It affects your judgement, your motor skills, slows your reaction time and so can the withdrawal symptoms.
I'm not even going to link a page because it's that easy to find supporting information that is credible.
I've been smoking for many years, and have many friends that do. The majority of stoners will tell you that it's very safe to drive while high. Why? When you're high each and every one of your senses are heightened, making you more 'aware' of things going on around you, making you a safer driver. And in addition, the feeling of slight paranoia usually makes you drive pretty slow and careful. I have no studies to back that up, but most stoners generally agree on this.
Never heard of one single incident of smoking and driving causing accidents. Alcohol and driving on the other hand, oh boy, been there done that, never doing that again (irresponsible teenager at the time). Vision blurry, misjudgement is distance, etc., very dangerous side effects for driving usually only associated with alcohol, and not marijuana.
Btw, I'm one of those productive stoners, can't stay away from my programming hobbies when I'm high. I enjoy marijuana, but I'm not exactly an advocate of legalization. There are many dangers with Marijuana from my experience, but driving certainly isn't one of them.
My own anecdote: sitting passenger in a car with someone stoned and thinking traffic was going too fast, racing by him. On I-95 in the middle lane, going approximately 25 mph. And he insisted he was actually safer when driving stoned due to that slight paranoia you'd mentioned. I mention this here only to underscore the reliability (or lack thereof) of the source you're using.
The study’s lead author, Eduardo Romano, a senior research scientist at the Pacific Institute for Research and Evaluation, said that once he adjusted for demographics and the presence of alcohol, marijuana did not statistically increase the risk of a crash.
In addition, the abstract's conclusion starts "Although overall, drugs contribute to crash risk regardless of the presence of alcohol, such a contribution is much lower than that by alcohol." (Emphasis mine.) This seems to contradict your statement that there is not an increased risk with marijuana use. (The NYT article seems to confuse the two when it talks about "risk of a fatal accident" in one paragraph then "risk of a crash" in the second.)
Instead, I think the paper you want, which is a bit older (from 2012) but not behind a firewall, is http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3276316/ . It says:
> Experimental studies have shown modest functional impairment, but debate exists over how well these experimental studies translate into real-life driving situations (41). Epidemiologic studies, however, have shown contradictory results (39, 42, 43). ... it is unclear whether marijuana plays a significant role in crash causation.
The paper itself is a meta-analysis. They write:
> Results of this meta-analysis indicate that marijuana use by drivers is associated with a significantly increased risk of crash involvement. Specifically, drivers who test positive for marijuana or self-report using marijuana are more than twice as likely as other drivers to be involved in motor vehicle crashes.
It then very emphatically points out that this conclusion is not strong enough for public policy decisions, and that there are many possible confounding factors.
This conclusion is also mentioned in the NYT link you posted: "Still, it is clear that marijuana use causes deficits that affect driving ability, Dr. Huestis said. She noted that several researchers, working independently of one another, have come up with the same estimate: a twofold increase in the risk of an accident if there is any measurable amount of THC in the bloodstream."
Thus, I think it's enough to suggest that your first line is likely incorrect.
Specifically, drivers who test positive for marijuana or self-report using marijuana are more than twice as likely as other drivers to be involved in motor vehicle crashes.
Without having read the study, do you know if this statistic controls for age or anything else? If so, I stand corrected.
Thanks for your polite tone, it's not every day I get called a "fucking moron" before 7 a.m. :)
The study I linked to says: The data were stratified and analyzed according to study design, type of drug assessment, study time period, study location, or age of the study subjects. A more than 2-fold increased crash risk associated with marijuana use was found in each of the subsets of studies
Table 2 shows the age breakdown as "<25" and "all ages." See http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3276316/table/tb... .