The link you are pointing to is a blog post, not a study.

Also, if you read the underlying studies you'll see that the ones where they give drivers marijuana under controlled conditions usually don't result in any increased risk at all, whereas with alcohol it's very easy to see a huge increase in risk using the exact same methodology). The second kind of studies they generally use are observational studies, which tend to peg the underlying risk increase from marijuana at around 2x (as compared with alcohol, which gives on average a 3x increase at the legal limit). However, the authors of these observational studies themselves say that this is most likely actually an overestimate of the risk, not an underestimate. Here are the caveats from one recent metastudy:

"Although this meta-analysis provides compelling evidence for an association between marijuana use and crash risk, it is impossible to infer causality from these epidemiologic data alone. A more rigorous assessment of the nature of the relation between marijuana use and crash risk is complicated by several factors. Among them are bias from measured and unmeasured confounding and the difficulty posed by polydrug use. Although most of the studies included in this meta-analysis considered some confounding variables in evaluating the association between marijuana use and crash risk, adjustment was usually limited to a few measured covariates, such as age, sex, and alcohol use. Additional variables that may confound the relation between marijuana use and crash risk, such as exposure to driving and risk-taking propensity, are often difficult to mea- sure and thus are not readily available. Other factors confounding the relation between marijuana use and crash risk are divergent definitions and assessments of marijuana use across studies.

Different methods of assessing marijuana use (e.g., self- report, urine tests, and blood tests) may have different levels of validity and reliability (39). Most of these screenings determine whether marijuana was used within the past few weeks, whereas acute impairment in driving skills from marijuana use lasts only 3–4 hours (39). Furthermore, because marijuana is an illicit drug in most countries, it is possible that drivers in the comparison groups might be less likely than those involved in crashes to submit to testing, which could lead to overestimation of the effect of marijuana use on crash risk (43).

Polydrug use represents another challenge to determining the role of marijuana in motor vehicle crashes. Polydrug use by drivers is common, with up to a quarter of drivers injured in crashes testing positive for 2 or more drugs (including alcohol) (42, 73, 74). Although it is necessary to understand the effect of individual drugs on driving performance, the high prevalence of polydrug use by drivers makes it difficult to do so. On the other hand, assessing interaction effects on driving safety of different drug combinations based on epidemiologic data would require very large study samples, comprehensive drug testing data, and tremendous financial and other resources (42–44, 75, 76). One of the studies included in the meta-analysis evaluated the effect of marijuana in combination with alcohol on crash risk and found that the combination of marijuana and alcohol confers an exceptionally heightened risk to driving safety (68). Experimental studies show that cannabis and alcohol affect different cognitive functions that may interact with driving performance (41). Specifically, cannabis seems to impair automatic behaviors, such as tracking, at low doses and impair ability to perform more complex tasks at higher doses, whereas alcohol seems to more readily affect functions requiring cognitive control (41). While both marijuana and alcohol can impair driving performance, there appears to be greater variations in effects of marijuana compared with alcohol (41)."

source: Marijuana Use and Motor Vehicle Crashes

Mu-Chen Li, Joanne E. Brady, Charles J. DiMaggio, Arielle R. Lusardi, Keane Y. Tzong, and Guohua Li