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Half of cannabis users think they can drive safely while high – are they right?

It’s been several years since recreational cannabis was made legal in some US states. What effect has it had on road safety?

Driver in Colorado where recreational use of cannabis has been legal since 2014. Photograph: Alamy

Driver in Colorado where recreational use of cannabis has been legal since 2014. Photograph: Alamy

Last modified on Fri 26 Apr 2019 12.02 BST

People who are stoned often think they’re being funnier than they actually are, now we know they overestimate their driving ability too.

Almost half of cannabis users believe it’s safe to drive when you’re high, according to a new study by PSB Research and Buzzfeed News. Perhaps unsurprisingly, those who abstain from weed, take a different view – only 14% believe someone who’s stoned can drive safely.

The dangers of driving while intoxicated have been so well established that it’s easy to assume it’s the abstainers who are right and pot-smokers are simply failing to recognize the danger they pose to themselves. But a number of studies into the issue have produced a murkier picture.

It’s true that THC, the psychoactive ingredient in cannabis, can impair a person’s levels of attention and their perception of time and speed, important skills you might think for driving a car. One meta-analysis of 60 studies found that marijuana use causes impairment on every measure of safe driving, including motor-coordination, visual function and completion of complex tasks.

But a 2010 analysis published in the American Journal of Addiction found that while “cannabis and alcohol acutely impair several driving-related skills … marijuana smokers tend to compensate effectively while driving by utilizing a variety of behavioral strategies”. The authors concluded that while marijuana should, in theory, make you a worse driver, in tests it doesn’t seem to. “Cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect,” they wrote.

A federal report to Congress, conducted by the National Highway Traffic Safety Administration, came to similar conclusions in 2017. In one test, volunteers were given either marijuana, alcohol or both and then used a driving simulator. The researchers found that the stoned drivers were actually more cautious, exhibiting “reduced mean speeds, increased time driving below the speed limit and increased following distance during a car following task”, although they did find it more difficult to maintain position within a lane.

Both of these studies come with the caveat that the amount of THC consumed and the user’s tolerance levels had an impact on results, with heavy smokers likely to be more greatly impaired. Users are often unaware of how much THC they have consumed – it’s easy to track the difference between one bottle of Budweiser or two, but harder to know how much THC is in each puff of a joint.

A cannabis dispensary in Portland, Oregon. Photograph: Pure Green

For that reason, this kind of research has only limited applicability to the bigger question of whether stoned drivers are actually likely to cause more accidents in the real world. Perhaps the more pertinent question is whether states where cannabis has been legalised have seen an increase in crashes and collisions.

A 2017 study found that fatal collisions have not risen in states where weed has been legalised, compared with control states where it remained criminalised. However, two further studies have shown that accidents, in general, are more common since weed became legal in certain states.

The Highway Loss Data Institute found a 12.5% increase in insurance claims on collisions in Colorado following legalisation and a 9.7% increase in Washington. But using the same methodology, they found no observable increase in accidents in Oregon (the authors suggest this may be because legal cannabis use is not continuing to increase in Oregon as it is in the other two states).

Another study by the same organisation found an average increase of 5.2% in police reporting of crashes in states where cannabis is legal compared with control states.

So it seems that further research is needed to work out the amount of weed that is dangerous and what exact effect it has on driving ability (and don’t those studies sound fun). While most studies suggest that drinking is more dangerous than smoking when it comes to driving ability, there is at least a correlation between increased cannabis use and car crashes.

It’s been several years since recreational cannabis was made legal in some US states. What effect has it had on road safety?

How Does Marijuana Affect Driving?

Chance of Crashing Increases If You Choose to Drive High

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Steven Gans, MD is board-certified in psychiatry and is an active supervisor, teacher, and mentor at Massachusetts General Hospital.

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Using marijuana can impair your judgment, motor coordination, ability to concentrate, and slow your reaction time. Therefore, it can impair your driving skills. Anytime the skills needed to drive safely are impaired, even slightly, the chances of having an auto crash increase.

Specifically, studies have found that marijuana use affects the driver’s concentration and ability to perceive time and distance. This may lead to poor speed control, drowsiness, distraction, and the inability to read road signs accurately.

The Chances of Crashing Increases

More than one research study has found a direct link between THC (the psychoactive chemical in marijuana) concentration in the blood and impaired driving skills. An analysis of several studies has found that the risk of being involved in a motor vehicle crash significantly increases after using marijuana.  

Another meta-analysis estimates that the risk of a crash that results in serious injury or death doubles after marijuana use.   In the 2015 “Traffic Safety Facts: Drug and Alcohol Crash Risk” report, the National Highway Traffic Safety Administration (NHTSA) notes that THC increased crash risk by 1 to 3 times more than sober drivers.  

Some studies also note that high-risk groups for car accidents are those most likely to use marijuana.   Most notably, this is young men in their late teens and 20s. This could play a factor in some of the statistics as well.

Higher Levels, Higher Risks

When drivers are involved in auto crashes, the drivers with THC in their blood are more likely to be the driver responsible for the accidents. This is compared to drivers who were not using drugs or alcohol. It is particularly true when THC is found at higher levels.

When marijuana use is combined with alcohol, the risk of having a highway mishap is significantly greater — much greater than with either drug used by itself. With the two combined their effect on driving skills are not added, they are multiplied, research shows.

Driving While Stoned Common

Research from the NHTSA indicates that when drivers are killed in motor vehicle crashes, drugs and alcohol are involved about 11% of the time.   The NHTSA report also shows an increase in the number of drivers who have tested positive for marijuana. They state that one in four drivers tested had THC in their system.

Increases in driving while under the influence of marijuana is attributed to the recent legalization and popularization of medical and recreational cannabis in many U.S. states.

Some drivers who use marijuana claim that smoking weed actually improves their concentration and therefore, their driving skills. Researchers have concluded that this might be true for the first few minutes of driving. However, marijuana users can soon become weary, bored, or distracted and their attention can begin to drift.

Problems With These Studies

The NHTSA, CDC, and almost every researcher that studies this issue notes that there are concerns with the marijuana-impaired driving statistics. One glaring problem is the testing procedures because THC can be detected in a person’s system for days or weeks at a time.

Unlike roadside tests for blood alcohol content, the tests currently used for marijuana can pick up traces even if the person is not impaired at the time of a crash. They may have smoked the day before or even earlier and the tests will still be positive.

The CDC notes that multiple drugs in a person’s system make it difficult to determine which contributed to a crash.   Also, drivers are not always tested for drugs, particularly if they already have an illegal blood alcohol concentration level. All of these factors can throw off data used in scientific studies.

The Risks Do Remain

Despite these concerns, the research does show that marijuana impairs a person’s ability to drive.

Also, though the laws vary by state, it is illegal to drive impaired in the United States. Using marijuana and driving while impaired can have serious legal consequences.

As a conclusion, the CDC says that “the safest option is not to have any alcohol or drugs in your system at all.”  

Research says that marijuana impairs driving. Explore what studies say about your chances of crashing if you choose to drive while high. ]]>