What do studies really say about driving and marijuana?

This is tricky, since much research on marijuana and driving is, out of necessity, not truly “in real life.” Many of these contradictory studies tested Marijuana in low potencies nowhere near what we see today.

What we do know is that marijuana impairs reaction time and the ability to shift focus when there are multiple/competing stimuli surrounding the person (which, from a driving standpoint, would include stoplights, other cars, people, or other unanticipated issues).  

We also know that marijuana when combined with alcohol has an even greater impairing effect on driving.

The authors of I-502 included a per se limit for driving under the influence (DUI) with marijuana.  A per se limit means that independent of how people feel or think they’re doing, a value about the cutoff is considered in violation of the law (for example, for people over 21, we have a per se limit of DUI for alcohol of .08%).  The DUI limit for marijuana was set at 5 nanograms of THC per milliliter of blood for people over 21 years of age, and any positive amount for people under 21.  Why 5 nanograms of THC per milliliter of blood?  This cutoff was selected because research indicated this was the point at which impairment occurs.