An article by Quinton James and Johnkeria Kinglocke titled "The Truth About Marijuana" published on the website of the Hanley Foundation (an anti-drug abuse organization in Florida) claims (my emphasis):
Would you allow someone to drive you while they are high? According to the Department of Children and Families (DCF), marijuana-impaired driving fatalities have more than doubled since marijuana use has become legalized. Did you know one in five drivers are under the influence of marijuana?
It is unclear if the authors intended to source their claim that 20% of drivers are under the influence of cannabis on research from DCF (or rather was sourcing DCF only to support the doubled fatalities claim), but, in any event, no DCF resources are cited in the article's bibliography.
Are 20% of automobile drivers under the influence of marijuana?
I would consider a claim that 20% of drivers have some trace of cannabis metabolites or other evidence of past drug exposure in their system that could be detected with a sufficiently sophisticated medical laboratory to be plausible, but that is not what the claim is. Rather, I find the idea that 20% of drivers one encounters on the road on any random day, time, or place are literally so whacked out on weed that they cannot exercise ordinary safe driving practices to be absurdly high (pun intended).
I did find a 2019 article "Driving Under the Influence of Marijuana and Illicit Drugs Among Persons Aged ≥16 Years — United States, 2018" from the US Centers for Disease Control (CDC) that reports that it found that 4.7% of those 16 or above reported driving under the influence of marijuana in the past year. 4.7% is much lower than 20%, and still even isn't the same claim. The CDC is reporting on any instance of stoned driving in the past year even if the person drove sober (or not at all) on the other 364 days of the year, while the Hanley Foundation appears to claim that 20% of drivers on the road right now (or on average) are high.