The Colorado State Patrol (CSP) reports that marijuana was involved in about 12 percent of DUI citations issued by its troopers in 2014, the first year during which it was legal to sell cannabis for recreational purposes. Out of 5,546 total DUI citations, 354 (6.4 percent) involved marijuana alone, while 320 (5.8 percent) involved marijuana along with alcohol or another drug. It looks like alcohol was involved in nearly nine out of 10 DUI citations by the CSP.
You may wonder how these numbers have changed since 2013, when possession, sharing, and home cultivation of marijuana were legal but sales had not started yet; since 2012, when voters approved legalization; or since the years before then. We don't know, because the CSP started keeping track of marijuana's DUI share only last year. We also don't know how many of the 674 marijuana-related DUI citations resulted in convictions, or how many of the convictions involved drivers who were actually impaired by marijuana, since Colorado presumptively equates a blood THC concentration of five nanograms per milliliter with driving under the influence of a drug (DUID), even though many regular users can safely operate a vehicle at that level.
If there were an upward trend in marijuana-related citations, that could reflect an increase in the number of dangerously stoned drivers, the broadened definition of DUID adopted in 2013, an increase in the number of drivers who have detectable amounts of THC in their blood but are not necessarily impaired, an increase in police attention to marijuana-impaired drivers following legalization, or some combination of those four things. For what it's worth, the CSP has not released accident data for 2014 yet, but in 2013, the first full year following legalization, CSP troopers investigated 480 crashes involving alcohol- or drug-impaired drivers that resulted in death or injury, compared to an average of 531 in the previous four years.
More on stoned drivers here.