Is Driving High Dangerous? How Does Weed Affect Driving?

driving high cheech and chong

Driving high is a bad idea.

Marijuana intoxication has a far less catastrophic effect on driving ability, coordination, motor control and risk taking when compared with alcohol and some other drugs, but that doesn’t mean driving stoned is safe.

Though getting high won’t ruin your motor skills, multiple studies from the early 1970s through 2016 show it can ruin your attention span, slow your reaction time to unexpected events and impair your cognitive ability and executive function — such as decision making and route planning. High drivers also had trouble staying in the center of their lanes.

Anyone who has driven sober knows it only takes one of these factors a split second to turn you, another motorist or or both into a bloody pulp.

Driving High: How does weed affect your driving?

Researchers mostly conducted these studies in controlled laboratory driving simulators. They compared results from driving high and driving sober from the same subjects to make sure the results were accurate — a standard practice when researching aspects of driver function for all sorts of different factors.

Interestingly, about half the studies on driving high show no statistically significant impairment, but exact dosing for Delta-9 tetrahydrocannabinol (THC, the main psychoactive ingredient in marijuana), wasn’t recorded for most of the studies.


driving high THC concentration
Figure from the 2017 report to Congress showing no direct correlation between THC concentration and driving performance.

“It is not clear why this is the case,” states a 2017 Congressional report by the National Highway Traffic Safety Administration. “It may stem from different THC doses, different time lags between doses and testing or driving, differences in the tasks used to assess the effects, tolerance developed through frequent use, and the different dependent measurement employed.”

Another notable finding of stoned driving research over the past decades is that stoned motorists tend to follow traffic rules more fastidiously — people driving high tend to maintain a greater following distance between themselves and other vehicles, and they then to drive under the speed limit. Researchers assume this results from the driver compensating for perceived impairment.

People drunk on alcohol, in contrast, tend to speed and follow too closely, in addition to the inability to stay in their lane, slowed reaction time and compromised motor skills.

How do cops measure how stoned you are when driving?

Unfortunately, there isn’t yet any way to reliably determine how stoned someone is when they are driving. Washington state, one of the first states to legalize recreational cannabis, defines driving stoned based on levels of THC in the blood. The trouble is, multiple studies have shown levels THC in the bloodstream show no correlation with real-time impairment. This is because THC is fat soluble and tends to hang around in fatty tissues long after its effects have worn off — up to a month in some cases.

This means someone who is a heavy smoker may have above the legal THC limit even if she hasn’t smoked for days, while someone who never smokes may be stoned out of her gourd while showing blood levels far below the legal limit.

Furthermore, a judge in New England recently ruled that the results of a standard field sobriety test can’t be used as evidence of marijuana intoxication in court. (The standard field sobriety test is the one where you have to follow the pen with your eyes, walk in a straight line, say the alphabet backwards, etc.)

But police agencies are working on finding better ways to catch people who are driving high. In summer of 2018, police agencies in Massachusetts are using a pilot program to test for THC in saliva with mouth swabs. Saliva levels of THC fluctuate more in tandem with consumption and are proving to be better indicators of impairment.

Most promising, however, is an effort by Hound Labs in Oakland, Calif. They have a pre-production model of their new marijuana breathalyzer that measures THC in breath vapor. The results of their studies showed that THC breath vapor levels peak shortly after consuming pot, and then fall over the curse of the next three hours, more closely following the course of an actual high.

driving high hound labs
Hound labs is touting a marijuana breathalyzer set to hit the market soon.

This new marijuana breathalyzer will have to be independently verified, tested in the field and the results established as reliable through legislation and court precedent before police agencies adopt the technology wholesale, however.

About Adam Townsend 50 Articles
Adam Maxwell Townsend has been a journalist for 15 years. His reporting has covered science, technology and medicine. Currently, he edits and writes medical and pharmaceutical articles for consumer websites, including WoahStork.

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