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;
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:
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.
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.