Report: Fatal marijuana-related crashes up where drug is legal

Study: Increase in deadly crashes involving m... 02:04

It's been nearly three years since Mary Gaston's son Blake was hit and killed at a Bellevue, Washington intersection by a driver who was high on marijuana.

Mary Gaston and her son Blake Mary Gaston

"He was laying in the intersection, on the ground, had a massive head injury. So he was bleeding out," she said.

In 2012, Washington became one of the first states to legalize marijuana. Now a new study by AAA has found the number of fatal accidents involving drivers who'd recently used pot more than doubled between 2013 and 2014.

The study also found that there is currently no reliable test to determine the amount of marijuana in the blood stream that leads to driver impairment, reports CBS News' Kris Van Cleave.

"Biologically, cannabis and alcohol are very different. I think policymakers trying to do the right thing are trying to establish something like a .08 for cannabis, and there's just not science to support it," said Jake Nelson, the AAA foundation's director of research.

Researchers instead encourage training officers to detect drug impairment. One dash cam video shows a speeding driver cutting through traffic and struggling to maintain his lane.

CBS News

"Now let me ask you this, where's the weed at," the officer asked after pulling the car over. The driver admitted to smoking marijuana.

A 2015 federal government report found the crash risk from drugs like pot was highly influenced by other factors including age and gender. More than a dozen states are considering legalizing marijuana.