That there would be greater incidents of car crashes involving marijuana users is unsurprising.
What it doesn’t tell me is if the overall auto fatality rate has increased or decreased, as the total fatality rate would be a more telling correlation of marijuana’s effects
State law does not require coroners to test deceased drivers specifically for marijuana use in fatal wrecks — some do and some don’t — and many police agencies say they don’t pursue cannabinoid tests of a surviving driver whose blood alcohol level is already high enough to charge them with a crime.
Marijuana is figuring into more fatal crashes overall. In 2013, drivers tested positive for the drug in about 10 percent of all fatal crashes. By 2016, it was 20 percent.
More drivers are testing positive for marijuana and nothing else. Of the drivers involved in fatal crashes in 2014 who tested positive for cannabinoids, more than 52 percent had no alcohol in their system. By 2016, it had grown to 69 percent.
https://www.denverpost.com/2017/08/25/colorado-marijuana-traffic-fatalities/