Impairment is the big question surrounding the enforcement of driving under the impairment of cannabis. It varies much more than alcohol based on BMI, age, and use history. Here are a few good resources for understanding cannabis impairment and how it compares to alcohol.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2722956/ https://mass-cannabis-control.com/wp-content/uploads/2019/01...

One of the most prolific researchers in this space is Marilyn Huestis. Anything by her is really good.