Here's an article that sounds really scary: "Drugged driving now bigger threat than drunk driving." A report by the Governor's Highway Safety Association and the Foundation for Advancing Alcohol Responsibility is the basis for the headline. To quote from the article:
Marijuana was the most common drug found in fatally-injured drivers. More than one third -- 36.5 percent -- tested positive for marijuana...The reported cited a study which found heavy marijuana use can double the risk of motor vehicle crashes resulting in serious injury or death.
OK so let us look at the evidence about marijuana use and car crashes, shall we?
The National Institute on Drug Abusepage about marijuana and driving tells quite a different story. Although some studies show that there is an impairment of driving with higher levels of marijuana in the blood, this page finishes by quoting a 2014 study by the National Highway Traffic Safety Administration (NHTSA) that found "no significant crash risk correlated with cannabis" when all other confounding factors were accounted for.
This NHTSA study, worth the read because it is a really well designed study, found:
...analyses incorporating adjustments for age, gender, ethnicity, and alcohol concentration level did not show a significant increase in levels of crash risk associated with the presence of drugs.
Let's say that again, there was NO significant increase in crash risk associated with marijuana or other drugs when you account for age, sex, and alcohol use.
It is a fact that marijuana remains in the body for weeks after use, so "testing positive" for marijuana means simply that one has used marijuana some time in the past few weeks. Note how "testing positive" gets morphed into "drugged driving" in the article.
It is also worth mentioning that the Foundation for Advancing Alcohol Responsibility is funded almost exclusively by companies that produce and market alcohol. It is a fact that alcohol producers view recreational marijuana as a threat to their profits and have responded by funding anti-marijuana "public service" announcements in many states.
Can marijuana impair driving? Of course. Is the danger of marijuana impaired drivers bigger than alcohol impaired drivers? Despite the propaganda, the answer is a clear no.