“Evidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments. Another way THC seems to differ qualitatively from many other drugs is that the formers users seem better able to compensate for its adverse effects while driving under the influence.”
H. Robbe. 1995. Marijuana’s effects on actual driving performance. HHMRC Road Research Unit, University of Adelaide. 1995.

“Alcohol impaired performance relative to placebo but subjects did not perceive it. THC did not impair driving performance yet the subjects thought it had.”
DOT HS 808 078, U.S. Department of Transportation, National Highway Traffic Safety Administration, Final Report, November 1993

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The theory that crazy people self-select as drug users cannot be stressed enough. A chronic behavior designed to alter one’s state of mind must be predicated on dissatisfaction with one’s natural state of mind – clinically, a chronic psychiatric disorder.

A few points about the gateway drug effect: I would dismiss any study that included one-time-only users of lsd, mdma, basic pharmacy opiates, and anything else I forgot that’s well-understood as having basically zero injury risk for first time users. There is definitely a class of people who just try things and lsd is substantially safer than hang gliding. At minimum you need to include only participants who started with a chronic cannabis habit and developed an additional chronic habit. I would also seriously consider segregating users who moved on only to psychadelics such as lsd, mushrooms, peyote, ayahuasca and salvia divinorum. There is definitely a distinct class who smokes marijuana and uses these drugs, occasionally or heavily, but would never consider even trying cocaine, opiates, amphetamines, etc. Aside from mushrooms, the psychadelic drugs are relatively safe and are simply used for reasons separate than the reasons explaining heroine use. Exploring the gateway drug effect brings us instantly into contact with the question of which other drugs should be scrutinized for potential decriminalization or legalization – but we can improve our statistics by selecting a few of these drugs and seeing whether this class can emerge from clinically feasible samples. Anecdotally I could put you onto a full contiguous city block of them, all middle class.

If you’d like me to focus my energy and attention on how marijuana affects driving, it’s my layman’s opinion that it’s an extremely small or even nonexistent problem. From the link you provided: “After alcohol, THC (delta-9-tetrahydrocannabinol), the active ingredient in marijuana, is the substance most commonly found in the blood of impaired drivers, fatally injured drivers, and motor vehicle crash victims. Studies in several localities have found that approximately 4 to 14 percent of drivers who sustained injury or died in traffic accidents tested positive for THC.” Blood tests for THC can be positive for months after the last consumption, and will light up like an oil rig fire long after the hour or so after smoking when a person might be markedly impaired. The percentage of fatally injured drivers with THC in their blood *should* be exactly the same as for all the people driving through the area – but in fact it seems to be substantially LOWER. Perhaps though this merely points to the blindingly obvious fact that people who smoke weed often drive slower and more carefully than average to avoid being pulled over – a fact which will disappear with legalization.

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It does not increase the risk in adults without specific, uncommon, vulnerabilities.

“Cannabis acts as a component cause of psychosis, that is, it increases the risk of psychosis in people with certain genetic or environmental vulnerabilities, though by itself, it is neither a sufficient nor a necessary cause of psychosis”
Parakh P & Basu D. Cannabis and psychosis: have we found the missing links? Asian J Psychiatr. 2013.

It can even help people with schizophrenia.
“Effect size differences in cognitive performance in the schizophrenia group as a function of cannabis use were in the small to medium range, denoting superior performance in cannabis-using patients.”
Rabin RA. The effects of cannabis use on neurocognition in schizophrenia: a meta-analysis. Schizophr Res. 2011.

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“…existing research suggests an alternative confounding model based on time-varying effects of socioeconomic status on IQ. A simulation of the confounding model reproduces the reported associations from the [August 2012 study], suggesting that the causal effects estimated in Meier et al. are likely to be overestimates, and that the true effect could be zero“.
-Ole Rogeberg. Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status.

In addition:
-It is likely regular teen use will drop with legalization as it did in Portugal.
-The loss was only alleged in teens who were heavy users and continued heavy use into adulthood.
-The heavy users only abstained for 1 week. It can take longer for effects to wear off in such users [Fried. 2005]
-Abstention from using was not verified.
-The study chart actually shows 5.23 point drop, not 8, in the heaviest using group.

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