Does marijuana impair driving?

Question

Is the use of cannabis/marijuana/THC associated with risks when driving a motor vehicle, similar to impairment from the use of alcohol?

The conclusions of public studies seem inconsistent. For example, norml.org's Marijuana and Driving: A Review of the Scientific Evidence:

Although cannabis intoxication has been shown to mildly impair psychomotor skills, this impairment does not appear to be severe or long lasting. In driving simulator tests, this impairment is typically manifested by subjects decreasing their driving speed and requiring greater time to respond to emergency situations.

Nevertheless, this impairment does not appear to play a significant role in on-road traffic accidents. A 2002 review of seven separate studies involving 7,934 drivers reported, “Crash culpability studies have failed to demonstrate that drivers with cannabinoids in the blood are significantly more likely than drug-free drivers to be culpable in road crashes.” This result is likely because subject under the influence of marijuana are aware of their impairment and compensate for it accordingly, such as by slowing down and by focusing their attention when they know a response will be required. This reaction is just the opposite of that exhibited by drivers under the influence of alcohol, who tend to drive in a more risky manner proportional to their intoxication.

This appears to be well supported by a number of studies including the well reputed Canadian Senate Special Committee on Illegal Drugs. 2002. Cannabis: Summary Report: Our Position for a Canadian Public Policy. Ottawa. Chapter 8: Driving Under the Influence of Cannabis, which states:

Cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving. Cannabis leads to a more cautious style of driving. However it has a negative impact on decision time and trajectory. This in itself does not mean that drivers under the influence of cannabis represent a traffic safety risk;

and

UK Department of Environment, Transport and the Regions (Road Safety Division). 2000. Cannabis and Driving: A Review of the Literature and Commentary. Crowthorne, Berks: TRL Limited, which according to norml.org states:

There is no evidence that consumption of cannabis alone increases the risk of culpability for traffic crash fatalities or injuries for which hospitalization occurs, and may reduce those risks.

And a golden oldie by the US DOT in 1993 "Marijuana and Actual Driving Performance", states:

THC's adverse effects on driving performance appear relatively small.

However, other studies have found otherwise:

"The impact of cannabis on driving." Bédard M, Dubois S, Weaver B., Public Health Program, Lakehead University, Thunder Bay, ON. [email protected] states:

Cannabis had a negative effect on driving, as would be predicted from human performance studies. This finding supports the need for interventions to decrease the prevalence of driving under the influence of cannabis, and indicates that further studies should be conducted to investigate the dose-response relationship between cannabis and safe driving.

Science Daily "New Study Shows Cannabis Effects On Driving Skills" (Mar 1, 2013)

Cannabis is second only to alcohol for causing impaired driving and motor vehicle accidents. ... These cannabis smokers had a 10-fold increase in car crash injury compared with infrequent or nonusers after adjustment for blood alcohol concentration.

and

Science Daily "Cannabis Use Doubles Chances of Vehicle Crash, Review Finds" (Feb 10, 2012)

Drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a vehicle collision as those who are not under the influence of drugs or alcohol, claims a paper published recently on the British Medical Journal website.

It is possible that the conclusions of the above studies are consistent, perhaps because although THC only slightly impairs motor skills that slight impairment leads to a significant increase in the risk of a motor vehicle accident. Otherwise it would be helpful to understand the nature of the inconsistency.

Answer 1

This is a thorny question, both because it is a politicized issue, and because the effects of cannabis use while driving are less well studied than the effects of alcohol, and appear to include a number of confounding factors.

Overall, my survey of the literature suggests that there are two questions, which have (surprisingly) different answers:

1. Does marijuana consumption make it more difficult for a driver to operate a motor vehicle? The answer to this question is Yes.
2. Does marijuana consumption prior to operating a motor vehicle increase the risk of serious injuries or death during operation? The answer to this question is probably No, and at best that much more study is needed. Certainly it is nothing like the same risk increase as driving drunk.

Justification for claim 1: There are a number of studies on the subject, and you've cited several of them in your question. The WHO report here offers a reasonable overview of the state of research circa 1997, and concludes:

There is sufficient consistency and coherence in the evidence from experimental studies and studies of cannabinoid levels among crash victims to conclude that there is an increased risk of motor vehicle crashes among persons who drive when intoxicated with cannabis.

However, as noted in a 1999 meta-study out of John Hopkins (which I consider to be an exceptionally well done paper in general, not just on this topic), the studies considered in the WHO report

...were for experimental studies and descriptive studies of cannabis prevalence in drivers. In themselves, such studies do not establish a casual association between cannabis use and motor vehicle crashes.

That is, the studies cited by the WHO report were either experimental in nature (e.g., giving subjects controlled doses of cannabis and then measuring their driving abilities in a controlled environment), or studies that simply showed "lots of dead drivers have cannabis in their system" (which cannot show that lots of cannabis consumers who drive end up dead). The John Hopkins study cites another meta-study (Smiley 1986), which I cannot find an online source for, indicating that among other things, THC impairment of driving skills is present only during an extremely brief time after consumption. This is consistent with the measured half-life of 0.1 hours in blood mentioned slightly earlier in the paper. However, the meta-study does confirm impairment during acute exposure, and this result appears to be broadly upheld in more recent research.

Interestingly on this point, studies attempting to find a safe limit for driving while under the effects of THC (e.g., here) suggest a limit of 7–10ng/ml. For reference, a typical inhaled dose used in studies takes about 2 hours to decay to this value. Interestingly, alcohol does not have a half-life in blood, and is instead removed at a linear rate, possibly explaining the difference in effect lengths (see here).

So what we can conclude from this is that the experimental and descriptive studies in the WHO report and elsewhere do show some evidence that drivers will be impaired briefly following the consumption of cannabis.

Justification for claim 2: The main reason that there doesn't seem to be an increase in traffic accident rates after consuming cannabis, even though there is a measurable decrease in ability to drive safely, is that such drivers appear to drive more cautiously. The John Hopkins meta-study establishes this by considering a very large number of epidemiological studies together, along with evidence from experimental studies like those mentioned in part 1 above and, after accounting for other risk factors and confounding, concludes that:

1. There is no evidence that cannabis consumption increases the risk of culpability for injury or death while driving, and that in fact, such risks may be slightly reduced. (CI for the odds ratio includes 1.0, but with mean below it.)
2. There is evidence that there may be an interaction between cannabis and alcohol, which impairs drivers more than either drug alone, and leads to increased accident risk. However, this relationship is unclear, in part because of confounding factors like frequent misclassification caused by police preferentially attributing fault to drivers under the influence (as opposed to the other party in an accident), and to delays in blood testing which make it difficult to determine whether a party was in fact experiencing the symptoms of cannabis exposure at the time of the accident, or not (the exponential decay means that even small errors in the test equipment can produce large errors in the estimated time of consumption).
3. Cannabis consumption might lead to an increased risk of minor accidents (these being less rigorously reported and recorded). However, the evidence for this claim is not conclusive.

This result is in broad agreement with other more modern studies. For example, a 2005 study finds that while a naive analysis shows a strong relationship between cannabis consumption and accident rates, this relationship disappears when other risk factors (seat-belt use, alcohol consumption, speed, and sleepiness) are taken into account, and cannabis is instead found to have a mild (and not significant) 0.8 OR protective effect. In contrast, studies that do not account for other risk factors, like this one continue to find correlations between cannabis use and accident rates, but I would consider their methodology flawed because they do not account for known confounders.

Another interesting 2010 study supports all these findings as well: cannabis does impair driving ability immediately following exposure; effects do not last long; drivers compensate strongly for their impairment; alcohol and cannabis together may increase accident risk more than either alone. Its recommendation is that users be advised by doctors to wait an hour or two after consumption before driving.

So overall, it looks like driving while high is probably not a good idea (frankly, I can't imagine it being a fun experience), but is probably not risking your life or the lives of other people in anything like the same way as driving drunk. There are however a lot of confounding factors, and indisputably driving safely will be more difficult immediately following consumption, for 1-2 hours.

Answer 2

The National Highway Traffic Safety Administration recently conducted a large study that consisted of over 9,000 drivers and concluded that Cannabis use does not increase the chance of accident. They of course took into account different variables such as sex, age, etc.

Should all of us get high as a kite and drive around? Absolutely not. There are many studies that indicate that heavy users do not experience the impairment that new users do, which may or may not indicate that there are psychological factors involved.

The question is far from black and white. For example, cell phone use distracts drivers. However, is the risk of crash increased when a driver is on a highway, with nobody in sight, in good driving conditions, and takes two seconds to call somebody using his bluetooth? If indeed it is, the increased risk may not even be measurable. It may be equivalent to the risk he takes while turning up his radio. If a person takes two hits from a joint and drives, what is the level of impairment? It's very low if any. What if he takes 10 hits? How dangerous is he compared to the driver that just had four shots of 80 proof whiskey? Most every legitimate study says he is much safer than the drinker.

Also, one must be careful of the research they look at. For example, the study cited above by Science Daily seems highly flawed. It concludes that "Cannabis users "had a 10-fold increase in car crash injury..." Not only is this hard to believe, I couldn't find any solid evidence in support of their findings. Not a shred. If this were the case, we would often hear of Cannabis users causing accidents and testing positive for THC only (not alcohol). The fact is, we don't hear of this happening, hardly ever.

It is interesting to note that I've attempted many times to find any accurate statistics on the number of fatal accidents caused by drivers under the influence of Cannabis only, and to no avail. If anyone can find anything on that, I would love to see it. To my knowledge, these figures don't exist. I have done some pretty intense research into this, but it's possible I didn't look in the right places.