization in Washington and Colorado and subsequent changes in motor vehicle crash fatality rates in the first 3 years after recreational marijuana legalization.


CANNABIS USE AND DRIVING: Evidence Review Canadian Drug Policy Coalition (CDPC) Simon Fraser University


An increase of cannabis use involved in fatal car crashes has been seen in Washington State after the implementation of legalization regulations; however the presence of cannabis cannot be said to indicate impairment, nor be the main contributor to the crash risks.

• Overall, the rates of fatal crashes have declined in states with medical cannabis laws.

• The rate of cannabis-related driving offences in Canada remains low in comparison to the rate of driving offences associated with alcohol.


NHTSA The Incidence and Role of Drugs in Fatally Injured Drivers 1992


The responsibility rates of drivers with THC-only or cocaine-only were not higher than the drugfree rates.

NHTSA Marijuana and Actual Driving Performance 1993


Drivers under the influence of marijuana retain insight in their performance and will compensate where they can, for example, by slowing down or increasing effort. As a consequence, THC's adverse effects on driving performance appear relatively small.



Based on alcohol and drug testing of the full range of patients presenting to emergency departments for treatment of motor vehicle crash injury, including patients treated and released as well as those admitted to hospital, alcohol is clearly the major drug associated with serious crashes and greater injury.

With crash severity, age, and seat belt use taken into consideration, alcohol is still associated with more severe injury. However, patients testing positive for illicit drugs (marijuana, opiates, and cocaine), in the absence of alcohol, were in crashes very similar to those of patients with neither alcohol nor drugs.

When other relevant variables were considered, these drugs were not associated with more severe crashes or greater injury.




SENATE SPECIAL COMMITTEE ON ILLEGAL DRUGS CANNABIS : SUMMARY REPORT - 18 - Given what we have seen, we conclude the following:

ÿ Between 5% and 12% of drivers may drive under the influence of cannabis; this percentage increases to over 20% for young men under 25 years of age;

ÿ Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving. Cannabis leads to a more cautious style of driving. However it has a negative impact on decision time and trajectory. This in itself does not mean that drivers under the influence of cannabis represent a traffic safety risk;

ÿ A significant percentage of impaired drivers test positive for cannabis and alcohol together. The effects of cannabis when combined with alcohol are more significant than is the case for alcohol alone;


Psychomotor Performance, Subjective and Physiological Effects and Whole Blood D9 -Tetrahydrocannabinol Concentrations in Heavy, Chronic Cannabis Smokers Following Acute Smoked Cannabis


In the present study, we found minimal performance changes in critical tracking and divided attention tasks 1.5 –5.5 h after smoking 700 mg/kg (range 480–1,000) THC (Table I).


Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption


The first full year after coming into effect, legalization is associated with an 8–11 percent decrease in traffic fatalities. The impact of legalization on traffic fatalities involving alcohol is larger and estimated with more precision than its impact on traffic fatalities that do not involve alcohol. Legalization is also associated with sharp decreases in the price of marijuana and alcohol consumption, which suggests that marijuana and alcohol are substitutes. 


See the full list of studies here:



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