The study, commissioned by the AAA Foundation for Traffic Safety, found that laws in six states that legally assess impairment by measuring how much THC (the active ingredient in marijuana) is in a person’s blood are not supported by science.

Lawmakers in those states looked to policies on drunken driving for cues on how to legislate against driving while high. But the body absorbs alcohol and cannabis in different ways, the study said. While drunkenness directly correlates to alcohol in the bloodstream, cannabis impairment takes place only when THC makes its way into the fatty tissue of the brain.

So what did they decide?

Over a dozen states are considering legalizing marijuana in some form in 2016, but it is not clear how they would institute laws to prevent people from driving while high.

California’s Adult Use of Marijuana Act, which proposes to legalize recreational marijuana, includes a provision for $15 million to be given to the California Highway Patrol over five years to “develop protocols and best practices for determining when a person is driving while impaired, including from marijuana use.”

The AAA study echoes the recommendations of many experts who call for the improvement of technology to evaluate drivers’ saliva for cannabis use.

Sean O’Connor, the faculty director of the Cannabis Law and Policy Project at the University of Washington, said that there was promising research into detecting cannabis through saliva and other techniques, but that it was being stymied by the drug’s legal classification as a Schedule 1 substance.

“We are hamstrung by the fact that you can’t do legitimate scientific research unless you have a Schedule 1 license,” he said.

His argument underlines the difficulty for states trying to write coherent policy when the drug is still illegal under federal law. Douglas Berman, a law professor at Ohio State University who edits a blog on marijuana law, policy and reform, said that even our knowledge about the way people drive when high is confounded by the reality that marijuana was illegal when some of that data was collected.

“People had an extra reason to be cautious drivers when they were aware that they smelled like marijuana,” he said. “Now that it’s legal in some jurisdictions, absolutely that may affect not just the willingness of people to get behind the wheel after they’ve smoked, but also how much they’ll worry about getting pulled over

Still seems like it's really hard to tell how much it impairs driving. 

Accident rates have been going down in areas that have legalized... not clear if stoners drive less or self regulate (i.e. drive slow)

Probably some of each.