Cannabis and Driving Impairment: the Truth About Stoned Drivers

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The truth about cannabis and driving impairment can be difficult for many people to accept.  That's because, like many topics surrounding cannabis, the truth runs counterintuitive to what we have been taught, and what most people believe.

Although freedom of speech, assembly and religion are always trotted out when speaking of freedom, the one freedom Americans value most is the freedom to get into their cars and drive, drive, drive.

Like all freedoms, the freedom to drive comes with responsibilities, and one of those responsibilities is to not drive impaired.  

Naturally a lot of people, in addition to alcohol, are concerned about stoned drivers hitting the highways. 

Whenever marijuana measures are on a ballot, you can bet that the rhetoric on the dangers of cannabis and driving impairment will reach a resounding crescendo of death, doom and destruction by those who oppose the initiatives.

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Studies show the truth about cannabis and driving impairment is not what you think

The fact is when marijuana is widely and affordably available, the consumption of alcohol will significantly decrease as the consumption of cannabis increases.

This will have major implications for the health and safety of the community. Driving is one of them.

Unlike studies relating to the health benefits of marijuana, which have been systematically thwarted by our federal government, driving studies are fairly abundant.

The studies all come to pretty much the same conclusion—the use of marijuana does not significantly affect driving ability. In fact some studies actually show an improvement.

Using a double-blind, placebo-controlled clinical trial, a study published in the March issue of Journal of Psychoactive Drugs reported that drivers performed the same after smoking marijuana as they did at baseline on every single task.

The only difference found was that “Participants receiving active marijuana decreased their speed more so than those receiving the placebo cigarette during a distracted section of the drive” and “No other changes in driving performance were found.”

Excessive speed is one of the major causes of auto accidents. The study found that use of marijuana did not affect any other driving ability.  It could be argued that marijuana, which leads drivers to reduce their speed without interfering with any other measure of driver performance, can potentially reduce the likelihood of having an accident.

Supporting such a conclusion, a study in May 2008 conducted by Hebrew University and the University of Negev in Israel found that “Smoking THC cigarettes caused drivers to drive slower in a dose-dependent manner, while alcohol caused drivers to drive significantly faster than in ‘control’ conditions.”

The reason drivers drive slower when they use marijuana, according to the study, is “subjects seemed to be aware of their impairment after THC intake and tried to compensate by driving slower.”

The National Highway Traffic Safety Administration (NHTSA) has known this for a long time.

 In November 1993, the NHTSA released a study titledMarijuana and Actual Driving Performance, which concluded “THC’s adverse effects on driving performance appear relatively small” and “Evidence from the present and previous studies strongly suggests that alcohol encourages risky driving, whereas THC encourages greater caution.”

A study published in 2007 by the Canadian Journal of Public Health and an earlier French study published in 2005 found that drivers who tested positive for alcohol were three to four times more likely to engage in unsafe driving and be involved in a fatal accident than drivers who tested positive for marijuana.

Always Err on the Side of Caution

We are in no way advocating driving while impaired.  

Like many other drugs, both prescription and over-the-counter, consumption of marijuana does cause changes in one’s motor abilities.  Likewise, the same cautions that would be undertaken when taking any drug must be exercised when using cannabis.

Cannabis consumers need to heed the same precautions printed on the little white slips of paper with itsy-bitsy printing included with many over-the-counter medications such as Benadryl or NyQuil that warn “be careful when driving a motor vehicle or operating machinery.”

Marinol is synthetic THC whose properties are identical to natural THC found in marijuana, although many patients find Marinol to be discombobulating.

In the Nursing Spectrum Drug Handbook, nurses are advised to “Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.”

Once a patient understands how the drug affects him, it’s OK to drive as long as he takes the impairment into account.

That is exactly what happens when people use marijuana—they seem to take it into account that they have been consuming cannabis and they drive slower.  By driving slower they are less likely to be involved in an accident.

I am not implying that smoking marijuana and driving is OK.

Smoking cannabis and driving is not OK and should be discouraged just like driving should be discouraged when taking Benadryl, Zoloft or Ambien.

Drivers especially need to recognize that driving after ingesting both cannabis and alcohol can significantly increase their risk of accident above what it would be for alcohol alone.

Marijuana is not alcohol and is not in the same league requiring dire warnings and active law enforcement actions to penalize people who use it.

Caution needs to be exercised when cannabis is consumed, but it is not the same as alcohol and those who compare it to alcohol are distorting the truth and falsifying the research.

 These are the facts about cannabis and driving impairment, so speak up whenever our opponents compare marijuana to alcohol like they are two peas in a pod.

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About the author 

Lanny Swerdlow

Lanny Swerdlow, RN is host of the Internet radio show Marijuana Compassion & Common Sense and founder of the Marijuana Anti Prohibition Project and the Brownie Mary Democratic Club. Contact him at

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