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Self-Perception Doesn't Predict Readiness to Drive After Cannabis Use

New research can help cannabis users make safer decisions.

Key points

  • Although THC is present in almost as many fatal car accidents as alcohol, it is much less certain when cannabis contributes to a crash.
  • Simulated driving studies confirm smoked cannabis impairs driving for no more than four and a half hours.
  • After one use of cannabis, THC can be present in biological fluids for several days, well beyond its acute effect.
  • Cannabis users often believe they can drive safely well before they are no longer impaired.

In a hilarious Cheech and Chong movie episode, the two were stoned and, aware of being impaired, were driving as carefully as possible in order not to be noticed by police, only to realize they were going only 5 MPH on a busy highway. Current research is now measuring the degree of impairment from recent cannabis use and comparing it to the level of perceived impairment. When understood, this research can help people use cannabis more safely.

Recent data regarding driving and cannabis use

A recent article containing a white-out blizzard of data stated the scary statistic that “next to alcohol, cannabis is the second most frequently found substance in the bodies of drivers involved in fatal MVAs [Motor Vehicle Accidents].”(1) Despite this stark fact, the authors probably sympathize with the phrase Mark Twain made famous: “There are three kinds of lies: lies, damned lies, and statistics.” Their article bluntly states their opinion that “ … the legal cart is currently significantly ahead of the scientific horse.”

The legal cart they are referring to consists of the zero-tolerance and per se levels of THC in blood or saliva that have been used as legal evidence of impaired driving. With the increasing legalization of medicinal and non-medicinal (i.e., recreational) use of cannabis, public safety concerns about driving under the influence of cannabis (DUIC) have heightened. In response to the proven fact that acute intoxication with cannabis impairs reaction times and driving skills and the fact that cannabis is the second most frequently found substance in fatal crashes, lawmakers raced ahead of the science to equate the mere presence of THC with proof of impairment. Zero-tolerance laws equate any amount of THC with proof of impairment, and per se laws set a pre-determined level of THC as proof of impairment, similar to the use of blood alcohol levels. But a single dose of cannabis can linger in detectable amounts for several days after its acute effects have worn off, and no reliable measure of the acute use of cannabis exists. This vastly complicates the problem of detecting cannabis use as the cause of any given car accident.

A simulated driving study confirmed “there appears to be a poor and inconsistent relationship between [the] magnitude of impairment and THC concentrations in biological samples, meaning that per se limits cannot reliably discriminate between impaired from unimpaired drivers. There is a pressing need to develop improved methods of detecting cannabis intoxication and impairment.” The study found impairment can be minimal in the presence of a positive THC result.(2)

Now a new article in JAMA Psychiatry (January 26, 2022) provides a useful guide for avoiding impairment after cannabis use.(3) In a double-blind randomized study, regular cannabis users took a simulated driving test after smoking cannabis that contained 0.02%, 5.9%, or 13.4%THC. They were instructed to smoke enough cannabis to achieve their normal high. Driving scores were measured at various time periods after smoking, along with each individual’s perception of impairment. Compared to the placebo 0.02% THC, both the 5.9% and 13.4% THC cannabis produced significantly decreased driving scores 30 minutes and 1 hour 30 minutes after being smoked. After 3 hours 30 minutes, there were borderline differences from the placebo and no differences at 4 hours 30 minutes.

Importantly, perceptions of impairment did not match driving scores, as 68.6% felt ready to drive at 1 hour 30 minutes despite impairment being nearly as bad as at 30 minutes. This inaccurate perception of driving safety means that to assure safety, individuals smoking cannabis should wait 4 hours 30 minutes before driving. Data does not yet exist for the length of driving impairment following vaped or edible cannabis. THC blood concentrations, especially in the heaviest regular cannabis users, did not predict driving score decrements.


There are two conclusions to be reached from this recent data, one for lawmakers and one for cannabis users. First, zero-tolerance and per se limits are unscientific. While they may be earnest attempts to discourage DUIC, they are arbitrary and have no defensible evidence base. Second, cannabis users need to recognize they face the same conundrum as alcohol users. Neither can rely on their self-perception of ability to drive safely. Prudence dictates cannabis should be used only in situations when no driving is necessary for the next 4 hours 30 minutes. While waiting this long assures driving is almost certainly not impaired, it will unfortunately not protect against unscientific traffic laws that consider having THC in your body is a punishable offense even when not impaired.


1. Pearlson G et al, Cannabis and Driving, Frontiers in Psychiatry, 2021 Sep 24: 12: 689444.

2. Arkell T, et al, The failings of per se limits to detect cannabis-induced driving impairment: Results from a simulated driving study, Traffic Inj Prev, 2021;22(2):102-107.…

3. Marcotte, T, et al, Driving Performance and Cannabis Users’ Perception of Safety, JAMA Psychiatry. 2022 Jan 26;e214037.

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