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How Marijuana Affects Driving Performance: Is It Safe To Drive High?

A favorite pastime of the college generation that I grew up with was to get high and go on roadies. In case you don’t know what a roadie is, it basically involves getting high in a car with friends, while blasting some crazy music, and driving around for fun – like a mini road trip. What most people want to know is the answer to the following question:

Does marijuana affect your driving performance?

Yes, the main reason marijuana impairs an individual’s ability to operate a motor vehicle is due to the Delta 9-tetrahydrocannabinol, commonly referred to as THC.

Most research indicates that it does have an effect on one’s ability to drive, but the effect is dose-dependent. Additionally there seems to be a great deal of disparity between marijuana smokers due to some other factors listed below.

  • Smoking technique: How was the marijuana ingested? Smoke, vapor, edibles, etc.
  • Potency of marijuana: How strong was the weed?
  • Amount ingested: How much was ingested?
  • Tolerance: Did the individuals consuming the marijuana have a high or low tolerance?
  • Natural coordination: How good of a driver was the person drug-free?
  • Driving experience: Does the driver have adequate experience on the road?

How Marijuana Affects Driving Performance

Below is the THC equivalent to blood alcohol concentration.  You can see how much THC it takes to equal a blood alcohol concentration of 0.05 (the legal limit).  If you ingest this amount of THC, you will experience some similar difficulties in driving performance associated with the amount of alcohol listed.  Since most smokers are not aware of the exact microg/kg of THC that they’ve ingested, they are putting themselves at risk if they take the wheel.  For more information compared to alcohol, read Alcohol vs. Marijuana: Driving Under the Influence.

  • 300 microg/kg THC = blood alcohol concentration (BAC) >/=0.05 g/dl.

The equilibrium and brake latency data with 3.95% THC are similar to prior results in our laboratory in participants with breath alcohol concentrations near 0.05%.

Automated driving tasks

Marijuana primarily worsens the ability to perform automated driving tasks. Unlike alcohol which significantly worsens a person’s ability to perform more complex tasks, marijuana affects automatic functions such as: road tracking control.

Slowed reaction time

The slowing of reaction time is estimated to be dependent upon the factors listed above. In one study it was found that marijuana increased bodily sway and resulted in a slowed reaction time when it came to hitting the brakes. The slowed brake latency was by an average of 55 ms – which in the real world is equivalent to a 5 feet delay at a speed of 60 mph.

Source: http://www.ncbi.nlm.nih.gov/pubmed/9862085

It is hypothesized that reaction time may be further slowed as the amount of marijuana consumed is increased.

Psychomotor skills

Marijuana has been found to have a detrimental influence on psychomotor skills and functioning. Psychomotor skills are skills that involve a combination of mental and physical functioning such as tying your shoes or playing baseball catch. When smoking marijuana, these skills diminish – which could make operating a motor vehicle less safe.

Source: http://www.ncbi.nlm.nih.gov/pubmed/17332811

Culpability surveys: Marijuana increases accident risk

Surveys have shown that drivers who tested positive for THC (especially in higher doses), are 3 to 7 times more likely to be responsible for an accident than sober drivers. More recently was a finding which suggests that risk of fatal or injury-resulting accidents is linked to marijuana dosage and THC blood concentration.

Source: http://www.ncbi.nlm.nih.gov/pubmed/17332811

Recency of marijuana usage is also linked to crash risk. The more recent the marijuana was consumed, the more likely there was to be an accident. Past use of the drug was not a factor.

Source: http://www.ncbi.nlm.nih.gov/pubmed/14725950

So does marijuana affect driving performance? Yes.

The degree to which it affects driving is unknown, but is likely linked to the amount that was ingested. Although much research still needs to be conducted in this area to determine safe blood levels of THC for driving, it’s best to avoid the practice altogether.

If you look at the studies carefully, you will notice that there is an increased risk of an accident while driving high on marijuana. As a society it is important to be mindful of this issue and develop appropriate regulations for THC blood concentration.

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