During the campaign to legalize recreational marijuana in Colorado, marijuana opponents had a talking point that they hit over, and over - if marijuana becomes legal, there will be mayhem on public roadways. According to their campaign rhetoric, marijuana opponents wanted voters to believe that a massive epidemic of stoned drivers would surely follow the passage of marijuana legalization. The numbers are in for 2014, and marijuana was not the largest cause of DUIIs, not by a long shot. Per The Cannabist:
For 2014, The State Patrol reported that troopers issued 5,546 citations for driving under the influence of drugs or alcohol. Of those, 674 --- about 12.2 percent --- involved suspected marijuana use, either alone or in combination with other intoxicants. For 354 of those citations --- about 6.4 percent of the total, or one in every 16 --- marijuana was believed to be the only substance involved.
The State Patrol did not provide statistics on the marijuana blood levels found in the cases or how often the citations led to convictions. The numbers also don't provide a complete picture on stoned driving enforcement in Colorado because they don't contain tallies from local police and sheriff's departments.
I think it's safe to say that not all of those cited for a marijuana-only DUII were convicted, so the statistics quoted in the article above are actually even higher than what the end result actually was. Due to per se marijuana limits that are not based on science, marijuana DUIIs are fairly easy to beat considering it's rare that the person was actually impaired due to marijuana consumption at the time of the DUII. It will be much harder for marijuana opponents to make the 'stoned driver' argument in the future when they try to fight marijuana reform in other states, which is obviously a great thing. If marijuana opponents truly feared DUIIs, they would go after substances that are far more likely to be the cause of a DUII. However, I doubt they will ever lead the charge against those substances.