Driving after smoking pot can double a person's risk of being in a serious or fatal car crash, according to a review of studies published Thursday in the British Medical Journal.

While it may seem like the review is just reinforcing advice to put it in park when under the influence, it's actually the first review of studies to look at the risk of crashing under the influence of marijuana independent of any other substances such as alcohol, according to the authors.

The results of the review "provide a more definitive statement on the direction that efforts in public policy and intervention should take in addressing road safety," the researchers wrote.

The findings did not change the policy of NORML, an organization advocating the legalization of marijuana.

"Just like alcohol, there's a difference between use and abuse," said Allen St. Pierre, executive director of NORML.

The organization condemns driving under the influence, he said.

Researchers in Canada reviewed nine studies that totaled nearly 50,000 people and found those who smoked within three hours before driving were twice as likely to get into a serious car accident. The risk was even higher among drivers aged 35 or younger.

Marijuana plays less of a role in car crashes than alcohol, but smoking pot, even in low doses, can put drivers at risk, researchers found. The studies didn't clarify what role marijuana plays in minor car crashes.