According to polls, chances are high that California voters will pass the recreational marijuana ballot initiative. A big concern to opponents is the potential for more impaired drivers on the roads. As California braces for the uncertainties posed by legal weed, questions loom.

Does cannabis impair driving the way alcohol does?

It depends on who you ask. Some point to studies such as one by a Colorado law enforcement group indicating a 48 percent increase of cannabis-related traffic deaths since the state legalized cannabis in 2013, as reported by the Los Angeles Times, which also referenced an AAA report finding that the number of drivers involved in fatal accidents who had recently used cannabis had more than doubled to 17 percent between 2013 and 2014.

However, a 2015 National Highway Traffic Safety Administration study found that after adjusting for age, gender, race and alcohol use, drivers testing positive for marijuana were no more likely to crash than those who hadn’t used any drugs or alcohol. This report noted alcohol is still the most dangerous substance to use behind the wheel.

This study echoed results of a 1993 Department of Transportation study that noted marijuana caused slight impairment, but that drivers under its influence retained insight into their performance and compensate by slowing down or increasing effort. “As a consequence, THC’s adverse effects on driving performance appear relatively small.” However, the study cautioned that it should not be inferred that drugged driving is risk-free.

Is it legal to drive while impaired by cannabis?

Under California law, it is (and would remain if Prop 64 passes) illegal to drive under the influence of cannabis.

Which leads to the question: Is there an effective way to test for cannabis impairment?

Not yet. THC is harder to measure than alcohol by unit in the blood. Users are most affected when THC concentration is highest in the brain, and roadside brain tissue testing isn’t a thing yet, according to KCRW.


Has a level of THC concentration determined to render a driver unsafe been determined?

No, it hasn’t!

A study commissioned by the AAA said there is no scientific basis to the testing currently in use by law enforcement to determine whether someone is too impaired by cannabis to drive.

Incidentally: Prop 64 sets aside $3 million a year from 2018 to 2023 to the California Highway Patrol to study detection of drugged driving impairment.

If you are paranoid you're being followed and you haven't left your driveway yet, should you be driving?

Only you can make that call, bro. But signs point to no.