New study on marijuana and driving in Colorado

Following up on yesterday's post about driving and marijuana legalization, the Cannabist reports that a new study has been released showing an increase in the number of Coloradans in fatal car crashes that tested positive for marijuana.  The study focused on the period from 2009-2011, when Colorado's commercial medical marijuana market came into being.  It does not include post-legalization data.  

Like other studies on marijuana and car fatalities, the study's tests cannot determine whether the drivers were actually impaired or whether they had smoked marijuana at some earlier date.  As a result, we don't know whether the positive tests are simply the result of increased use or indicative or an increase in impaired driving.  

Adding to the complexity, the story notes that traffic fatalities in Colorado decreased overall during the relevant time period.  If there had been a significant rise in marijuana-impaired drivers on the road (as opposed to a rise in people testing positive because of a general increase in use), we might imagine that it would have resulted in an overall increase in traffic fatilities.  Of course, it could be that marijuana impaired driving led to an increase in fatalities but that the increases were more than offset by other developments (e.g., innovations in car safety, effectiveness at deterring other forms of reckless driving, etc.). 

In any event, it will be interesting to see if any future studies are able to tease out whether (and to what extent) legalization is resulting in more marijuana impaired driving.  Here's the beginning of the Cannabist story:

One study shows that more drivers involved in fatal car accidents in Colorado are testing positive for marijuana — and that Colorado has a higher percentage of such drivers testing positive for marijuana than other states even when controlled for several variables. But the data the researchers use do not reveal whether those drivers were impaired at the time of the crash or whether they were at fault.

 

“[T]he primary result of this study may simply reflect a general increase in marijuana use during this … time period in Colorado,” the study’s authors write.