I would like to stress that all of the statements and facts presented here are supported by references, Most of which are from UK government studies and Universities across the world.
Marijuana and Driving: A Review of the Scientific Evidence
It is well established that alcohol increases accident risk. Evidence of marijuana's culpability in on-road driving accidents is much less convincing.
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.
Today, a large body of research exists exploring the impact of marijuana on psychomotor skills and actual driving performance. This research consists of driving simulator studies, on-road performance studies, crash culpability studies, and summary reviews of the existing evidence. To date, the result of this research is fairly consistent: Marijuana has a measurable yet relatively mild effect on psychomotor skills, yet it does not appear to play a significant role in vehicle...